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UNIVERSITY OF HAWAI'I LIBRARY The Enigmatic Surface of(3200) Phaethon: Comparison with cometary candidates
A THESIS SUBMITTED TO THE GRADUATE DIVISION OF THE UNIVERSITY OF HAWAI'I IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF
MASTER OF SCIENCE IN ASTRONOMY
August 2005
By Luke R. Dundon
Thesis Committee:
K. Meech, Chairperson S. Bus D. Tholen To my parents, David and Colleen Dundon.
III Acknowledgments
lowe much gratitude to my advisor, Karen Meech, as well as the other members of my
thesis committee, Dave Tholen and Bobby Bus. Karen, among many other things, trained
me in the art of data reduction and good observing technique, as well as successful writing
of telescope proposals. Bobby helped me perform productive near-IR spectral observation
and subsequent data reduction. Dave provided keen analytical insight throughout the entire process of my project. With the tremendous guidance, expertise and advice of my
committee, I was able to complete this project. They were always willing to aid me through
my most difficult dilemmas. This work would not have been possible without their help.
Thanks is also due to numerous people at the !fA who have helped me through my
project in various ways. Dave Jewitt was always available to offer practical scientific advice,
as well as numerous data reduction strategies. Van Fernandez allowed me to use a few of
his numerous IDL programs for lightcurve analysis and spectral reduction. His advice was
also quite insightful and helped focus my own thought processes. Jana Pittichova guided
me through the initial stages of learning how to observe, which was crucial for my successful
observations of (3200) Phaethon in the Fall of 2004. Henry Hsieh was there to offer practical
advice about my project or about planetary science in general.
Jim Heasley also aided me in my momentous first and only observing night on the Keck
telescope, along with his collaborator, Lisa Prato from UCLA. Lisa provided me with her
REDSPEC data reduction software, which made possible the first successfully obtained L
band spectra on (3200) Phaethon. Successful observation and subsequent data reduction
with NIRSPEC would also not have been possible without the invaluable aid of support
iv astronomer Jim Lyke. At UKIRT, support astronomer Andy Adamson helped me both on the mountain and for numerous days and weeks after my observing run, always providing advice on successful spectral reduction strategies. His expertise aided me in detecting the near-IR spectral variation in (3200) Phaethon. At the IRTF Bobby Bus was there as support astronomer and committee member, always keeping an eye on my observing technique and ensuring that I obtained the best possible data for my target asteroid. With his practical advice I was able to take the appropriate steps to ensure that accurate data was obtained in my spectral observing runs.
Outside expertise was also provided by Paul Abell of the Johnson Space Center, who provided advice on my L-band spectral analysis of Phaethon. Petr Pravec of the Ondrejov
Observatory provided many thoughts and analytical advice on my photometric lightcurves of (3200) Phaethon and 1999 JD6. To both of them my thanks is due.
Besides scientific expertise, moral support and practical advice were constantly provided by my current officemate, Scott Dahm, as well as my previous officemates Lisa Chien and
Nick Moskovitz. My other classmates Hai Fu, Dave Harrington, Jeyan Kartaltepe, Mark
Pitts, Steve Rodney and Bin Yang also provided tremendous support. Bin was always there to offer both scientific advice and moral support through my project, being herself another planetary graduate student.
I must also extend thanks to the administrative staff at the HA, who helped in many countless ways, such as my travels to Chile, requests for interlibrary loans, and usage of audio-visual devices, to name a few. The computer support staff, including Pui Hin Roads,
Naryan Raja, George Miyashiro, and Phillip Cahela, all provided tremendous help through my numerous computer-related dilemmas. Equally helpful were the support staff for IRAF
(NOAO), who helped me through many data reduction strategies.
Last but definitely not least, I am forever indebted to my family, Mom & Dad, Maureen,
Mark and Molly, for their prayers and moral support through my graduate school adventure here at the Institute for Astronomy.
v Abstract
An investigation is presented on the surface properties of (3200) Phaethon via
photometric and spectroscopic observations. (3200) Phaethon is a unique member of the
near-Earth Asteroid population, as it is a remarkably blue (B-V=0.587) asteroid and the sole asteroidal parent body of a meteor stream, the Geminids. Numerous attempts have
been made to explain its previous history, including ejection from a cometary nucleus,
an asteroidal collision, or a close planetary encounter. We present results that show 0.3
0.9JLm and 0.8-2.5JLm hemispherical color variation across rotation. These color fluctuations
correlate with the asteroid's lightcurve variation. An anti-correlation exists between the visible and near-IR color variation. The cause of the color variation may be a combination of particle size effects and compositional variation, possibly a result of space weathering.
Additional asteroids and comets were observed to compare Phaethon's dynamical (orbit, rotation period) and physical (surface) properties. Ultimately, (3200) Phaethon may be evidence of a unique "transition object" that once contained a variable mixture of volatiles
and rocky debris.
VI Table of Contents
Acknowledgments v
Abstract ... vii
List of Tables viii
List of Figures IX
Chapter 1: (3200) Phaethon . 1
1.1 Introduction. 1
1.2 Observations 12
1.2.1 Imaging 12
1.2.2 Imaging Results 14
1.2.3 Image Analysis 20
1.3 Spectroscopy ..... 29
1.3.1 Spectroscopic Analysis. 36
1.4 Discussion . 46
1.5 Conclusions/Summary 52
1.6 References...... 55
Chapter 2: Comparison Objects. 64
2.1 Introduction. 64
2.2 Imaging 71 2.3 Results. 76
2.3.1 1999 JD6 77
Vll 2.3.2 (142) Polana 80
2.3.3 1997 SEs .. 83
2.3.4 49P/ Arend-Rigaux 86 2.3.5 (1566) Icarus 88
2.3.6 (944) Hidalgo. 88 2.3.7 Imaging Analysis . 90
2.4 Spectroscopy ...... 95 2.4.1 Spectroscopy Analysis 101
2.5 Conclusions 104
2.6 References . 105 Chapter 3: Conclusions 111
3.1 (3200) Phaethon: an unusual object 111
3.2 References ...... 118
Vlll List of Tables
1.1 Properties of (3200) Phaethon . 5
1.2 Possible absorption features 10
1.3 Observing Instruments ... 12
1.4 Extinction Coefficients[mag/airmass] for (3200) Phaethon 13
1.5 Observing Conditions 14
1.6 (3200) Phaethon colors. 15
1.7 . Colors of (3200) Phaethon over multiple phase angles. 26
1.8 Observing Instruments . 30
1.9 Spectroscopic Conditions for (3200) Phaethon . 30
1.10 Average near-IR spectral slopes ...... 38
2.1 Comparison Objects .. 68
2.2 0 bserving Instruments . 73
2.3 Observing Conditions 74
2.4 Extinction Coefficients [mag/airmass] 75
2.5 Integration Times, SIN 75
2.6 1999 JD6 Colors .. 78 2.7 (142) Polana Colors 82
2.8 1997 SEs Colors .. 86 2.9 (1566) Icarus Colors 89
2.10 Comparison Object Colors. 93
IX 2.11 Observing Instruments. 96 2.12 Observing Conditions . 96
3.1 Asteroidal vs. Cometary Origin of (3200) Phaethon . 118
x List of Figures
1.1 Near-IR spectrum of (3200) Phaethon obtained by Dumas et at. (1998). The
spectrum has been normalized to 2.2t-tm. A symbol in the upper left-hand
corner displays the average error in the data...... 9
1.2 Theta statistic plot for period determination of (3200) Phaethon. The
deepest minimum occurs at 3.6048hr, indicative of the most likely period
for (3200) Phaethon...... 16
1.3 Phased R-filter lightcurve for (3200) Phaethon at a period of 3.6048hr.
Overplotted are the V-R colors of (3200) Phaethon, to be discussed in the
text. For this figure, peaks in the V-R curve correspond to bluer areas, and
troughs correspond to redder areas. The lightcurve is phased to OO:OOUT, 19
November 2004...... 17
1.4 Lightcurve constructed from data taken in January 1995, phased to OO:OOUT,
November 19, 2004. The lightcurve amplitude is larger from 2004, along with
rotational changes in lightcurve shape. Caution must be taken in comparing
this lightcurve with the 2004 lightcurve, since the uncertainty in phasing (due
to uncertainty in the known period) is +/-2.4hr, or 0.67 rotational phase.. 18
1.5 Interpolated and phased V-R measurements of (3200) Phaethon, at a period
of 3.6048hr and zero phase OO:OOUT, 19 November 2004. There is an obvious
sinusoidal trend of the V-R data for all three nights of observation. Overlaid
are corrected field stars that were used for differential photometry in all
November 2004 data...... 19
Xl 1.6 R-filter lightcurve produced from 1997 data on (3200) Phaethon. Data
were phased to 3.6048hr at the zero phase of OO:OOUT November 19, 2004.
Uncertainty in phasing the 1997 data with the 2004 lightcurve is +/-1.7hr,
or 0.47 rotational phase...... 20
1. 7 B-V and V-R lightcurves from November 22, 1997. Calculations were phased
to 3.6048hr and zero phase 00:00 UT, 19 November 2004. 21
1.8 R-I and V-I lightcurves from November 22, 1997. Calculations were phased
to the same period and zero phase of Fig. 1. 7...... 22
1.9 BVRI broadband spectrum of (3200) Phaethon. The data is normalized to
the I-filter (0.826j.tm) measurement. 23
1.10 Airmass test for V-R color measurements made for (3200) Phaethon (same
color scheme as in Fig. 1.5). The second plot expands between airmass 1.05
and 1.2. It is evident the 2004 V-R colors are not a function of airmass. .. 24
1.11 Phase curve for (3200) Phaethon. Red tick marks indicate actual data, and
the blue fitted line represents a linear fit to those data points. The linear
1 slope coefficient is ,6=0.0259 mag deg- . Errors are smaller than the size of
the individual data points...... 25
1.12 B-V colors of (3200) Phaethon over multiple phase angles. There appears to
be a slightly linear trend of reddening with increasing phase angle, which is
fair evidence of phase-dependent color behavior for this asteroid...... 27
1.13 Composite image (North-Up, East-left, 50"x35") of (3200) Phaethon
constructed from ",,35 images (""1200 seconds integration time) on November
19, 2004. Surrounding field stars have been removed to make a better analysis
of Phaethon's profile. Imperfections in the non-sidereal tracking can be seen
by the non-spherical shape of the asteroid. Due to its primary elongation
in the east-west (left-right) direction, vertical crosscuts of both asteroid and
star were made for creating surface-brightness profiles...... 28
Xll 1.14 Surface-Brightness profile for (3200) Phaethon on November 19, 2004 (R
filter). No obvious coma can be detected based on the results of this and the
subsequent SB profiles derived from the November data...... 29
1.15 Surface-Brightness profiles for (3200) Phaethon on November 22, 2004 (R
filter). The PSF is better represented for the field star on the left side of the
plot than the right side. No coma has been detected...... 31
1.16 Surface-Brightness profiles for (3200) Phaethon on November 22, 2004 (V
filter). Two different field stars were used for these two profiles. We assume
from these results that Phaethon currently is not ejecting any volatile gases
or coma...... 32
1.17 SB profiles taken from individual60-second R(left) and V(right) filter images
on November 19, 2004. Although noise is stronger in these single-frame
profiles, there is still no sign of activity (coma) from the asteroid...... 33
1.18 Binned spectra obtained for (3200) Phaethon from SpeX, 0.8-2.5j.tm, at
R,....,100. These two spectra are separated in time by approximately 130
minutes (or roughly 60% rotational phase). The spectrum on the right is
distinguished by its stronger curvature, which periodically appeared in the
reduced spectra. All IRTF data have been normalized to 1.22j.tm. The upper
left hand corner displays a symbol showing the average error in the spectral
data points...... 34
1.19 Relative reflectance spectrum of (3200) Phaethon, using an ATRAN
atmospheric modeling program. Error bars have been incorporated into the
plot of this spectrum...... 35
Xlll 1.20 1.4-2.5{Lm spectra obtained from UIST at UKIRT in December 2004.
Rotational variations in this Rrv800 spectra are easier analyzed by comparing
their computed numeric slopes (%j{Lm). Spectra displayed here are separated by 20% in rotational phase, and are normalized to 1.6{Lm. The upper left
hand corner displays a symbol showing the average error in the spectral data
points " 36
1.21 NIRSPEC combined spectrum obtained for (3200) Phaethon. Weak signal
did not allow time-resolved coverage in this wavelength region. Overplotted
is the modeled thermal emission of (3200) Phaethon, using the NEATM
formulated by Harris (1998). A symbol plotted in the upper left-hand corner
displays the average error of the data points. 37
1.22 Spectral slopes (measured 0.8-2.3{Lm) of the SpeX data from Nov. 20, 2004
(zero phase OO:OOUT, 19 November and phased to a 3.6048hr period). Note
the bluer surface from 0.0-0.40 rotational phase, then sharp reddening from
0.50-0.70 rotational phase. The data are rather noisy from 0.80-1.0 rotational
phase. The overlaid Fourier fit has a 50-a probability. 39
1.23 Separate spectral slope plots covering two different regions of the SpeX data. The left-hand plot representing data from 0.8-1.8{Lm shows a stronger trend
of spectral slope with rotation than the right-hand plot representing data
from 2.0-2.5{Lm. The overlaid fit to the 0.8-1.8{Lm data has a 25-a certainty. 40
1.24 Spectral slopes (measured 0.8-2.3{Lm) of the SpeX data from Nov. 20, 2004,
taken from the ATRAN-reduced spectra...... 41
1.25 Spectral slopes of the UIST data from Dec. 17, 2004 (zero phase OO:OOUT, 19
November and phased period of 3.6048hr). The bluer surface is once again
revealed in the first 40% of rotation, and follows with a gradual reddening
in the second half of rotation. Overlaid is a Fourier fit to the spectral slope
variation, with a confidence of 12.5-a. These data represent the highest SjN
spectra obtained for (3200) Phaethon...... 42
xiv 1.26 Spectral slopes from UIST on Dec. 19, 2004 (zero phase OO:OOUT, 19
November). The bluer surface in the first half ofrotation is seen, albeit with
fewer data points. Cirrus clouds hampered successful observations across the
entire rotational phase on 19 December. The confidence of this fit is 3.5-CT. . 43
1.27 NIRSPEC 2.9-3.69j.lm spectrum of (3200) Phaethon, after subtraction of the
modeled thermal emission. Thermal emission dominated the reflected signal
after 3.4j.lm. Plotted in the upper left-hand corner is a symbol displaying the
average error of the data points. 44
1.28 Seeing tests for data from SpeX at IRTF and UIST at UKIRT in November
and December 2004. If the spectral slopes were a direct function of the
changes in average spectral intensities (normalized to counts/sec for UKIRT,
since different integration times were used), then the spectral variation in
(3200) Phaethon would not be intrinsic to the asteroid rotation...... 45
1.29 Airmass tests for data from SpeX and UIST in November and December.
Once again, there seems to be no direct correlation between the spectral
slopes and the airmasses of the individual observations. 46
1.30 Near-IR spectra of (3200) Phaethon and carbon-black. The match-up
between asteroid and mineral are quite close. CK chondrite does not match
here due to its reddened spectrum from an absorption feature at 1.1j.lm.
Plotted in the upper left-hand corner is a symbol representing the average
error of the data points for (3200) Phaethon. 47
1.31 L-band spectra of (3200) Phaethon, carbon-black, and a CK chondrite. The
similarity between Phaethon's spectra and the two compounds are close
until 3.4j.lm, where thermal emission dominated the spectrum of Phaethon.
Plotted in the upper left-hand corner is a symbol representing the average
error of the data points for (3200) Phaethon. 48
xv 1.32 Side-by-side plots of Fig.1.5 and Fig.1.24, which display (3200) Phaethon's V
Rand near-IR spectral variation, respectively. Data on the left were obtained on Nov. 19, 2004, and data on right were obtained on Dec. 17, 2004 (UT).
Both datasets are phased to the 3.6048hr period of Phaethon, and zero phase OO:OOUT, Nov. 19, 2004...... 53
2.1 Phased lightcurve for 1999 JD6 , phased to a period=7.668hr, at zero-phase of 08:15UT, 03 June 2004. The deviation in data from 10-20% rotation remains unexplained...... 79
2.2 B-V(left) and V-R(right) rotationally phased colors for 1999 JD6 from June 2004. The B-V colors represent data taken on June 9 & 11, and the V
R colors represent data taken on June 11 & 12. The decreasing color here
correlates with the first maximum of the lightcurve (10-15& rotational phase) shown in Fig. 2.1. 80
2.3 Broadband spectrum for 1999 JD6 , taken from its BVRI data points.The
data is normalized to the I-filter (0.826j,tm) measurement...... 81
2.4 V-filter lightcurve of (142) Polana, phased with the determined period of
9.764hr by Barucci et al. (1994). Zero-phase is at OO:OOUT, 18 June 2004.
The triple-peaked lightcurve differs from Barucci et ai.'s due to aspect angle
differences. 82
2.5 Broadband spectrum for (142) Polana, taken from its BVRI data points. The
data is normalized to the I-filter (0.826j,tm) measurement...... 83
2.6 V-I colors for (142) Polana, phased to period=9.764hr. No color variation could be detected within the error bars...... 84
2.7 Binned V-filter lightcurve for 1997 SE5 , phased with the 9.058hr period of Pravec et ai. (1998). Zero-phase was OO:OOUT, 18 June 2004...... 85
xvi 2.8 V-I colors for 1997 SE5 , phased to period=9.058hr. Since the June 21 data is probably due to instrumental effects (see text), no net color variation is
detected here...... 86
2.9 BVRI broadband spectrum for 1997 SE5 . The data is normalized to the I-filter (0.826p,m) measurement...... 87
2.10 R-filter lightcurve of 49PI Arend-Rigaux, phased to a 13.47hr period, which is 0.11 hr different from the period derived by Mills et at. (1988). Zero phase
OO:OOUT, 06 November 2004. 88
2.11 Binned V-I colors for 49PI Arend-Rigaux, phased to the 13.47hr period. Of the measurements that have high enough SIN, no color variation is detectible. 89
2.12 (1566) Icarus broadband spectrum. Typical to most S-types, there is steep
reddening until 0.75p,m, followed by a slight decrease, which is usually
evidence of a 1.0p,m pyroxene absorption feature. The data is normalized
to the I-filter (0.826p,m) measurement...... 90
2.13 Composite images for 1997 SE5 (left) and 1999 JD6(right). The image of
1997 SE5 (North-right, East-up) represents ",4500 seconds integration and is
94" square. The image of 1999 JD6 (North-up, East-left) represents ",3500 seconds integration and is 43" square. . 94
2.14 Surface-Brightness profile (V-filter) of 1997 SE5 . While nearby stars were removed from the composite image of 1997 SE5, the anomalous bump on the
left side is probably due to improper removal of nearby stars...... 95
2.15 Surface-Brightness profile (R-filter) of 1999 JD6. While B-V and V-R color
variations were detected on this asteroid, no coma is present...... 97
2.16 Spextool-reduced spectrum of (944) Hidalgo on 20 November 2005, at
08:00UT. Parts of the spectrum were removed due to residual atmospheric
absorption still present at 1.4 and 1.9p,m. Error bars (included with the data
points) are smaller than the data points. 98
XVll 2.17 ATRAN-reduced spectrum of (944) Hidalgo on 20 November 2004, at
08:00UT. Error bars (included with the data points) are smaller than the
data points...... 99
2.18 REDSPEC-reduced spectrum of (944) Hidalgo on 24 December 2004,
normalized to 3.3f-lm. Located in the lower left-hand corner is a symbol
displaying the average error in individual data points. Overplotted is a
modeled thermal emission spectrum of the asteroid. Plotted in the upper
left-hand corner is a symbol depicting the average error in the data points. 100
2.19 Spectrum of NEA 1998 XB, which displays a 2.0f-lm pyroxene absorption
feature characteristic of S-type asteroids. Spectrum is normalized to 1.6f-lm.
Persistent atmospheric contamination at 1.9f-lm has been removed for clarity.
Plotted in the upper left-hand corner is a symbol depicting the average error
in the data points...... 101
2.20 Spectrum of main-belt asteroid (757) Portland. Spectrum is normalized to
1.6f-lm, with atmospheric absorption again removed at 1.9 f-lm. Plotted in the
upper left-hand corner is a symbol depicting the average error in the data
points...... 102
2.21 Thermal-subtracted spectrum of (944) Hidalgo. Overlaid is a reflectance
spectrum calculated via the NEATM software. The discontinuity between
the two fits may be due to the attempt to use a near-Earth asteroid modeling
routine for a main-belt asteroid. Plotted in the upper left-hand corner is a
symbol depicting the average error in the data points...... 103
2.22 Thermal-subtracted spectrum of Hidalgo, overplotted with a CK chondrite
spectrum. Although there appears to be similarity between the two spectra
from 2.9-3.4f-lm, the curvature of Hidalgo's spectrum is more pronounced at
all wavelengths. Plotted in the upper left-hand corner is a symbol depicting
the average error in the data points...... 104
XVlll 3.1 B-V VB. V-R and V-R VB. R-I colors of (3200) Phaethon, comparison objects,
and other asteroids, from Yoshida et ai. (2004), as adapted from results of
the ECAS survey. Solid triangles refer to C-types, asterisks refer to S-types,
open squares refer to D-types and open stars refer to M-types. Phaethon is
shown in blue, while the other comparison objects analyzed in this project
are shown in red...... 112
3.2 Carbon and Graphite 0.4-2.5f,lm spectra, normalized to 1.0f,lm. Also
overplotted are the near-IR spectrum of (3200) Phaethon and its broadband
data points from the BVRI photometry. In the lower left-hand corner is a
symbol displaying the average error of the Phaethon data...... 116
XIX Chapter 1
(3200) Phaethon
1.1 Introduction
The distinction between cometary and asteroidal properties has been a contested issue for years. Understanding the intrinsic compositional differences between comets and asteroids is a challenge because these objects are often small, have low reflectivities and are usually far from the Earth. Studying cometary nuclei would be optimal, but these objects are usually quite faint when inactive. Cometary nuclei are consequently difficult to observe with earth based observations. However, some nuclei that are extinct may reside closer to Earth, and would allow for easier observations. The Near-Earth Asteroid (NEA) population may contain cometary members (e.g., Weissman et at. 2002). Although comets and asteroids have traditionally been distinguished by the presence or lack of comae, current data show that these objects can be dynamically or physically similar. Comets can exist very close to asteroids, such as the Jupiter Family Comets. Many Jupiter Family Comets (JFCs) are aptly named due to their orbital coupling with Jupiter. This coupling is reflected by the value of the Tisserand Invariant (TJ), which is usually less than 3.0 for comets and greater than 3.0 for asteroids. Those asteroids that do have coupled orbits with Jupiter
(and consequently have TJ < 3.0) are dynamically similar to comets. Asteroids with colors or spectra similar to comets are physically similar to comets.
1 Comets with orbits near Jupiter are theorized to form in the Kuiper Belt region beyond
Neptune. Kuiper belt objects (KBOs) form in situ. Comets found in the Oort Cloud beyond
Pluto originally formed closer, between Jupiter and Neptune, and eventually were perturbed outward by the giant planets. Upon entering the planetary region of the Solar System due to perturbations by galactic tides or passing stars, comets can interact with both the giant outer and terrestrial inner planets. The comet nuclei shed mass via sublimation of volatiles
(which drags dust from the nucleus and forms a tail) in their eccentric orbits around the
Sun. Eventually, these nuclei may become inactive after they form a refractory mantle, shed all their volatiles, be dynamically ejected from the Solar System altogether, or impact either a planet or the Sun.
The refractory components on 19PjBorrelly detected by spacecraft Deep Space 1 have helped explain the albedo variations previously detected on the surface of its nucleus (Nelson et at. 2004, Buratti et at. 2001). Models of Solar System formation may explain how comets could have heterogeneous compositions that could be detectable as color or albedo variations across their surfaces. As the Solar System formed, planetesimals that evolved to cometary nuclei experienced radial migration due to pressure gradients in the solar nebula (Weidenschilling 1997). Differences between comet orbital velocity and orbital velocity of the gas nebula produced frictional forces that could slow the comet, and consequently induced radially inward motion. Since compounds of different condensation temperatures formed at different radial distances from the Sun, the migration of cometary bodies incorporated a diverse assortment of material into their composition. Comets would consequently have heterogeneous compositions as a result of their formation. This heterogeneity is evidenced by jets of different gas species emanating from comae.
Many comets are known to go through periods of low activity or near-dormancy, as was observed for 28PjNeujmin 1 and 49Pj Arend-Rigaux (e.g., Birkett et at. 1987). While there obviously are significant abundances of volatiles on comets, a non-trivial amount of dust and rocky debris are also a part of their composition, as shown by spacecraft observations of IPjHalley (Grothues & Schmidt-Kaler 1996), 19PjBorrelly (Soderblom et at. 2004), and
2 81P/Wild 2 (Farnham & Schleicher 2005). Although asteroids conversely have primarily rocky compositions, there have been rare but interesting reports of ices located on large asteroids such as (1) Ceres (A'Hearn et at. 1992). By the end of their active lifetime, some
(i. e., a few hundred to a few thousand) comet nuclei may become stabilized in a localized zone nearby the inner terrestrial planets (Harris & Bailey 1998). The amount of time that a nucleus may remain in an inactive state (before disintegrating) within the terrestrial planet zone should be approximately 40% of the comet's active lifetime (Fermindez et al.
2002). The discovery of extinct nuclei would lead to initial classification as asteroids, due to their lack of comae. Such was the case for 107P/(4015) Wilson-Harrington, which was independently discovered as both a comet (in 1949) and an asteroid (in 1979), due to its weak release of volatiles. The photographs taken of this object in its discovery year of 1949 are the only images to date that positively show a coma (Cunningham 1949, Fermindez et at. 1997). 107P/Wilson-Harrington is now believed to be either a dormant or extinct comet, due to its lack of apparent activity. The tail in the 1949 images had bluer than solar colors
(B-Rrv-1.0, where B-Rsun=1.17; Allen 1973) which could be evidence of volatiles such as water ice or ionized gases, or a plasma tail.
Primitive, ice-rich objects found beyond the orbit of 107P/Wilson-Harrington have only just begun to sublimate their volatiles. Centaurs, the dynamical intermediaries between
Kuiper Belt objects and Jupiter Family comets, probably contain many frozen volatiles
(e.g., Dotto et at. 2003) that will sublimate and form comae only if they get close enough to the Sun. Several rotationally sampled color measurements of Centaurs have been made for which there have been claims of variable surface colors (AmrvO.05-0.1). Barucci et al. (2002) detected probable near-IR spectral variations on the surface of 2001 PT13, and Bauer et at. (2002) detected optical color variations across the surface of 1999 UG5 , both suggestive of possible compositional variation. Sekiguchi et at. (2002) additionally detected V-R heterogeneities across the surface of KBO 1996 T066 . Kern et at. (2000) reported similar fluctuation of albedo(rv 20%) and spectral shape(rv5%) in the Centaur (8405) Asbolus.
These results were initially surprising, because it was expected that surface composition
3 would be uniform from resurfacing by dust, cosmic ray or UV irradiation (weathering), and by micrometeorite bombardment (Housen et at. 1979).
Asteroids are expected to evolved objects (non-primordial) with uniform colors due to the same aforementioned weathering processes. However, even these objects (found in the main belt between Mars and Jupiter) have displayed rare instances of color variation, such as near-Earth asteroid (433) Eros, and main-belt asteroids (243) Ida and (951) Gaspra
(Murchie et at. 2002), where each had observed hemispherical color variations of 20-40%. Do some asteroids have color variations as a consequence of their formational histories?
While main-belt asteroids' formed in situ, near-Earth asteroids did not originate in the inner Solar System. They were scattered or gravitationally perturbed from the main belt
(e.g., Morbidelli et at. 2002). The asteroid main belt has a compositional gradient, whereby iron and silicate-rich objects are closer to the Sun than carbonaceous asteroids (e.g., Binzel et at. 2002). This trend of composition with heliocentric distance has been termed a 'mineralogical gradient' (Jones et at. 1990), as heavier metals were condensing out of the nebula closer to the Sun than lighter volatiles (Cruikshank et at. 1991). This gradient was caused by unique condensation temperatures amongst different compounds. Since silicate and iron-rich asteroids are concentrated in the inner part of the main belt, the near-Earth asteroid population is dominated by these iron-rich objects. Farther out are more carbon rich and darker asteroids. Even farther reside comets that have ices and organic compounds.
Asteroid and cometary formation sites may have been distant from each other in the past, but the evolution of these objects' orbits (as evidenced by 107PjWilson-Harrington) have brought members of the cometary population to mix with the near-Earth asteroids.
In addition to development of activity from volatiles, comets are classified dynamically, often having high eccentricities (e > 0.5). A few comets have moderate inclinations (i > 20°), unlike most asteroids that remain within the plane of the Solar System. Some asteroids share these dynamical properties, such as (944) Hidalgo, which has an eccentricity of 0.66 and inclination of 42.6°. Another dynamically cometary asteroid is (1566) Icarus, with e=0.83 and i=22.8°. Hidalgo remains within the main belt of asteroids and has a
4 Table 1.1 Properties of (3200) Phaethon ======::O:=r=:=b=:=it=a===l======Eccentricity 0.89 Inclination(O) 22.6 Semimajor Axis (AU) 1.24 Perihelion(AU) 0.14 Aphelion(AU) 2.6 TJ 4.5 Physical Diameter (km) 5.1+1-0.1b Rotation Period (hr) 3.604+1-0.001a Geometric Albedo (Pv) 0.11 +1-0.02b Color (B-V) 0.587+1-0.007c a Pravec et al. 1998. b Green et al. 1985. c This work.
perihelion of q=1.97AU. The small perihelion of q=0.18AU for Icarus is a consequence of its larger eccentricity and small semimajor axis (1.08AU).
Near-Earth asteroid (3200) Phaethon has no detectible coma, but does have distinct cometary characteristics, such as an orbital eccentricity of 0.89 and inclination of 22.6° (see
Table 1.1 for orbital and physical properties). Conversely, it has a very asteroidal Tisserand
Parameter of T J=4.5. Discovered by IRAS in 1983, Phaethon is the parent body of the
Geminid meteor stream (Davies et al. 1984). To date, every other known parent body of a meteor stream is a comet.
Meteor showers are believed to originate when the Earth passes through the orbiting debris left from active comets. In theory, an asteroidal collision could result in meteor stream debris. However, Hunt et al. (1986) and Fox et al. (1982, 1983) used dynamical models to show that only cometary dust ejection could produce the observed distribution of the
Geminid stream aphelia and perihelia. The most probable point of ejection of these meteors would have been at perihelion (0.14AU). An asteroidal collision would have ejected all particles simultaneously. All particles would have practically identical orbital parameters,
5 including similar aphelia. However, the aphelia of the Geminids are fanned out over a variety of aphelion distances, with a common perihelion (i. e., common point of ejection) of • 0.14AU, an unlikely location for an asteroid collision. This variation in aphelia indicates different times of ejection to their present respective orbits as well as perturbation from
Mercury, Venus, Earth and Mars. The dynamical age of the meteor stream is estimated to be between 1000-4000 years old, although the actual ejection of the particles from Phaethon probably occurred over a span of 1000 years (Beech 2002).
The Geminids represent a peculiar group of meteor particles. Their mass densities are higher than cometary meteors but less than average asteroidal densities. Cometary meteor densities average 0.5-1.5gjcm3 , while asteroidal meteor densities are 2.5-3.5gjcm3
3 (Halliday 1988). The Geminids' densities average 1.2gjcm . Asteroidal meteors have sufficient tensile strength to survive the descent to the Earth's surface, while cometary meteors more easily disintegrate in the upper the atmosphere. No meteorites have been discovered that originate from the Geminid meteor shower every December 13-14. They are more resistant to crumbling than other cometary meteors, as indicated by the Na D line in the fireballs of the shower (Halliday 1988). When comparing tensile strengths of fireball meteoroids, the Geminids are average. When comparing tensile strengths with meteorite material, the Geminids are 100 times weaker than average. The higher tensile strength of the Geminids in comparison to cometary meteors may be due in part to the thermal processing of the particles in their close passages around the Sun (Williams & Wu 1993).
The Geminid meteor stream will cease to intersect the Earth's orbit by the year 2100A.D.
(Gustafson 1989).
As a parent body of a meteor stream, Phaethon may have a high degree of macroporosity.
Britt & Consolmagno (2000) argued that the high porosity of interplanetary dust particles
(IDPs) may correlate to a parent body with high macroporosity, up to 30-50%. The mass density of the Geminids appears to be slightly higher than average cometary meteor streams, which implies a lower porosity amongst the particles than cometary particles.
Examples include the Perseids from 109PjSwift-Tuttle and Leonids from 55pjTempel-
6 Tuttle, whose mass densities average 0.5-1.0g/cm3 (Babadzhanov 1993). Consequently,
Phaethon's porosity may be slightly less than other cometary parent bodies of meteor streams, since the Geminids have an average mass density of 1.2g/cm3 ..
Besides association with meteor streams, comets can also be characterized by their general colors and/or spectral behavior. Cometary colors and spectral slopes are usually slightly redder than solar colors (Meech et at. 2004). Besides silicates or hydrocarbons, cometary red spectral slopes can also be caused by solar radiation (Strazzulla et at. 1991).
The red colors can also be caused by combinations of organic compounds. In contrast, blue colors can be indicative of icy or bodies with pure-carbon, or comets that are more active
(Jones et at. 1990, Cloutis et at. 1994). Active comets could display varying colors due to the behavior of outgassing vents (Mumma & Reuter 1989). Different species of outgassing compounds would exhibit colors unique to their compositions.
Many NEOs have red slopes into the near-infrared, due primarily to silicates and/or iron.
Asteroids most likely have no volatiles (except for asteroids such as the Trojans located in farther, colder regions), due to their closer proximity to the Sun as well as the radiogenic heat produced by radioactive minerals (such as A126). Diverse mineralogy on an asteroid, while not entirely diagnostic of a comet, may indicate that the object experienced radial migration, similar to comets in the forming stages of the solar nebula. Uniquely different minerals found on an asteroid may also be indicative of space weathering or differentiation, as found on (4) Vesta (e.g., Bottke et at. 2002). Taxonomic classification of an asteroid may also constrain its physical similarity to comets, as many D-type asteroids (such as 1997 SE5; Hicks et at. 2000) are hypothesized to be descendants of comets located nearby Jupiter's orbit.
The colors and spectral slope of (3200) Phaethon place it amongst the few known near
Earth asteroids with a blue featureless spectral slope, from the optical into the near-infrared.
Green et at. (1985) and Veeder (1987) obtained near-IR colors of J-H=0.27+/-0.01 and
H-K=-0.03+/-0.02 for Phaethon (solar JHK colors J-H=0.317+/-0.000 and H-K=0.072+/ 0.003; A'Hearn et at. 1984). Phaethon's near-IR colors are significantly bluer than most
7 asteroids (J-Havg =0.40+/-0.02,H-Kavg =0.05+/-0.04), but comparable to cometary near-lR colors (J-Havg=0.30+/-0.03,H-Kavg=-0.01+/-0.05). Red surfaces on asteroids and comets are easily explained by silicates, hydrocarbons, and particle size effects, but blue surfaces are much rarer and more difficult to constrain. They are usually explained by a combination of compositional and particle size effects.
Optical spectra of Phaethon reveal a steep negative (blue) slope of -13%/1000A (Luu
& Jewitt 1990). While this steep a slope has never been replicated, work by Lazzarin et at. (1996) also found that Phaethon has a (slightly less) negative optical spectral slope of
-3.9%/1000A. Average asteroidal slopes range from 6-20%/1000A. The near-lR spectrum of Phaethon obtained by Dumas et at. (1998), shown in Fig. 1.1, revealed no absorption features. The authors did find that the slope in the near-lR matched that of the dust continuum of the Centaur (2060) Chiron, which does have a coma (Meech et at. 1993).
The near-lR slope of Phaethon was slightly blue in the H-band, and neutral in the K band. No spectral features were detectible from 0.8-2.5p,m. They deduced that this spectral shape must correspond to a rubble mantle from which volatiles have escaped due to close approaches with the Sun (0.14AU). The data presented in this thesis also show a similar spectral slope and lack of absorption bands in the near-lR.
(3200) Phaethon is classified as an F-type (Tholen 1984). F-types are characterized by their negative spectral slopes and low albedo. B- and F-type asteroids might be matched with CK chondrite meteorites, since this form of chondritic mineral also has a negative optical spectral slope. CK chondrites, found most often in Antarctica (Campins & Swindle
1998), contain high abundances of magnetite, calcium-aluminum inclusions (CAls), and traces of Fe and Ni. These chondrites are very black and opaque (Carvano et at. 2003).
However, magnetite's spectral slope becomes positive (reddens) at 1.1p,m. This result is inconsistent with the present data on Phaethon, where the slope remains negative out to
2.0p,m. The exact cause of such a blue spectral slope for an asteroid has been a debated issue. A water ice surface would produce blue optical colors, but would also have absorption features at 1.5, 1.65 and 2.0p,m, as shown by observations of the ice-rich surface of the
8 2(3200) Phaethon-- Dumas et al. (1998)
I
O~'----'--'--'---'---'---L..-,--L..-,--L..-,--L.--L.--'------'------'------'------'------'------'------'----! 1.4 1.6 1.8 2 2.2 2.4 Wavelength (microns)
Figure 1.1 Near-IR spectrum of (3200) Phaethon obtained by Dumas et at. (1998). The spectrum has been normalized to 2.2,um. A symbol in the upper left-hand corner displays the average error in the data.
Uranian satellite Miranda (Bauer et at. 2002b; Brown et at. 1984). These absorption features are not seen on Phaethon's surface. The asteroid also approaches a perihelion
distance of q=0.14AU, where its equilibrium blackbody temperature should reach 750K;
this should have sublimated any water present on the surface of the object. Whether any
water still resides within is dependent on the thermal diffusivity of the inner constituents,
along with other physical properties. The blueness might possibly come from scattered
light from rough macroscopic surfaces or because of composition. The presence of a regolith
disagrees with the conclusions of Green et at. (1985) whose thermal data analysis found that Phaethon most likely has high thermal inertia and a solid rock surface. Usually,
objects with solid surfaces have high contents of silicates or iron, both of which are red.
The blue spectrum and colors thus observed on this object indicate a surface that should
9 Table 1.2 Possible absorption features ~:==:===:======Wavelength Feature 0.7 j1.m Phyllosilicates 1.0j1.m Pyroxene 1.1j1.m Magnetite, Fe304 1.1j1.m Olivine 1.5j1.m H2 0 (ice) 1.65j1.m H20 (ice) 1.73j1.m C-H stretch (organics) 2.0j1.m H2 0 (ice) 2.0j1.m Pyroxene 2.2j1.m CN 2.3j1.m C-H stretch (organics) 3.4j1.m C-H stretch (organics)
have a different mineralogical structure than its albedo and thermal inertia suggests from previous analyses. It may be possible to match Phaethon's spectral shape with that of known meteorites. Matching spectral properties of meteorites and asteroids, however, has proved to be a difficult challenge. Known meteorites may not be representative of the majority of asteroids found either in the near-Earth or main belt population.
Spectral slope variation present on asteroids like (3200) Phaethon may be due to either particle size effects or to compositional variations. Many absorption features are found in the 1-4j1.m region that are diagnostic of comets, such as C-H bands at 1.73j1.m, 2.3j1.m,
3.4j1.m, as well as possible (though improbable) CN absorption at 2.2j1.m. See Table 1.2 for a summary of these possible features, as well as common mineralogical features found on asteroids. The C-H absorption could be indicative of possible organic hydrocarbons
(Moroz et at. 1998). C-H bonds, however, are usually expected to be present in the form of volatiles, which are not expected to be present on Phaethon's surface. A previous search by Cruikshank et at. (2002) had difficulties detecting this C-H signature in a sample of
"" 10 low-albedo main belt asteroids, but the authors do agree that it should be present on primitive objects.
10 Color variations thi,l.t are caused by particle size effects naturally depend on the surface properties. Determination of the type of surface (rough, smooth, regolith, bedrock) depends on the assumed theory of reflective scattering. Hapke (1993) scattering theory calculates bi-directional reflectance for planetary surfaces as a function of observing geometry and the physical properties of the regolith. A major assumption in this model is that the average particle size of the regolith is significantly larger than the wavelength being used for observation. Asteroidal surfaces with large enough particles (>100j.Lm) should have flatter spectra with less pronounced absorption features. Hapke scattering theory also accounts for phase angle behavior such as phase reddening, where particles exhibit a decreased reflectance and reddened color at larger phase angles. This thesis relies on Hapke theory since Phaethon is known to have a densely packed regolith or solid rock surface (Green et at.
1985). However, this theory may be inaccurate or inappropriate when deducing properties of a particulate regolith on the surface. Mie theory, a simplified scattering model, assumes perfectly spherical and isolated particles that can be of any size relative to the wavelength used. Particles smaller than the wavelength observed will scatter blue, and particles larger will scatter red (Jewitt & Meech 1988). Such a theory is useful for studying dust grains located in a comet tail or coma. Mie theory will be assumed if we attribute any color variations to particle size effects. If the particles are of a soot-like substance much smaller than the wavelength observed, then Mie theory, though simplified, may be much more accurate to use than Hapke theory.
This thesis presents an analysis of the surface of (3200) Phaethon. Imaging was conducted for time-resolved colors from lightcurves, and spectroscopic observations were made to constrain possible absorption features or variations in spectral slope during the rotation of the asteroid. From these data and analyses, I show that (3200) Phaethon does indeed have a combination of optical color and near-IR spectral variations across the surface.
11 Table 1.3 Observing Instruments Date (UT 2004) Telescope Detector Filter(s) Apt Seef Conditions Nov. 19 UH 2.2 m TEK 2048 BVR 2.2 0.7 Photometric Nov. 21 UH 2.2 m TEK 2048 VR 2.6 1.0 Light Cirrus Nov. 22 UH 2.2 m TEK 2048 VR 2.2 0.7 Photometric t Photometric aperture radius [arcsec] j: Seeing [arcsec, FWHM]
1.2 Observations
1.2.1 Imaging
Imaging observations were made in November 2004 at the Mauna Kea Observatory, Hawaii.
We observed with the University of Hawaii 2.2m telescope at f/10 configuration and Tek
2048 CCD (see Table 1.3 for technical specifications and weather) with f/lO focal ratio.
While BVRI color measurements were made, the majority of images were interleaved with
V and R filters, in order to make time resolved color measurements. The B filter had lower throughput than the VRI filters. (3200) Phaethon was tracked at non-sidereal rates, due to its rapid westerly motion at approximately 100 arcsec/hr (north-south motion was comparatively small at 3-7 arcsec/hr). Unfortunately, the filter wheel on this telescope was in need of much repair. The faulty filter wheel hampered the interleaving of the filters between observations. The plate scale was 0.219" /pix, read noise was 6e-(rms), and gain was 1.74e-/ADD. The field of view for the square chip was 7.5 arcminutes across.
Overscan and trim correction, bias frame correction, fiatfielding and cosmic ray rejection were performed using standard IRAF routines. Flatfielding utilized twilight fiats, which more accurately represent the illumination pattern on the CCD than dome fiats, especially on the UH 2.2m. Landolt standard stars (1992) were used for atmospheric extinction, color correction, and zero point offset. Aperture photometry was performed with the IRAF
PHOT routine. The standards were measured with apertures twice the size of the stellar
PSF FWHM. Extinction coefficients for the three-night run at the UH 2.2m are given in
12 Table 1.4 Extinction Coefficients[mag/airmass] for (3200) Phaethon Night kB kv kR 19 Nov. 0.241+/-0.062 0.105+/-0.001 0.042+/-0.001 22 Nov. 0.137+/-{).001 0.084+/-0.001
Table 1.4. Because November 21 was non-photometric due to cirrus, calibration fields from November 22 were used.
Weather at Mauna Kea was mostly stable, with two photometric nights and one night with thin cirrus for the observing run on (3200) Phaethon. Seeing was equal to or less than
1.0" for each night. Table 1.5 describes the observing conditions for (3200) Phaethon.
While standard stars were used for calibration on photometric nights, differential photometry ("cloud correction") was also vital for accurate detection of color variations.
Passing cirrus clouds could interfere with lightcurve data (in either filter). Interference from clouds is not removed with standard star calibration, but can be removed by measuring nearby field stars in each of the frames with the asteroid. The weighted-average brightness of each field star (across all frames) was set as the true brightness for the respective star.
Groups of 10-20 field stars were measured in each image. Due to the motion of the asteroid and the finite size of the chip, the same set of stars could not be used for the entire night.
Instead, the set of stars used was slightly changed when at least one star would no longer be detectable on the chip. Overlapping stars between sets could be used to tie together the photometry for the entire night. All stars in each frame were then corrected to the weighted average brightness of each star. The average correction applied in each frame was subsequently also applied to the asteroid. While this procedure was most important on the second night, where light cirrus was present, differential photometry was also repeated as an added precaution to the first and third nights of the run.
The SIN for Phaethon remained high each night, ranging between 150 and 250. The non-sidereal tracking was unfortunately not working consistently, and occasionally caused deviations from an ideal circular PSF pattern in the asteroid. Circular PSF patterns would
13 Table 1.5 Observing Conditions Date RA Dec. r(AU) ~(AU) a(O)t lOt Moon(%) 2004 Nov. 19 0451 +3928 1.776 0.831 13.6 107.3 50 2004 Nov. 21 0444 +3923 1.759 0.805 12.5 80.0 72 2004 Nov. 22 0439 +39 19 1.750 0.793 11.95 66.5 81 tPhase Angle (Sun-Asteroid-Earth) [degrees] t E = Lunar Separation from asteroid [degrees] be crucial for detection of possible faint coma. Phaethon's rapid westerly movement across the star field caused occasional mergers with field stars. Because the NEO was close to opposition and at a low phase angle of rv12°, it was possible to maintain short integration times of 40 sec, which enabled more thorough temporal coverage of the lightcurve (and compensation for the sticky filter wheel).
1.2.2 Imaging Results
Lightcurve data were first corrected for phase angle, geometric and heliocentric distance, and light-travel time. The reduced magnitude m(l,l,a) at 1AU geocentric and heliocentric distances was computed using the relation
m(l, 1, a) = m - 5Iog(rD.) (1.1) where m is the observed magnitude, a is the phase angle, r is the heliocentric distance [AU], and ~ is the geocentric distance [AU]. The reduced magnitude is then normalized to zero phase angle (absolute magnitude H), by the H-G relation (Bowell et al. 1989):
m(l, 1, a) = H - 2.5Iog(1 - G)exp[-3.33tanO.63 (a/2)] + Gexp[-1.87tan1.22(a/2)] (1.2) where H is the reduced magnitude at phase angle a=O°, and G is the slope parameter (0.23 for Phaethon; Wisniewski et al. 1997). Table 1.6 contains the measured BVR colors for
14 Table 1.6 (3200) Phaethon colors Date at B-V V-R R-I 19Nov.2004 12.5 0.587+1-0.005 0.349+1-0.003 22Nov.1997 75.0 0.650+1-0.004 0.295+1-0.002 0.320+1-0.003 12Nov.1996 23.5 0.618+ 1~0.005 0.347+1-0.004 Solarf 0.665+1-0.000 0.367+1-0.000 0.338+1-0.000 t Phase Angle in Degrees(O). t Solar Colors, from Allen (2000).
Phaethon (also included are colors measured from data obtained in 1997 and 1996). Filters were interleaved R-B-R-V-R in order to correct for rotational lightcurve variation. This string of interleaved filters was run multiple times to average out systematic errors. From the colors in Table 1.6 we can deduce that (3200) Phaethon is bluer than Solar colors, which confirms the near-IR and thermal results of Green et ai. (1985) and Veeder (1987). The
2004 color V-R=0.349 (solar V-R=0.367; Rabinowitz 1998) is similar to results given by
Skiff et ai. (1996), who reported a V-R=0.34/-0.01.
Phase Dispersion Minimization (PDM; Stellingwerf 1978) was used to confirm the rotational period of (3200) Phaethon. PDM systematically tests lightcurve data for different periods, and subsequently displays the deviation ("theta-statistic") for each determined period (Bauer et ai. 2002). The lowest theta statistic corresponds to the most probable period for the data (Stellingwerf 1978; Bevington 1969). Fourier analysis, another period- determination method for lightcurve data, uses sine and cosine terms to fit a periodic function to the measurements. While Fourier analysis was also used as a redundancy to confirm the periods of each object, the phasing of each lightcurve was achieved with PDM.
The theta plot for Phaethon is shown in Figure 1.2. The deepest minimum occurs at
3.6048hr+I-0.0001hr, which confirms the 3.60hr rotation period measured by Pravec et ai.
(1998) and Krugly et ai. (2002).
Figure 1.3 displays the 2004 lightcurve of (3200) Phaethon, phased to a 3.6048hr period.
While the amplitude (b.m=0.12) of the lightcurve is comparable to published work (e.g.,
Wisniewski et ai. 1997), the shape of the maxima in the 2004 lightcurve are noticeably
15 3200 Phaethon Theta Parameter Nov04 1.5
0.5
O'------'------'-----'---'--'-----'---'-----'--'----'------'--~---J'------'------'------' o 2 4 6 8 Period (hr)
Figure 1.2 Theta statistic plot for period determination of (3200) Phaethon. The deepest minimum occurs at 3.6048hr, indicative of the most likely period for (3200) Phaethon.
different from these those in the lightcurve obtained by K. Meech in 1995 (see Fig. 1.4).
Even though it was plotted with the same zero phase as the 2004 lightcurve, the 1995 lightcurve must be interpreted with caution; the uncertainty in phasing over this interval of time (1995-2004) is +/-2.4hr, or 0.67 rotational phase.
While the maximum at 0.35 rotational phase occurs both in the 1995 and 2004 lightcurves, the amplitude varies by a factor of two. There are additional changes to the lightcurve shape that are most probably due to changes in aspect angle. The phased rotational period is identical for both the 1995 and 2004 data. Drastic changes in lightcurve shape have indeed been measured for (3200) Phaethon, over varying phase angles (Krugly et al. 2002).
Figure 1.3 only displays R-filter data; however, V-filter measurements were also interleaved with the R images. Time-resolved V & R filter data were interpolated (using
16 3200 Phaethon R-filter Nov. 2004
13.9
v '0 ::l ~ C llD III :::E 14 '0..v III... ,I:J .... • Nov 19 «i u • Nov 21 14.1 • Nov 22 V-R colors
o 0.5 1 Rotational Phase (0000UT,19Nov.2004)
Figure 1.3 Phased R-filter lightcurve for (3200) Phaethon at a period of 3.6048hr. Overplotted are the V-R colors of (3200) Phaethon, to be discussed in the text. For this figure, peaks in the V-R curve correspond to bluer areas, and troughs correspond to redder areas. The lightcurve is phased to OO:ooUT, 19 November 2004.
IDL) to make simultaneous V-R calculations. The V-R measurements were then phased to the same 3.6048hr period, and are plotted in Fig. 1.5. The amplitude of the V-R phased curve is Am-O.035-0.04, approximately one-third of the amplitude for the 2004 lightcurve. It is significant that the V-R variations measured for (3200) Phaethon can be phased to the 3.6048hr period. If there was no correlation between the color variations and the rotation of the asteroid, then the V-R points would scatter about a straight line across the phased color curve. The fine black line shown in Fig. 1.3 is an overlay of the V-R fit onto the R-filter lightcurve. The reddest part of the object is correlated with its brightest maximum, while the bluest part of the object is correlated with the darkest part of its lightcurve. The implications of this observed correlation will be discussed in Section 1.4, Discussion.
17 (3200) Phaethon R-filter Jan. 1995
14.1 - -
(j) '0 ;:i +-' ·~14.2 -9~ cO jJ:~" ... :::E :I '0 (j) " +-' E14.3 - ~ Cii u 14.4 - -
14.5 "---'----l_---'------'-_--L---L_-'------'--_-'---'---_L.-----'---_'---lI I I o 0.5 1 Rotational Phase (OOOOUT, 19Nov.2004)
Figure 1.4 Lightcurve constructed from data taken in January 1995, phased to OO:OOUT, November 19, 2004. The lightcurve amplitude is larger from 2004, along with rotational changes in lightcurve shape. Caution must be taken in comparing this lightcurve with the 2004 lightcurve, since the uncertainty in phasing (due to uncertainty in the known period) is +/-2.4hr, or 0.67 rotational phase.
Figure 1.5 displays an obvious periodic trend in all three nights' V-R measurements. Overlaid on the plot is a Fourier fit to the V-R plot. From the coefficients of the Fourier fit and their corresponding uncertainties, the confidence of the fit can be determined.
Lightcurves were also constructed from reduced observations made in November 1997 at the University of Hawaii 2.2m, using the same aforementioned Tek2048 CCD. From the reduced data and R-filter lightcurve shown in Fig. 1.6, it is obvious that the aspect angle between Phaethon's rotational axis and the Earth has significantly changed between
1997 and 2004 (although the uncertainty in phasing the 1997 data with the 2004 data is
+/-1.7hr). Aspect angle appears to have been more similar between the 1995 and 2004 datasets than the 1995 and 1997 sets. A search for color variations was made using multi-
18 3200 Phaethon V-R Nov. 2004 0.42 ,--....,.-----.--....----.-----,,--....,.-----r-.,....----.--,----r-----.--r----,
0.4
0.38 0:: I > 0.36
• 0 . 19 0.34 • \0 ~1 • ov 22 V-R fiL • 'I 1\
0.32 L-----'------'-_-'------'------''------'------'-_-'------'--_L-----'------'-_--'------' o 0.5 1 Rotational Phase (0000UT,19Nov.2004)
Figure 1.5 Interpolated and phased V-R measurements of (3200) Phaethon, at a period of 3.6048hr and zero phase OO:OOUT, 19 November 2004. There is an obvious sinusoidal trend of the V-R data for all three nights of observation. Overlaid are corrected field stars that were used for differential photometry in all November 2004 data.
filter photometry obtained on November 22, 1997. These color lightcurves are presented in Figs. 1.7-1.8.
From these colors and the colors from 2004 a broadband spectrum of (3200) Phaethon could be constructed, and is displayed in Fig. 1.9. This spectrum is even more blue than typical F-type spectra, as displayed in Tholen (1984). The cause of this blueness may be due to Rayleigh scattering of small particles or aerosols.
The B-V, V-R and B-R color plots in Figs. 1.7 and 1.8 were composed of interpolated data points obtained in the same manner as shown in Fig. 1.5. Possibly due to the change in aspect angle, there is no definite or consistent trend in color variation for the 1997
19 Phaethon R-filter Nov. 1997 13.9 r-.,----,-.L..-,---.------,--.---.----r-.------,---r-...-----r----.,
14
Q,) "0 ...::l ·~14.1 ...... • III ...... ::E " .. "0 ...... Q,) f 14.2 .. .0...... iii .. u ••• 14.3 ... .Nov 12 -. .Nov 21 .Nov 22
o 0.5 1 Rotational Phase (OOOOUT.19Nov.2004)
Figure 1.6 R-filter lightcurve produced from 1997 data on (3200) Phaethon. Data were phased to 3.6048hr at the zero phase of OO:OOUT November 19, 2004. Uncertainty in phasing the 1997 data with the 2004 lightcurve is +/-1.7hr, or 0.47 rotational phase.
data. However, the net amount of variation in the colors (Am=0.04) is comparable to the amplitude (Am=0.035) of the color curve in Fig. 1.5.
1.2.3 Image Analysis
Initial inspection of lightcurves can aid in determining the rough shape of an object. From the 2004 lightcurve amplitude of (3200) Phaethon we can determine an approximate ratio of its spatial axes, assuming that the lightcurve is influenced solely by shape and not albedo. Taking lO°.4.:lm as an approximation of the ratio, and using the amplitude of
Am=0.12, the ratio of axes is 1.12. If the shape is assumed spherical, then this value may instead correspond to a ratio of urla albedo maximum and minimum. This ratio is more likely a combined influence from shape and albedo. Possible albedo variation may
20 (3200) Phaethon B-V NOY.1997 (3200) Phaethon V-R NOY.1997 I I
0.7 0.35
I I II
0.6 0.25 -
I I o 0.5 1 o 0.5 1 Rotational Phase (OOOOUT, 19Nov.2004) Rotational Phase (OOOOUT, 19Nov.2004)
Figure 1.7 B-V and V-R lightcurves from November 22,1997. Calculations were phased to 3.6048hr and zero phase 00:00 UT, 19 November 2004.
be due to compositional or particle size variations. Pravec et at. (2002) discussed how asteroid rotation periods shorter than 4hr are more likely correlated with spherical bodies; consequently, the brightness variation on Phaethon may have a large influence from albedo.
Color variation on NEOs must be treated with caution. One must verify that the variation is not a function of parameters such as airmass or clouds. After cloud correction, we tested the V-R measurements from Fig. 1.5 by plotting them over airmass (see Fig.
1.10; the second plot expands between airmass of 1.05 and 1.2). The results indicate that the data are not a function of atmospheric extinction.
As also shown in Fig. 1.5, the V-R data for the corrected field star do not vary over Phaethon's rotation. This lack of star fluctuation again proves that the color variation is intrinsic to the asteroid. Fourier analysis was also employed to test the standard deviation of the V-R points from a fitted sinusoidal line to the data. The standard deviation is defined by 0":
(1.3)
21 (3200) Phaethon R-I Nov.1997 (3200) Phaethon V-I Nov.1997 0.4 ,..-r-I'-----',c.,-----,-----,-----,----,----,---,---,---,--,--, 0.7 ,-,--'--,--r-'--'-'-'-'--'--'--'-'-I'---'
I I I I 0.35 # # 0.65 f- II #I II #I II I II II I I I I I I I I III! III! 'I II I I I I II I I I 'I 0:: ;> I I I I IE IE 0.3 0.6 I I
0.55 ~~I~~--,--,--,I--,--,--,--,--,--,--, o 0.5 1 o 0.5 1 Rotational Phase (OOOOUT. 19Nov.2004) Rotational Phase (OOOOUT, 19Nov.2004)
Figure 1.8 R-I and V-I lightcurves from November 22, 1997. Calculations were phased to the same period and zero phase of Fig. 1. 7.
where N is the number of measurements, Xi are the values and x is the mean value. From the uncertainties in the lowest order Fourier terms, we can calculate the number of a, or confidence of match to the fit. The color variation over the three nights in 2004 were detected at varying levels of confidence: 12.7-a for Nov. 19, 6-a for Nov. 21, and 9-a for Nov. 22. A 6-a value corresponds to 99.9999% probability. The lowest sigma (lowest confidence level) occurred on November 21st, which was also the non-photometric night.
Even with cloud correction, weather undoubtedly affects the quality and SIN of the data.
Unfortunately, this second night was also hampered by a sticking filter wheel. The regular
V-R-V-R-V pattern achieved on the other two nights was much more difficult on this night.
Nevertheless, the same trend in color variation persisted on November 21.
Whereas lightcurve variation is either caused by albedo or changes in shape, color variation is usually caused by compositional variation or by particle size effects (as explained by Mie scattering theory; e.g., Jewitt & Meech 1988). Ifthe color variation is due to different particle sizes, then the presence of a regolith on the object can be confirmed by testing for
"phase reddening" (Magnusson et ai. 1996). This phase effect is more pronounced at shorter optical wavelengths «0.6p,mj Gradie & Veverka 1986), and much less so pronounced in the
22 (3200) Phaethon Brdbnd Spectrum
1.1 Q) C) I:: ttl -+-' C) ~ ...... 1.05 Q) 0:: "0 Q) N .~ 1 ta S.... zo 0.95
0.4 0.6 0.8 Wavelength (microns)
Figure 1.9 BVRI broadband spectrum of (3200) Phaethon. The data is normalized to the I-filter (0.826j.Lm) measurement.
near-infrared. Consequently, R and I filters would not work as well as B and V filters for tests of phase reddening. However, Gradie & Veverka (1986) demonstrate that spectra of laboratory meteorites redden with increased phase angle out to 1.2j.Lm. NEAR results on
(433) Eros also revealed that phase reddening can be detected out to near-IR wavelengths, though not as strongly as optical measurements (Clark et al. 2002).
Ultimately it is useful to study the effects of phase angle on both the albedo (relative reflectance) and colors of an object. Variations in either average brightness or color at small phase angles gives vital information on small-particle properties. Data taken at large phase angles can reveal coarse-particle properties (e.g., Gradie & Veverka et al. 1986).
R-filter lightcurves from 2004, 1997 and 1996 were measured at maximum brightness to obtain individual measurements for each year, spanning phase angles of 12°, 75°, and 23 0, respectively. These data were then reduced to lAD heliocentric and geocentric distance, and
23 3200 Phaethon V-R Nov.'04 TEST 3200 Phaethon V-R Nov.'04 TEST 0.42 0.42
0.4 0,4
0,38 0.38 a:, I a: > 11 I II, 1· ::L 0,36 111 0.36 l j. !~! Ff ~ 0.34 0.34 Hf !!f
0.32 0.32 1 12 1.4 16 LO 2 1.05 1.1 1.15 1.2 Airmass Alrma""
Figure 1.10 Airmass test for V-R color measurements made for (3200) Phaethon (same color scheme as in Fig. 1.5). The second plot expands between airmass 1.05 and 1.2. It is evident the 2004 V-R colors are not a function of airmass.
are plotted in Fig. 1.11. The measured phase coefficient, ,8=0.0259 mag deg-l, represents the linear fitted slope to these three data points. An interesting surge in brightness appears in the a=12° data point, which may be an early onset of opposition effect (e.g., Hapke 1993). However, one must be careful when interpreting brightness variations over multiple phase angles. (3200) Phaethon did indeed have different phase angles during the 1996, 1997 and 2004 apparitions, as shown by the uniquely shaped lightcurves from 2004 and 1997. Macro- and microporosity may be a influence on this opposition surge, since porosity is known to cause opposition surge, and is often characteristic of meteor stream parent bodies like Phaethon.
While darker and bluer objects such as Phaethon are not as sensitive to particle size effects as brighter and smaller objects (Kamei & Nakamura 2002), color measurements have also been made on this object over the same aforementioned range of phase angles to make a rough estimate of the object's possible phase reddening (see Table 1.7). Fig. 1.12 shows a plot of B-V over phase angle, and indicates a general (though non-linear) trend of phase reddening. The non-directional V-R behavior in Table 1.7 illustrates the weakened influence
24 •
15 & Data' «l -J:: & Fit 0. «i -. '-'- tlO 16 «l ::2 I Q::
17
20 40 60 80 Phase Angle (Degrees)
Figure 1.11 Phase curve for (3200) Phaethon. Red tick marks indicate actual data, and the blue fitted line represents a linear fit to those data points. The linear slope coefficient is 13=0.0259 mag deg-1. Errors are smaller than the size of the individual data points.
of phase effects at longer wavelengths. The B-V changes by ~m=0.063, which is greater than the rotational V-R change in 2004.
Whether or not the V-R color variation is caused by particle size effects, it is definite that there is a correlation between the 2004 lightcurve of Fig. 1.3 and the V-R variation of Fig. 1.5. Bluer areas correspond with darker fractions of the asteroid, and redder areas correspond with the brightest fractions of the asteroid. Overplotted the lightcurve of Fig.
1.3 is the V-R variation.
Surface variation on cometary nuclei could be evidence of their radial migration when forming in the solar nebula. This heterogeneity may be represented in the rotationally resolved colors. The detection of color variation on the surface of (3200) Phaethon may suggest a cometary body. A thorough test for coma was performed by making surface-
25 Table 1.7 Colors of (3200) Phaethon over multiple phase angles Date Phase Angle B-Y Y-R 2004 Nov. 19 12.0 0.587+/-0.005 0.349+/ -0.003 1997 Nov. 22 75.0 0.650+/-0.004 0.290+/ -0.002 1996 Nov. 12 23.0 0.618+/-0.005 0.347+ / -0.004
brightness (SB) profiles. Since Phaethon was tracked at non-sidereal rates, the field stars were trailed. SB profiling relies heavily on the PSF of both the object and the surrounding field stars. If the stellar PSF is trailed while the asteroid PSF is not, then an azimuthally averaged radial SB profile can not be made. Because of the trailed motion of the stars, asteroid and stellar PSFs could only be compared orthogonal to the direction of trailing
(see sample composite image, Fig. 1.13). If both PSFs match in the actual profile, then no coma is detected. The asteroid PSFs usually resemble stellar PSFs because no coma is reflecting light. If there is a deviation in the SB profile for the asteroid (particularly in the wings of the profile, away from the surface of the object) this may be evidence of a coma. Unfortunately, non-sidereal tracking was not always consistent, and consequently the asteroid occasionally became slightly elongated in the E-W direction. While this glitch was rare, careful examination of each image was required on each night. High-quality images were then shifted (sub-pixel shifts) and combined to increase the signal of both the asteroid and the field star. Sky subtraction was performed for each combined image by measuring the background sky in multiple locations around the target object (star or asteroid), and then subtracting the average value of background. Figure 1.14 displays the SB profile calculated for the R-filter on November 19, 2004. This plot represents the highest quality SB profile obtained for (3200) Phaethon. The subsequent nights had problems with inconsistent telescope tracking. Average net integration times of 1200 seconds are represented in the nightly composite images. Because Phaethon was near opposition (and at Y",16.0), even 1200 seconds of integration allowed for high SIN in each SIB profile.
Field stars were chosen with similar relative brightness to Phaethon. The peaks of both stellar and asteroid profiles were thus closely aligned with each other. Phaethon did not
26 Phaethon Phase Reddening Test o.7 ,------,------,-----,-,---,----,.-,--,---,-,------,----,--,-----,----,
0.65
:> I III I 0.6 I
20 40 60 80 Phase Angle
Figure 1.12 B-V colors of (3200) Phaethon over multiple phase angles. There appears to be a slightly linear trend of reddening with increasing phase angle, which is fair evidence of phase-dependent color behavior for this asteroid.
experience trailing like the stars; however, for consistency, profiles for both the asteroid and the star were only derived from crosscuts perpendicular to the stellar motion across the chip.
Non-sidereal tracking was not very successful on November 21. No SB profiles could be made from this night's data. An attempt was made to construct SB profiles in the V-filter and R-filter on November 22nd, as shown in Figures 1.15 and 1.16. Predictably, no coma was detected. If (3200) Phaethon was at one time active and ejecting the Geminids, it has ceased this activity, and is currently dormant or extinct. Because its surface has reached an approximate blackbody temperature of 750K while at perihelion (0.14AU), it is improbable that any ice or volatiles remain on the surface.
27 Figure 1.13 Composite image (North-up, East-left, 50"x35") of (3200) Phaethon constructed from ",35 images (",,1200 seconds integration time) on November 19, 2004. Surrounding field stars have been removed to make a better analysis of Phaethon's profile. Imperfections in the non-sidereal tracking can be seen by the non-spherical shape of the asteroid. Due to its primary elongation in the east-west (left-right) direction, vertical crosscuts of both asteroid and star were made for creating surface-brightness profiles.
For comparison with the composite-image profiles, an attempt was made to construct
SB profiles from single V and R images of only 60-second integrations. Fig. 1.17 displays these SB profiles. Unfortunately, the decrease in signal causes a corresponding increase of noise in the wings of the profiles. Nevertheless, it is obvious here too that the asteroid's profile is practically identical to the stellar profiles. From this preliminary investigation of SB profiles and the results given by Hsieh & Jewitt (2005), we conclude that (3200)
Phaethon has no detectable coma.
28 3200 Phaethon (R-filter) 041119
20 A Phaethon • star
26
28 1.-1--'----Io.----'_"--....L..---'--'-----'-----Io._I.----'---"--'----'----10.------' -4 -2 0 2 4 Radius (arcsec)
Figure 1.14 Surface-Brightness profile for (3200) Phaethon on November 19, 2004 (R-filter). No obvious coma can be detected based on the results ofthis and the subsequent SB profiles derived from the November data.
1.3 Spectroscopy
While photometry allows analysis of relatively high SIN data to determine lightcurves and time-resolved colors, spectroscopy can permit a higher resolution analysis of possible mineral or volatile absorption features on (3200) Phaethon, and/or spectral slope variation. Near-infrared spectroscopy was the focus of this investigation, due to the many known mineralogical and volatile absorption features given in Table 1.2. Tables 1.8 & 1.9 present the observing instruments and conditions of Phaethon while spectroscopy was performed. Measurements were made at the NASA Infrared Telescope Facility (IRrF), United Kingdom Infrared Telescope (UKIRr), and the Keele Observatory on Mauna Kea. Observations made on November 20 from the IRrF utilized the medium-resolution instrument SpeX (Rayner et al. 2003), a 0.8-5.4/-Lm spectrograph. The resolution of the
29 Table 1.8 Observing Instruments
Date (UT 2004) Telescope Instr.a Res. b Timec Slitd Scalee RNt a9 Seeh Cond' Nov. 20 IRTF 3.0m SpeX 100 120/1 0.8 0.15 50 15 0.5 LC Dec. 17 UKIRT 3.8m mST 800 120/2 0.84 0.12 40 15 0.5 P Dec. 18 UKIRT 3.8m mST 800 180/2 0.84 0.12 40 15 1.2 TC Dec. 19 UKIRT 3.8m UIST 800 150/2 0.84 0.12 40 15 0.7 LC Dec. 20 UKIRT 3.8m mST 800 150/2 0.84 0.12 40 15 1.2 0 Dec. 24 Keck II 10.0m NIRSPEC 1300 1.0/120 0.76 0.18 25 5.0 0.7 LC a Instrument (spectrograph) used. b Resolution (b.>../ >..) of spectrograph.
C Integration time(s)/Number of Coadds d Slid width [arcsec] e Plate Scale [arcsec/pix] f Read Noise [e-] 9 Gain [e- / ADU} h Seeing (arcsec,FWHM) i Weather Conditions; LC=Light Cirrus, P=Photometric, TC=Thick Cirrus, O=Overcast
Table 1.9 Spectroscopic Conditions for (3200) Phaethon Date(UT 2004) Filt.a RA Dec. r(AU) b.(AU) a(o)b foc Moon(%) IRTF Nov. 20 JHK 0447 +3925 1.768 0.818 13.1 95.2 62.1 UKIRT Dec. 17 HK 0233 +3018 1.499 0.616 26.1 70.0 35 Dec. 18 HK 0228 +2937 1.488 0.615 27.5 55.0 45.0 Dec. 19 HK 0224 +2856 1.476 0.614 28.8 41.4 56 Dec. 20 HK 02 19 +28 17 1.465 0.613 30.0 28.1 66.5 Keck II Dec. 24 L 0202 +2530 1.418 0.613 35.3 31.6 95.6 a Filter(s) used b Phase angle (Sun-Asteroid-Earth), degrees
C Lunar separation from asteroid [degrees]
30 3200 Phaethon S-B prf 041122 (R)
20
..-. "t 22 o (II VI o s... (\l l:lD S 24 '--' 0: S 26
28 '---'---'---'---'_-'--...L---l...--'-_L----'----'---'--"'---'---'---l -4 -2 0 2 4 Radius (arcsec)
Figure 1.15 Surface-Brightness profiles for (3200) Phaethon on November 22,2004 (R-filter). The PSF is better represented for the field star on the left side of the plot than the right side. No coma has been detected.
0.8-2.5j.£m observations was R""lOO. SpeX is beneficial for time-resolved work because it can obtain individual spectra covering 0.8-2.5j.£m, without any separate filters. The nodding was set at 7.5" separation. The beam pattern was an ABBA sequence. The slit was oriented to match with the parallactic angle, to account for differential refraction in the atmosphere. Integration times were limited to 120 seconds to avoid saturation of sky lines. Cirrus clouds were approaching out of the northwest, but remained distant from the asteroid. The reflectance spectra of Solar System objects can display some absorption features and spectral slopes relative to the solar spectrum. The sharpness of these features can be weakened by thermal heating, carbonaceous compounds, and other factors. The G2V standard star HIP 22536 was used for the dual purpose of telluric correction of atmospheric absorption and removal of the reflected solar spectrum. Flatfielding and extraction of 1-D spectra were
31 3200 Phaethon S-B prf 041122 (V) 3200 Phaethon S-B prf 041122 (V)
• Phaelhon .I'hl< lhon • slar • t lr 22 22
N, ., .," ~ ~ 24 ..~ 24 .." .. .§." E e 26 e 26
28 28
-4 -2 0 2 4 -4 -2 0 2 Radius (aresee) Radius (aresee)
Figure 1.16 Surface-Brightness profiles for (3200) Phaethon on November 22, 2004 (V-filter). Two different field stars were used for these two profiles. We assume from these results that Phaethon currently is not ejecting any volatile gases or coma.
applied using the Spextool package written by Cushing et at. (2004). Spectral combination and telluric correction was performed with standard IRAF routines. The data were analyzed both individually and in grouped units averaging eight to ten spectra. Sample grouped and reduced spectra from SpeX are given in Fig. 1.18.
A slightly different attempt at spectral reduction of the IRTF data was also used as described in Rivkin et at. (2004). This involved a combination of lRAF and IDL routines. Flat-fielding, combination of data frames to increase signal, and subsequent extraction of the spectrum was performed with standard IRAF routines. After all spectra were extracted, they were then run through an IDL routine that performs subpixel shifts to maximize the wavelength match of the object and standard spectra, while simultaneously making telluric correction using the ATRAN model atmosphere (Lord 1992). This model depends on three parameters: altitude of the observer, precipitable water, and zenith angle. Altitude and zenith angle are independent variables, while precipitable water is varied until the best fit is obtained. From these values the atmospheric contribution is removed from each object and standard star frame. Consequently, all standard stars throughout the night can be
32 3200 Phaethon (R-filter) 041119 3200 Phaethon (V-filter) 041119 16
16 16
~ 18 • Phaelhon t 20 ~t .," ., • i1J' .." .." ~ to 22 20 OIl to •• to S \ . S e 24 e 22 '\r'';. 26 24
28 -4 -2 0 2 4 -4 -2 0 2 Radius (arcsec) Radius (arcsec) '"
Figure 1.17 SB profiles taken from individual 60-second R(left) and V(right) filter images on November 19, 2004. Although noise is stronger in these single-frame profiles, there is still no sign of activity (coma) from the asteroid.
utilized for calibration. Fig. 1.19 displays a sample spectrum of (3200) Phaethon using this "ATRAN method", which better preserves the signal from 2.0-2.5pm. Each individual spectrum obtained represents a rotational phase of the asteroid. This phase is reported at the bottom of each plot.
A month after observing on the IRTF, four nights of spectral observations were made in December 2004 from UKIRT with UIST, a 1-5pm imager and spectrometer with a 1024x1024 InSb array (similar to SpeX). Initial resolutions were R",1300 that were later re-binned to R"'800. Typical exposure times were similar to SpeX times. The HK filter was used, which has a grism with coverage from 1.4-2.5pm. Flatfielding and dark subtraction were performed with an online spectral reduction package, ORAC-DR, written by the support staff for the Joint Astronomy Centre. This package reduces raw data to estimated wavelength-calibrated spectra. From these partially reduced spectra we repeated wavelength calibration using the Argon lamp arcs taken throughout the night. Next we corrected for telluric absorption and solar reflected spectrum using G2V star HIP 10679 and standard IRAF routines. Non- sidereal rates for tracking Phaethon were greatly increased from the November rates, now
33 (3200) Phaethon Spex 20Nov04 (3200) Phaethon Spex 20Nov04 2 I I I 2,------l--=-T::....;:.L,.:-:;=-;:..::.;:-=..;..:----;::..<:,=;.-.=r::,:";"::,,,:,,,:::-r--;---, I
~ 1.5 ..'l" () ~ p:;" - '0 .!!" 0; 8... ,g 0.5
Rot Phase~20% Rot Phase~80%
1.5 2 2.5 1.5 2 2.5 Wavelength (microns) Wavelength (microns)
Figure 1.18 Binned spectra obtained for (3200) Phaethon from SpeX, 0.8-2.5pm, at R,....,lOO. These two spectra are separated in time by approximately 130 minutes (or roughly 60% rotational phase). The spectrum on the right is distinguished by its stronger curvature, which periodically appeared in the reduced spectra. All IRTF data have been normalized to 1.22pm. The upper left hand corner displays a symbol showing the average error in the spectral data points.
at 150" /hr West, 100" /hr South. The weather was initially perfect on December 17 but soon deteoriated to patchy thick cirrus on subsequent nights. December 17 was the only "photometric" night of all nights used for spectroscopy. Figure 1.20 displays a subset of spectra obtained with UIST. Numerical calculation of spectral slopes [%/pm] allow for more accurate analysis of spectral behavior over asteroid rotation.
The last observing run for this program was on NIRSPEC on December 24, from the Keck Observatory at Mauna Kea. NIRSPEC is a cross-dispersed echelle grating spectrograph with a 1024xl024 InSb array. The filter used was the KL filter, so that neon and xenon arc lamp lines could be more easily identified in the K-band region for wavelength calibration. The Cross Disperser Grating (CDG) was set at 32.86°, which allowed for wavelength coverage from 2.85-3.69pm. L-band spectra of (3200) Phaethon were obtained with NIRSPEC in an effort to further verify its negative spectral slope, but also to discern any possible C-H hydrocarbon bands that may be contributing to its primitive,
34 3200 Phaethon SpeX 041120 2 '-'---'I...::..,'---=--,=-----=;-=-:::,.=.:-.cT1==r.=..::..:r--:-=T-J=-:;"'=-::'I--=----;;:""=-:;:-':"::-"';:--'-----'
Rot Phase=45% 0 I I I 1 1.5 2 2.5 Wavelength (microns) Figure 1.19 Relative reflectance spectrum of (3200) Phaethon, using an ATRAN atmospheric modeling program. Error bars have been incorporated into the plot of this spectrum. featureless spectral behavior. C-H hydrocarbon bands are difficult to detect (Emery & Brown 2004) in primitive objects, but are diagnostic of composition. Integration times were only 1.0 second in duration, but were coadded 60, 90, or 120 times. Flatfielding, bad pixel masking, and wavelength calibration were performed using the REDSPEC reduction package, written by L. Prato of UCLA. Standard star HIP 9197 was utilized for extinction correction, telluric correction, and solar spectrum removal with standard IRAF routines. Humidity remained high but steady at 70%, which allowed for partially successful sky subtraction between nodded pairs of images. Figure 1.21 shows the combined spectrum of (3200) for the observing run on NIRSPEC. Note the marked change in spectral slope to a slightly reddened spectrum, different from the other negative sloped spectra in the near-IR (and optical, as shown by Lazzarin et at. 1996). The reddening seen here is due to thermal 35 2 (3200) Phaethon UIST 17 Dec.'04 2 (3200) Phaethon UIST 17 Dec.'04 I Rot Phase= 10% Rot Phase=30% o ~~_.L...L1--,---.L...L.....i---'----'---L-L..-.L...L~I_'__...L.....i_~1 o ~-'-~-'---L1-L..--'---L1-L..--'--'--L...L-L-L..-.L-LI--,---.L...L-'-J 1.4 1.6 1.8 2 2.2 2.4 1.4 1.6 1.8 2 2.2 2.4 Wavelength (microns) Wavelength (microns) Figure 1.20 1.4-2.5f..Lm spectra obtained from UIST at UKIRT in December 2004. Rotational variations in this R",800 spectra are easier analyzed by comparing their computed numeric slopes (%/f..Lm). Spectra displayed here are separated by 20% in rotational phase, and are normalized to 1.6f..Lm. The upper left hand corner displays a symbol showing the average error in the spectral data points. blackbody emission, which needs to be subtracted out if the true reflectance spectrum is to be examined. 1.3.1 Spectroscopic Analysis Near-IR spectral slope alone can be a useful indicator in the classification of a Solar System body, as shown in Table 1.10. The average near-IR spectral slope of -10%/f..Lm measured for Phaethon makes it unique from both average cometary (0-10%/f..Lm) and asteroidal (0 15%/f..Lm) slopes. This negative slope is four times less negative than the optical spectral slope obtained by Lazzarin et ai. (1996). The slope apparently becomes less blue (less negative) as the spectrum extends to longer wavelengths. While most cometary slopes are either neutral or slightly red, there are a few known blue asteroidal optical slopes, such as those members of the Themis family (e.g., Juric et ai. 2002). Overall, most comets and asteroids have reddened near-IR spectra due to hydrocarbons, silicates or space weathering. 36 (3200) Phaethon NIRSPEC Dec.'04 4,----j-----.------r------,------r-,--.--.-----.------.-,---.,.-.....,....----.,..-----.--; I "./' " " ~ . ".,... 2 ... ~4/" .. " c: ..,.~.. ~ . .. h ...o ."1' . v ;;:: v c::: 0 "tl III .!::'! ~e o z'"' -2 • Phaelhon • Thermal Emission (NEATM) 3 3.2 3.4 3.6 Wavelenglh (microns) Figure 1.21 NIRSPEC combined spectrum obtained for (3200) Phaethon. Weak signal did not allow time-resolved coverage in this wavelength region. Overplotted is the modeled thermal emission of (3200) Phaethon, using the NEATM formulated by Harris (1998). A symbol plotted in the upper left-hand corner displays the average error of the data points. Because (3200) Phaethon is thus far a featureless object at all measured wavelength regions, time-resolved measurement of the spectral slope is the only logical method to constrain any possible compositional and/or albedo variations. Time-resolved spectral slopes were measured from the IRTF and UKIRT spectra. All spectral slope measurements were then phased to the 3.6048hr period ofPhaethon, with zero phase at OO:OOUT, November 19 2004. Comparing spectra across different telescopes, even for the same object, can be difficult for many reasons. All spectra in this analysis were normalized and then converted to units of [%/I'm]. From these values, slope trends could be compared more accurately between telescopes. 37 Table 1.10 Average near-IR spectral slopes =====:======:::===::::;::;::::===== Object Slope (%1 j.J.m) (3200) Phaethon -10+1-0.05 Comets (avg) 0-10+/-0.1 Asteroids (S-type) 15+/-0.05 Asteroids (C-type) 0-5+1-0.05 Sun 0+1-0.00 Fig. 1.22 displays the measured rotational IRTF spectral slopes phased over Phaethon's rotation. Spectra were measured for slopes individually and in grouped units (8-10 spectra median-combined to one). Slope that becomes more negative with rotation is indicative of a bluer surface, while a redder surface induces a more positive (or less negative) slope. The majority of the slopes remained negative (to some extent) throughout rotation. The near-IR spectral slopes appear to become bluer through the initial 0.40 rotational phase. There is a sudden unusual spike in slope (or reddening) from 45-70% rotation. Color or spectral variations are usually believed to be gradual hemispheric changes across the surface of asteroids or comets due to craters or resurfacing from collisions (e.g., Murchie et ai. 2002). The noise from 0.80-1.0 rotational phase makes analysis of spectral slope in this part of the phase difficult, though there appears to be a vague semblance of reddening. Superimposed over the data is a Fourier fit to show the general trend in spectral slope variation. In order to constrain the source of the noise in these spectral slope measurements, separate plots of spectral slope measurements were constructed that represented fractions of the 0.8-2.5p,m data. Fig. 1.23 displays two separate plots of IRTF spectral slopes over rotation. The left-hand plot presents slopes measured from 0.8-1.8j.J.m, and the right-hand plot presents slopes measured from 2.0-2.5j.J.m, using the same SpeX data as in Fig. 1.22. The plot from 0.8-1.8j.J.m shows a more systematic trend of spectral slope variation, and is consistent with the higher SIN that was obtained in this wavelength region. There still exists a slight reddening halfway through rotation, which may be intrinsic to the rotation of the asteroid in the near-IR. 38 ...... 10 ~ • 0 .. l-o .. .." .~ .. J' . J' ...... o.o .. E . "'l..o ", .. "-~ 0 '-' Q) ~ 0 ai'. ~ en -10 to -....,l-o .... () Q) ~ rt) -20 • Individual • Grouped Fourier Fit -30 0 0.5 RotaLiona] Phase Figure 1.22 Spectral slopes (measured 0.8-2.3j.£m) of the SpeX data from Nov. 20, 2004 (zero phase OO:OOUT, 19 November and phased to a 3.6048hr period). Note the bluer surface from 0.0-0.40 rotational phase, then sharp reddening from 0.50-0.70 rotational phase. The data are rather noisy from 0.80-1.0 rotational phase. The overlaid Fourier fit has a 50-a probability. Measurements of spectral slope were also made from the grouped ATRAN-reduced spectra, as shown in Fig. 1.24. A repeated progression of bluer spectral slope with rotation exists, followed by a sudden reddening at the end of the rotation. The decrease in color is similar to the lRAF-reduced spectra from Figs. 1.22 and 1.23, but at a slightly shifted rotational phase. The reason for the discrepancy is unknown. Decrease in color also occurred in these spectra for 60% ofrotation, a larger fraction than obtained with the SpextooljlRAF reduced spectra. The average slope values (-15 to -5 %/ j.£m) from the ATRAN-reduced data are comparable to the slopes measured from the 0.8-1.8j.£ffi slopes in Fig. 1.23. Time-resolved slopes could also be supplemented with the UIST measurements from UKIRT. Fig 1.25 displays the spectral slope plot for the first night, December 17. 39 3200 IRTF' Slope (0.85-1 8 microns) l: -5 l: 0 o .. .. # ~ 100 E" •.....\,0/' . E " ~-IO . . .' ~ ' . ... ' . ., "..:- ...... c. . ."". ... 0 " , . , , ;." e...... -..'\. ... ~* ... iii '\. . "' ..:,...... ,."" a; -15 '. b ., " Individual "c. CIl_ZO , Grouped -100 • Individual Fourier Fit ... Grouped -Z5 0 05 o 05 Rotational Phase Rotational Phase Figure 1.23 Separate spectral slope plots covering two different regions of the SpeX data. The left-hand plot representing data from 0.8-1.8j..Lm shows a stronger trend ofspectral slope with rotation than the right-hand plot representing data from 2.0-2.5j..Lm. The overlaid fit to the 0.8-1.8J.Lm data has a 25-a certainty. Results from UIST confirm the decrease in near-IR color in the first half of Phaethon's rotation. The second half is followed by a gradual reddening. December 17 was the clearest night of the spectroscopic observing runs, and allowed for high-quality background subtraction within spectral pairs. UIST was also used December 18-20; unfortunately, the weather did not cooperate these nights. December 19 was the only other night where partially decent data were obtained. The spectral slopes measured from this night are displayed in Fig. 1.26. Patchy cirrus prevented complete coverage of rotation on this night. Unfortunately NIRSPEC could not be used for time-resolved analysis, due to weaker signal and difficulty in removing noise from cirrus. Only a composite spectrum could be constructed from this dataset. Additionally, the true reflected spectrum was hampered by thermal emission that began to dominate the reflected signal at 3.5j..Lm. An IDL routine, written by Y. Fernandez and based on the Near Earth Asteroid Thermal Model (NEATM) by Harris (1998), models the expected thermal blackbody curve of the asteroid along with the expected reflected signal, based on the observing geometry and thermal properties of Phaethon. The reflected and emitted components were modeled given the albedo of 40 3200 Phaethon IRTF Slope -6 -8 a. -14 o 0.5 Rotational Phase Figure 1.24 Spectral slopes (measured 0.8-2.3J.'m) of the SpeX data from Nov. 20, 2004, taken from the ATRAN-reduced spectra. 0.11 (Green et aI. 1985) and optimal beaming parameter (17) of 1.6 (Harris 1998), radius of 2.505km, and corresponding orbital geometry. The beaming parameter (77) takes into account the enhanced sunward thermal emission of the asteroid due to surface roughness (Delbo et aI. 2003). The Near Earth Asteroid Thermal Model (NEATM) written by A.W. Harris (1998) produces the best matched models to thermal data of near-Earth objects. Other thermal models, such as the Standard Thermal Model (STM) or Fast Rotator Model (FRM), work better for main-belt asteroids than for near-Earth objects. This modeled thermal spectrum was then subtracted from the reduced spectrum shown in Fig. 1.21. After subtracting the modeled thermal emission, the reflected spectrum could finapy be analyzed (Fig. 1.27). As can be seen in the thermal-subtracted spectrum, Phaethon has a slightly negll.tive/neutral sloped spectrum into the L-band. It is interesting 41 (3200) UIST Slope 041217 .. Individual .. Grouped """' 10 fit I:: Fourier o l-o .Sl 6 ...... ~ 0 .. '-" QJ .. .. c...... o .. ~ -10 .. ro l-o -+-' U c..QJ Ul -20 o 0.5 1 Rotational Phase (OOODUT, 19Nov.2DD4) Figure 1.25 Spectral slopes of the UIST data from Dec. 17,2004 (zero phase OO:OOUT, 19 November and phased period of 3.6048hr). The bluer surface is once again revealed in the first 40% of rotation, and follows with a gradual reddening in the second half of rotation. Overlaid is a Fourier fit to the spectral slope variation, with a confidence of 12.5-0". These data represent the highest SIN spectra obtained for (3200) Phaethon. to note that the modeled reflectance shown, calculated from the NEATM, is quite similar to Phaethon's thermal-subtracted spectrum. A continual negative reflected spectral slope from 0.8-3.5j.tm is a significant finding. Currently the only known mineralogical cause of a featureless negatively-sloped spectrum from 0.5-3.5j.tm is synthetic carbon black. Synthetic carbon black is inorganic and much bluer than hydrocarbon or graphitic compounds (Cloutis et al. 1994). The presence of spectral slope fluctuation can be caused by more than the rotation of the asteroid. Seeing variations can cause variable color-dependent slit losses of photons in the detector. A simple test is to plot the spectral slope over average spectral intensity (or average counts/sec, to normalize different exposure times). The seeing tests for SpeX on 42 (3200) UIST Slope 041219 • Individual .Grouped -c:: 10 Fourier fit o I.. ·sCJ ~ 0 '"" Cll oCo .. U) -10 ...... f ~.. .. CJ .. Cll .. Co en -20 .. / o 0.5 1 Rolational Phase (OOOOUT. 19Nov.2004) Figure 1.26 Spectral slopes from UISTon Dec. 19,2004 (zero phase OO:OOUT, 19 November). The bluer surface in the first half of rotation is seen, albeit with fewer data points. Cirrus clouds hampered successful observations across the entire rotational phase on 19 December. The confidence of this fit is 3.5-0'. Nov. 20 and UIST on Dec. 17 are shown in Fig. 1.28. Fortunately it does not seem that slit losses were a direct cause of slope variation at either IRrF or UKIRr. Even though standard stars are used to normalize for atmospheric extinction, a similar plot was made of spectral slopes over airmass (Fig. 1.29), to verify extinction correction worked successfully. The spectral slopes were neither a function of seeing or airmass. From these tests we can conclude that the spectral variations observed are intrinsic to the rotation of (3200) Phaethon. Near-earth asteroids fortunately have a better tendency than main-belt asteroids to match with laboratory meteorite spectra (Hicks et al. 1998). From comparisons with these meteoritic spectra, compositions of NEAa can be constrained. Unfortunately, the spectra 43 (3200) Phaethon NIRSPEC 24Dec.'04 4r-'-...----,-....,..---,-..---.------,---.--,----,..---.,----,----.---,-,----, I ~ 2 c: ....~ (,) II> -II> P:: 0 '0 II> ....N etl -S s... o Z -2 • (3200) Phaethon • Modeled Reflectance (NEATM 3 3.2 3.4 3.6 Wavelength (micron) Figure 1.27 NIRSPEC 2.9-3.69~m spectrum of (3200) Phaethon, after subtraction of the modeled thermal emission. Thermal emission dominated the reflected signal after 3.4~m. Plotted in the upper left-hand corner is a symbol displaying the average error of the data points. obtained for (3200) Phaethon make classification difficult due to the lack of absorption bands from 0.8-3.5~m. Searches were made amongst the carbonaceous chondrites, which are known for their neutral, featureless spectra. However, even some chondrites with hydrocarbon compounds display C-H stretch features in the L-band, which are absent on Phaethon. In addition, many carbon-based spectra redden into the near-IR. Only one compound, found in Cloutis et al. (1994), has matched well with Phaethon's near-IR and L band reflectance spectra. This compound is termed carbon-black, and is a soot-like synthetic substance, with normally small-grained «0.2~m) particles. The form of carbon in this compound is different than traditional crystalline graphite or even the hydrocarbons found in organic compounds. Carbon-black has an amorphous, disordered molecular tructure, 44 20 20 Seeino Test (3200) 041217 10 • ? ? · 0 10 ...0 · ... -30 -20 2 3 4 5 1.6 1.8 2 2.2 Average counts Avg counts/sec Figure 1.28 Seeing tests for data from SpeX at IRTF and UIST at UKIRT in November and December 2004. If the spectral slopes were a direct function of the changes in average spectral intensities (normalized to counts/sec for UKIRT, since different integration times were used), then the spectral variation in (3200) Phaethon would not be intrinsic to the asteroid rotation. and can result from heating hydrocarbons to high enough (rv500-700K) temperatures. A hydrocarbon phase can have its hydrogen driven off by a combination of UV irradiation and high temperatures. The result is a progressively more carbon-rich material on the asteroid surface, and eventually a soot-like amorphous substance such as carbon-black. Only extremely high temperatures (> 1500K) can produce the ordered crystalline carbon structure of graphite. However, graphite can begin to slowly form at temperatures as low as 700-800K. Vilas & Sykes (1996) experimented with heating of Murchison meteorite extract. When the sample was heated to 750K (rv500°C) in the laboratory, the spectra lost absorption features and became slightly bluer. Part of the Murchison meteorite is believed to contain a fraction of this carbon-black. The carbon-black spectra from Cloutis et at. (1994) is overlaid (see Fig. 1.30) with the IRTF spectrum taken from Fig. 1.18. Fig. 1.31 displays the NIRSPEC spectrum of Phaethon (thermal emission subtracted) overlaid with both carbon-black and a CK chondrite. CK chondritic material is spectrally different from Phaethon at 0.8-2.5Jtm with 45 20 ,-----f-Allir!-,ffi#-"!.a",.,Ss",-LT~e s""t~~(3Lf'2,-,,0T"0+-'>')01"'4:.+1",-2f.-17.t,-r-r-,--, "2 10 . .. "2 o ...... 0 10 - ... () () . .&.a • § :§. 0 . '- ~ .. ~ '" 01- '"o ""0 ... 'iil-l0"" ill 1:: .. b '"() '"() '" .. . ~-10 en"" -20 en 1.2 1.4 1.6 1.2 1.4 1.6 1.8 Airmass Avg counts/sec Figure 1.29 Airmass tests for data from SpeX and UIST in November and December. Once again, there seems to be no direct correlation between the spectral slopes and the airmasses of the individual observations. a red slope and an absorption band present at 1.1p.m due to magnetite. However, from 2.9-3.4p.m it is obvious that the asteroid and two compounds share similar featureless, neutral spectral slopes. Carbon-black gradually begins to slope upwards at 1.5p.m, which periodically occurred as well with the Phaethon spectra. The curvature in the Phaethon spectra was seen more readily in the individual and grouped spectra reduced with IRAF and Spextool than was seen in the grouped spectra reduced with the ATRAN-based model. 1.4 Discussion While many NEAs are collisionally-derived fragments from the asteroid belt, comets are believed to be composed of more primordial volatiles and dust particles from the solar nebula. If (3200) Phaethon is an extinct cometary nucleus, then its density should be lower than that of typical near-Earth or main-belt asteroids. Sheppard & Jewitt (2004) discussed the minimum spin period of a body with a given density: Pc = a(1000jp)1/2 (1.4) 46 I ~ 1.5 l:: ...,~ o ~ Q) 0:: 1 '0 Q) .~ ~e ol-o z 0.5 Rot Phase=80% 1 1.5 2 2.5 Wavelength (microns) Figure 1.30 Near-IR spectra of (3200) Phaethon and carbon-black. The match-up between asteroid and mineral are quite close. CK chondrite does not match here due to its reddened spectrum from an absorption feature at 1.1J.Lm. Plotted in the upper left-hand corner is a symbol representing the average error of the data points for (3200) Phaethon. where Pc is the critical period, a is a parameter that is dependent on the shape of the object (a=3.3hr for a spherical object). Solving for density and setting the critical period equal to Phaethon's period of 3.6048hr, we obtain a "critical density" of 838kg/m3 , less dense than water. This is a lower limit to the density of the asteroid, assuming it is spherical. There is a similar equation from Sheppard & Jewitt (2004) to calculate the critical rotational period for a spherical strengthless object: . Tc = (3II/Gp)1/2 (1.5) From this equation we calculate a minimum density of 839kg/m3 using Phaethon's known rotational period of 3.6048hr. This Iowa density would imply a highly porous 47 (3200) Phaethon/Carbon/CK Chondrite 4 I I I I I ~ 2- - l:: ~ a a •• C.I ~ ~ a •• ~ ... • a-r a '• ..... a Q) ~ 0 "0 Q) .~ ~ E 1-0o Z -2 I- a (3200) Phaelhon - o Carbon- black • CK chondrile _ 4 '-----'---'1'-"--'--'-_-'--'-'---'------'-_IL----'---'---'-----I.I_'--J 3 3.2 3.4 3.6 Wavelenglh (micron) Figure 1.31 L-band spectra of (3200) Phaethon, carbon-black, and a CK chondrite. The similarity between Phaethon's spectra and the two compounds are close until3.4p.m, where thermal emission dominated the spectrum of Phaethon. Plotted in the upper left-hand corner is a symbol representing the average error of the data points for (3200) Phaethon. asteroid. Assuming Phaethon's density is similar to that of the thermally processed Geminid meteor stream (1200kg/m3 ), then the overall mass of Phaethon is 2.65xlO13 kg. Naturally, this assumed density is significantly shape dependent. It is possible that Phaethon is fairly spherical, based on the lightcurve amplitude, spin period, and measured a:b axial ratio (1.12). Naturally, a more in-depth analysis of the apparitions at different aspect angles would be required to verify this possibility of a spherical shape. A present mass of 2.65xlO13 kg is within the same order of magnitude of Phaethon's derived mass loss from cometary ejection (Williams & Wu 1993). Phaethon consequently is estimated to have been twice as massive as it is now, whereby loss of volatiles and particles (Geminids) has decreased its size and mass. 48 With color and spectral variation detected, we must ask if Phaethon does indeed have a regolith on its surface. Structural regolith variations could cause the variations in spectral slope and photometric color. However, the presence of a low thermal inertia regolith is contrary to the 5-20j.tm results of Green et at. (1985), who deduced that the surface must have high thermal inertia, and consequently a solid rock surface. Besides solid rock, high thermal inertia could also result from a coarse-particle regolith (Delbo et at. 2003). Price (2004) stated that if smaller asteroids do have regoliths, they should have coarser regoliths than large (>1O-20km) asteroids. Could any particles gravitationally survive on the surface of Phaethon? Assuming a simplified balance of gravitational and centripetal forces between Phaethon and a test particle on the surface, centripetal force induced by the 3.6048hr period is not large enough to overcome the gravitational force ofthe asteroid. However, introducing the gravitational force of the Sun at perihelion (0.14AU) or at 1AU is strong enough to pull these objects off the asteroid surface. The particles might not be simply held on the surface gravitationally, but also could be bound by an indurated (cemented) layer. The induration may be due to heating and fusing particles on the surface. If water ice played a role in the induration of the regolith, no hydration absorption features any longer exist for (3200) Phaethon. At perihelion (0.14AU) its surface temperature approaches 750K (Carvano et at. 2003), and would remove any signs of aqueous hydration at 0.7j1.m, 1.5j.tm, 2.0j.tm, or 3.0j.tm. This result is common with high-temperature processing of Solar System bodies. Indeed, Hapke (1993) explains how increasing temperature on the surface of an asteroid will weaken or altogether remove evidence of any absorption bands on the surface. He also describes the effect of particle size on spectral reddening. Larger particles will cause spectra to be flatter and have less pronounced contrast in absorption features. Smaller particles on a carbonaceous chondrite regolith surface (Carvano et at. 2003) will conversely induce reddening of the reflected continuum and increase the albedo (it e.g., French et at. 1988), if small iron-based molecules exist within the mineral matrix. Magnusson et at. (1996) describe how, if color variations are due to particle size effects, one can verify the dependence by observing the colors over broad phase angles. The surface color should 49 redden with increasing phase angle, if any particles exist on the surface. Helfenstein & Veverka (1989) show that data at large phase angles can reveal properties about surface roughness (i.e., coarse particles), while data taken near opposition will reveal small particle characteristics. Emery & Brown (2004), however, warn that phase angle dependence on color variation will not be as pronounced in near-IR wavelengths, and does not act as strongly on C,F, or G-type asteroids as it does on S-type asteroids. Consequently, it is wisest to make short-wavelength (i.e., U, B or V-filter) observations to determine phase-reddening on darker, more primitive objects. While the B-V measurements of Phaethon increase with phase angle, the V-R colors do not follow a trend with phase angle. This trend is consistent with phase reddening being more dependent on shorter-wavelength measurements. Color variation over different phase angles must be interpreted with caution, especially for (3200) Phaethon, which exhibits both rotational color variation and evidence of phase reddening. The observed color variation in the phase-reddening plot of Fig.1.12 may simply be due to colors observed at different rotation phases. The 2004 data were obtained at 0.25 rotational phase, while the 1997 and 1996 data were taken at 0.96 rotational phase (zero phase OO:OOUT, Nov. 19, 2004). These values of rotational phase over the different years of data were calculated assuming a zero phase ofOO:OOUT, 19 November 2004; the uncertainties in the phasing between years was unfortunately non-trivial. If we accept these rotational phases at face value (i.e., ignoring the errors in phasing), then the colors measured in 1996, 1997 and 2004 all correspond to roughly the same V-R shown in Fig. 1.5. Ultimately, the color variation is greater than that depicted in Fig. 1.5: e.g., B-V variation of 0.063 and V-R variation of 0.059 between 1997 and 2004. The V-R variation from the rotationally phased 2004 data is only .6.m=0.035. Thus a coarse-grained, high thermal inertia regolith may indeed be present on the surface of (3200) Phaethon, and is causing the detected phase reddening. (3200) Phaethon's rotational lightcurve shares a correlation with the color variation. Bluest areas of Phaethon in the optical correspond with the darkest part of the lightcurve, while reddest areas correspond to the brightest part of the lightcurve. The converse effect 50 occurs in the near-IR, relative to the opticallightcurve. The near-IR spectral slope becomes bluest at the brightest part of the optical lightcurve. Bluer optical fractions of Phaethon may correspond to areas with coarse-grained particles, as shown by the evidence of phase reddening on Phaethon. Coarse-grained particles (Carvano et ai. 2003, Emery & Brown 2004) are known to decrease (darken) reflectance and make surfaces appear bluer in the optical. Conversely, finer and smaller-grained particles of identical composition could induce redder observed colors, if iron is present in the smaller particles. Naturally, this variation would be evidence of compositional variation vice particle size effects. More measurements will be required to verify these effects of phase reddening, which would confirm the presence of a particulate surface. Because particle size variations are less effective in changing near-IR measurements, color variation from 0.8-2.5I-£m may be evidence of varying combinations of carbon black with hydrocarbons, crystalline graphite, or semi-transparent silicates that slightly change the spectral slope. If we assume Mie scattering theory, the continual blue/neutral slope from 0.4-3.5I-£m may imply very small «O.4l-£m), featureless particles. The L-band spectral slope on (3200) Phaethon may be unique, as other main-belt F type asteroids have measured L-band spectra that are reddened. Examples include F-type (704) Interamnia, and taxonomically similar B-type (2) Pallas (Jones et ai. 1990). However, the lowered albedo of these larger objects and the presence of a small-particle regolith may have caused significantly more thermal emission than the amount detected on Phaethon. Jones et ai. (1990) did attempt to subtract the thermal emission from their spectra, using the Standard Thermal Model (STM) for blackbody calculations. The decreased size of carbonaceous particles in a regolith (assuming that dark carbonaceous regoliths exist on low albedo F- and B-types) can cause a reddening (Clark et ai. 1992) in the infrared spectrum. Iron-rich silicates locked within the carbon grains are released and are more open to reflect solar radiation when the regolith is composed of such pulverized small particles. Likewise, a lack of these fine-grained particles may decrease the overall reflectance and slope of the spectrum. It is unlikely that Phaethon has a substantial iron component on its surface, due to the lack of a UV absorption feature or a significantly reddened spectral slope. 51 At all wavelengths, the spectral match of (3200) Phaethon with any terrestrial minerals is closest with synthetic carbon black. Unfortunately, the albedo of carbon black (",0.02) does not reconcile with Phaethon's albedo of 0.11. Some agent would be required to raise the asteroid's albedo, if it was composed of this amorphous substance. Private communication with E. Cloutis revealed that transparent silicates are not unusual on asteroidal surfaces. Aubrite meteorites contain iron-free enstatite that is essentially transparent, which can incorporate carbon or graphite-type inclusions. Combinations of such silicates may indeed raise Phaethon's albedo to the level detected by Green et at. (1985) without changing the spectral slope. Another possible factor that could increase albedo are the Calcium Aluminum inclusions (CAls) commonly found in CK chondrites. These inclusions are shown by Kamei & Nakamura (2002) to increase albedo in meteorite samples. 1.5 Conclusions/Summary (3200) Phaethon is intriguing since it is a probable extinct cometary nucleus, along with the 107PjWilson-Harrington (which is also a spectrally similar CjF type asteroid). Although its diameter of 4km and rotational period of 3.56hr (Harris & Young 1983) is similar to Phaethon, 107PjWilson-Harrington is more dynamically cometary than Phaethon, due to its Tisserand Parameter of TJ=3.087. Phaethon's blue spectral slope is likely caused by an amorphous, carbon-rich substance. The anti-correlation of spectral variation in the near-lR with the color variation in the V and R filters may be due to multiple causes (see combined plots of V-R and near-lR spectral slopes in Fig. 1.32). Particle size variations from a coarse regolith may cause the V-R variations, while the near-lR variations may be of a compositional nature. Coarse particles in a regolith could maintain a high thermal inertia (as was measured by Green et at. in 1985), yet still cause the color variations detected in the optical. Further short-wavelength data analysis will be required to confirm phase reddening. Unfortunately, time-resolved B-filter coverage was not obtained during this observing program. The compositional variation in the near-lR might be due to space 52 weathering by UV irradiation, cratering and micrometeorite bombardment. The color variation may be evidence of a large crater on the surface. The fresh exposed surface may contain some primitive hydrocarbon material that is redder than the rest of the irradiated and thermally processed carbon-black matrix. These patches may consequently cause a reddening in the near-IR spectral slope. Price (2004) suggested that color variations on a hypothetical 5km asteroid are more probably due to fresh and old surface differences (due to either cratering or space weathering) than due to dramatic compositional variation. Nelson et at. (2004) discussed how the presence of carbon-rich material may be seen as an indication of evolution of cometary nuclei, as hydrogen is driven off the surface by heating and UV irradiation. The thermal processing on a close approacher to the Sun like Phaethon unfortunately removes all absorption features and possible aqueous hydration features, and consequently complicates positive identification of definite compounds or molecules. While active cometary bodies are most easily identified by the existence of volatiles, extinct nuclei may be intrinsically featureless. (3200) Phaethon may be a "normal" devolatized cometary nucleus. (3200) UIST Slope 041217 0-4.2 ,...... ,,...::3:.:;2~0.::,0-'P....:,h.:.:a:.:;e;.::.;l h:..:;o::.:n..:.,:.V....-..:.;R:..,.N:.:.:;.ov:...;.....;;.2::.:0:.;:0;.:.4...... , .. Individual .. Grouped 0.4 <: 10 Fourier fit ..o '6" 0.36 ? 0 Q: ., ' I 0. : . > o 0.36 tii -10 • ¥ : OJ b .Nov 19 ., "0. 0.34 .. 'tIP en -20 • Uy 22 V-Fl m .1"; u I I o 0.5 I o 0.5 I Rotational Phase (0000UT.19Noy,2004) Rotational Phase (OOOOUT. 19Nov.2004) Figure 1.32 Side-by-side plots of Fig.1.5 and Fig.1.24, which display (3200) Phaethon's V-R and near-IR spectral variation, respectively. Data on the left were obtained on Nov. 19, 2004, and data on right were obtained on Dec. 17, 2004 (UT). Both datasets are phased to the 3.6048hr period of Phaethon, and zero phase OO:OOUT, Nov. 19, 2004. 53 Carbon-black may exist in an indurated regolith with varying particle sizes across the regolith. When coupled with the albedo-raising transparent silicates, these carbon-black particles may be causing color variations simply as a result of their varying particle sizes. Certain areas may contain higher concentrations of carbon-black that reflect bluer in the near-IR (more diagnostic of composition than the optical), while changing particle sizes may induce redder reflectance in the optical. Interspersing of silicate particles or hydrocarbons would slightly alter the optical and near-IR colors. If (3200) was a "near-Earth comet", it must have formed farther out from its present orbit, and consequently was perturbed inward to its present orbit. Phaethon's present Tisserand Parameter of T J=4.5 strongly decouples it from Jupiter's orbit. This decoupling is dynamically dissimilar from many known cometary orbits. Phaethon may not be dynamically similar to comets, but its eccentricity is still quite high, and the color variation may still be evidence of prior radial migration (if not due to cratering or space weathering). Without any absorption features, it is difficult to prove compositional variation. Ifit formed in situ like the other main belt asteroids (Cruikshank et at. 2001), then a decrease in reddening in the near-IR spectral slope plots should be due to space weathering (Yoshida et at. 2004), and Phaethon's history and evolution would be entirely unique from other known meteor parent bodies. If its surface is a cometary mantle, then it might be possible to excavate holes in the surface. Volatiles are probably no longer part of Phaethon's composition, due to its repeated heating at perihelion. A combination of particles (causing optical color variation) gardening a fresh surface (causing near-IR color variations) is the most realistic scenario for this asteroid. (3200) Phaethon seems to be a unique "cometary" body that contained an unusually high amount of debris (rock), and may consequently have been a true "transition" object between asteroids and comets. If it were always just an asteroid, then the Geminids must have resulted from a collision with another asteroid at the perihelion of 0.14AU-an unlikely scenario. As a parent body of a meteor shower, Phaethon should have a high degree of macroporosity (30-50%) in its structural makeup (Britt & Consolmagno 2000). Indeed, 54 porosity may be quite common on all meteor parent bodies. In either event, it is very probable (e.g., Hunt et al. 1986) that the Geminids were produced by cometary ejection. We may have evidence in Phaethon that comets can become members of the near-Earth asteroid population, and may still be present as extinct nuclei masquerading as asteroids. 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(1997), Icarus, 126, 395-449. Yoshida, F., Dermawan, B., Ito, T., Sawabe, Y., Haji, M., Saito, R, Hirai, M., Nakamura, T., Sato, Y., Yanagisawa, T., Malhotra, R (2004), PASJ, 56, 1105-1113. 63 Chapter 2 Comparison Objects 2.1 Introduction The unique spectral and photometric properties of (3200) Phaethon should be compared to other objects dynamically (via orbit or rotation) or physically (via surface properties) related to this possibly extinct cometary nucleus. (3200) Phaethon may be quite blue, but this may be common of other objects that have perihelia close to the Sun. (3200) Phaethon has color variation across its surface, which has also been detected in other Solar System bodies and is often correlated with varying surface composition. As discussed in Chapter 1 (Section 1.1), spacecraft observations of bare nuclei surfaces have been able to reveal evidence of color heterogeneities, such as that seen on 19P/Borrelly by Deep Space 1 (Nelson et at. 2004). This color variation is expected based on the theory of comet nucleus migration in the formational stages of the early Solar System (Weidenschilling 1997). Could these variations also be detected via ground-based observations? The unique evidence gathered for Phaethon seems to support the possibility. By comparing Phaethon's colors, lightcurves and spectra to other dynamically or physically similar asteroids or comets, we can better constrain the uniqueness of Phaethon's properties. Comparison objects that have been chosen are dynamically similar in orbital properties or rotation rates, and/or have similar colors or taxonomic classifications. Nearby low-activity comets were also chosen in 64 hopes that they would have non-existent, or extremely weak comae. These comets would thus allow close examination of their nuclei. A challenge in planetary science involves the determination of basic spectral and photometric properties of cometary nuclei, which are best observed when inactive. Comets are inactive only when they are far (greater than 5-10AU) from the Sun (Meech & Svoren 2005). Dormant comets are very faint due to a combination of their small sizes and low albedos (reflectivities) and are thus difficult for observations. Besides high-priced spacecraft missions, ground-based observation of the bare nuclei is the best way to gather data about their nuclear compositions, because there is no interference from a gas or dust coma. The gas or dust in the coma of an active comet is usually compositionally altered by UV irradiation from the Sun, and further because of the outgassing thermal stratification of the upper layers, is not likely to be representative of the bulk makeup of the nucleus. The true structure and colors of the surface of the comet are obscured until the outgassing or coma subsides. There are only a few known "low-activity" comets that display minimal outgassing and dust comae close to the Sun. When these comets are close to the Sun and Earth, they are usually brighter and easier to observe. Examples include 49Pj Arend-Rigaux and 28PjNeujmin l. Objects that are discovered without any comae are officially designated as asteroids. Many objects in the Kuiper belt object (KBO) population, though officially assigned asteroidal nomenclature, probably have high contents of water ice and other volatiles, based on models of solar nebula formation. However, these volatile-rich objects are too far from the Sun for water-ice sublimation to form comae (although more volatile ices could). Due to the higher temperatures in the inner Solar System, the asteroids located in the main belt between Mars and Jupiter are believed to have no accessible volatiles. Highly elliptical or perturbed bodies from the KBO and Oort Cloud populations could venture from the outer to the inner Solar System, and consequently could sublimate their volatiles. Comae develop on these objects that are consequently known officially as "comets" or Centaurs. As comets progress in their orbits around the Sun, they may evolve to a volatile-poor state, 65 and may either disintegrate, impact the Sun, or form a mantle of refractory debris that prevents further sublimation. Those dormant comets that survive but do not any longer shed their volatiles when approaching the Sun may appear to be asteroids, as they have no characteristic comae. These nuclei could become hidden as members of the near-Earth or main belt asteroid population. Extinct nuclei would probably maintain their dynamically cometary properties, such as an orbital coupling with Jupiter's orbit. This orbital coupling can be quantified by the Tisserand Parameter(TJ), which is described by: (2.1) where aJ is Jupiter's semimajor axis, a is the asteroid or comet's semimajor axis, e is the eccentricity of the asteroid or comet, and i is its inclination. Objects with T J<3.0 have orbits strongly coupled to Jupiter's orbit and are usually classified as comets. Objects with T J>3.0 are weakly coupled to Jupiter's orbit and are usually classified as asteroids (Fernandez et at. 2005, in press). A few objects originally classified as asteroids but later detected with faint dust trails or gas comae (i.e., the case for 133P!Elst-Pizzaro; see Hsieh et at. 2004) have been re designated as comets. Spacecraft PIONEER VENUS detected a possible (yet faint) gas trail (Kerr 1985) originating from the surface of near-Earth asteroid (2201) Oljato that interacted with the magnetic field near Venus. Objects such as (2201) Oljato and 133P!Elst-Pizarro are making the distinction between cometary and asteroidal properties less clear. Whereas color variations on cometary surfaces may be expected (see Introduction to Chapter 1), reports of color variation on asteroids are rare and usually unsubstantiated, primarily due to their more uniform compositions. While many comets may have radially migrated when forming in the solar nebula (Weidenschilling 1997), asteroids formed in situ, and consequently have fairly homogeneous compositions, save for occasional heterogeneity from space weathering. Wavelength-dependent albedo variations seen on the Martian satellite Deimos (French 1988) have shown that particle size variations can influence possible 66 surface heterogeneity on asteroids. Smaller silicate-rich particles reflect more strongly at redder wavelengths. This reddening is due to silicate grains trapped within darker carbonaceous shells. When crushed, the red grains are more open to sunlight. The albedo increase is similar to basaltic dust on Mars reflecting with higher albedo than solid, darker bedrock. Near-Earth asteroids consist of many reddened objects, where many have little or no regolith (Carvano et at. 2003, Price 2004, Pravec & Harris 2000). The absence of a regolith is due to multiple factors, such as the smaller average size of NEAs and their higher rotation rates. Many of these objects are also believed to be fragmented objects from collisions in the main belt. The NEAs usually have lower eccentricities and inclinations than comets. Their semimajor axes are also smaller; consequently, these asteroids are heated for longer periods of time by the Sun. Possible color variations across near-Earth objects may be indicative of either particle size variations in a regolith or compositional variations of the surface itself (the parent body of the near-Earth object may have been a heterogeneous or differentiated object). Near-Earth objects that have reported color or spectral variations, coupled with higher than average eccentricities or inclinations, are usually classified as "cometary candidates", due to their dynamical or physical similarity with cometary properties. These properties include neutral or slightly red spectra, highly eccentric or inclined orbits, moderate rotation periods, surface heterogeneities, and occasional coma formation. (3200) Phaethon, as presented in Chapter 1, has convincing evidence of color variation. As the parent body of the Geminid meteor stream, it was already hypothesized to be an extinct or dormant cometary nucleus. Combined with the results of this investigation and previous research, Phaethon indeed is a unique object that may also be an extinct cometary nucleus. If Phaethon is a remnant nucleus, are there others like it in the NEA population? Asteroids were chosen that were either dynamically similar to Phaethon or shared at least one physical property. The similarity might be fast rotation like Phaethon, or high eccentricity with small perihelion. Other similarities might be color, taxonomic 67 Table 2.1 Comparison Objects Object ql p 4 B-V (3200) Phaethon 0.140 0.89 22 3.604 0.11-0.34 0.59a 0.11 F 2.50 (944) Hidalgo 1.950 0.66 42 10.063 0.35-0.60 0.74b 0.06 D 19.0 c 1999 JD6 0.324 0.63 17 7.680 0.7-1.2 0.67 0.15 K 0.6 1997 SE5 1.244 0.67 1.2 9.058 0.23 UNK 0.15 D 1.9 (142) Polana 2.089 0.14 2.2 9.764 0.11 0.62d 0.04 F 27.6 (1566) Icarus 0.189 0.83 22 2.273 0.05-0.22 0.80e 0.33 S 0.5 1998 XB 0.590 0.35 14 510.0 > 1.0 UNK ",0.25 S 1.5 (757) Portland 2.110 0.11 8.2 6.58 0.45 0.67! 0.14 M 16.1 28P/Neujmin 1 1.550 0.78 14 12.67 0.56 0.459 0.03 N/A 10.7 49P/ Arend-Rigaux 1.369 0.61 18 13.56 UNK 0.499 0.03 N/A 4.7 'UNK' refers to an unknown quantity 1 Perihelion distance [AU] 2 Orbital eccentricity 3 Orbital inclina.tion (degrees) 4 Rotation period [hr] 5 Lightcurve amplitude 6 Geometric albedo 7 Taxonomic class, as first formulated by Tholen (1984) 8 Ra.dius of object [km] a This work, Chapter 1 b Wisniewski et al. (1997) C Bus et al. (2002b), Binzel et al. (2001) d Cellino et al. (2001) e Tedesco, E.F. (1989) f Harderson et al. (2005) 9 Comet colors are V-R, taken from A'Hearn et al. (1988) and Birkett et al. (1987) class, or albedo. Some objects were additionally chosen for their unique dynamical or physical cometary properties. A list of these objects and their known rotational and physical characteristics is displayed in Table 2.1. (3200) Phaethon is listed first for comparison. Not all objects were able to be observed thoroughly in this observing program, due to time allocated on telescopes or weather. Main-belt asteroid (944) Hidalgo was chosen due to its dynamical association with Jupiter Family Comets (JFCs), as evidenced by its low Tisserand invariant (TJ )=2.068. Hidalgo also has a low albedo (Pv=0.06), another characteristic of cometary· nuclei. It regularly crosses Jupiter's orbit, similar to comet 63P/Wild 1. Howell and Lebofsky (1991) reported color variation of ~m=0.15 for Hidalgo. This object would have been a prime target for rotationally resolved color comparisons; unfortunately, it was only up when (3200) 68 Phaethon was visible in 2004. Instantaneous color and spectral measurements were made for (944) Hidalgo. 1999 JD6 is an Aten asteroid discovered by LONEOS at Flagstaff. Aten asteroids have semimajor axes within Earth's orbit, which implies that their surfaces are easily heated to high temperatures, much like Phaethon at perihelion (0.14AU). As a K-type asteroid (Bus et ai. 2002b), 1999 JD6 may be composed of CV chondritic material (Doressoundiram et ai. 2001). CV chondrites are carbonaceous meteorites that contain Calcium-Aluminum Inclusions (CAls), which are primordial minerals from the early Solar System. Similar to (944) Hidalgo, near-Earth asteroid 1997 SE5 also has a low Tisserand Parameter (2.656). Weissman et ai. (2002) classified this object as the second most probable near-Earth asteroid to dynamically originate from the Jupiter Family Comets (JFC). While 1997 SE5 does not have any cometary emission or absorption features, most D-type asteroid spectra are very similar to cometary nuclei spectra (Hicks et ai. 2000). (142) Polana is a main-belt asteroid that is one of the parent bodies of the Nysa-Polana asteroid family. This family contains a substantial number of rare F-type asteroids like (3200) Phaethon. Searches have been made (Cellino et ai. 2001) to detect 3p,m hydration features on the surfaces of the F-type Polana family asteroids, with negative results. To date, no near-infrared measurements have been made of the Nysa-Polana members. (142) Polana was easy to observe, given its relatively large diameter. (1566) Icarus is a near-Earth asteroid dynamically similar to Phaethon in its eccentricity, inclination, rotational period, and perihelion distance. Unfortunately, it also very quickly traversed the sky in 2004 (rv250" /hr), which made it difficult for lightcurve observations. 1998 XB is an Aten S-type asteroid that has no r~ported near-IR spectral information (Binzel et ai. 2004). It has an unusually long rotation period of approximately 51Ohr. Ultra-slow rotators may simply represent the tail end of the distribution of asteroid spin periods (Paolicchi et ai. 2002). While most asteroids are accelerated to periods of 6-24hr via interactions and collisions with other asteroids or planets, some asteroids experience 69 relatively few external forces and maintain low rotation rates. Depending on the scenarios, collisions can also conversely slow the rotation period of asteroids. (757) Portland is a relatively fast rotator (6.58hr period) amongst the other main-belt asteroids (Lagerkvist et ai. 1998). It is a member of the M-type asteroids, which are metallic in nature, have moderate albedos, and constitute about 10% of the total asteroid population. (757) Portland and 1998 XB were chosen as last-minute alternate objects during the spectroscopic run on UKIRT. None of their dynamical or physical properties are as close to Phaethon's as the other members of this sample. In retrospect, these two objects were poor choices for comparison to (3200) Phaethon. The data were retained, however, for completeness of the historical record. 28PjNeujmin 1 is a low-activity comet with weak CN emission and fairly red JHK colors of J-H=0.49 and H-K=0.17 (Birkett et ai. 1987). Little dust is expected on its surface. Campins et ai. (2003) argued that there are probable color variations (Llm=0.1) in the VRI bands for Neujmin 1. 49Pj Arend-Rigaux, also a low-activity comet, may contain a dustier and less gaseous coma than 28PjNeujmin 1. At times of lowest activity it has been compared to D type asteroids, being equally red at J-H=0.53, H-K=0.17 (Veeder et ai. 1987). Its dust production rates are controversial, but it appears that a significant fraction of the infrared flux from Arend-Rigaux stems from heated dust emission. 49PjArend-Rigaux has been observed by Birkett et ai. (1987) and Lowry & Fitzsimmons (2001) in the heliocentric range of r=2-3AU, but without any appearance of a coma. This chapter presents imaging and spectroscopic observations made on the aforementioned sample of comparison objects to determine surface properties from time resolved color measurements and spectra. From these data we hope to determine properties that might be shared with (3200) Phaethon's unique physical properties. 70 2.2 Imaging All of the objects in this sample were observed in June and November 2004, either at the Cerro Tololo Interamerican Observatory in Chile, and/or at the Mauna Kea Observatory, Hawaii. Observing conditions and instruments are listed in Tables 2.2 and 2.3. The observations in Chile were made with the LOrn telescope and Ohio State University AP7 512K CCD. Kron-Cousins BVRI filters were used in June, whereas only R and I filters were used in November. The B filter had much lower throughput than the VRI filters. The field of view for the detector was 4 arcminutes. The objects were tracked at both sidereal and non-sidereal rates. If an object was moving slowly enough across the sky, it was usually easier to track it sidereally and use short exposure times. The CTIO LOrn had rarely been used for non-sidereal tracking, and did occasionally malfunction during observations. The plate scale for the AP7 CCD was 0.469" /pixel, read noise was lle-(rms) , and gain was 5e-/ADU. Observations were also made at Mauna Kea from the University of Hawaii 2.2m telescope and Tek 2048 CCD (f/lO configuration). Tek2048 offered a field of view of 7.5 arcminutes. The plate scale was 0.219"/pixel, read noise was 6e-(rms), and gain was 1.74e-/ADU. Unfortunately, the filter wheel on this telescope was in need of much repair, and consequently was difficult to maneuver during each night. Tek 2048 also supplied Kron-Cousins BVRI filters. Non-sidereal tracking was easier on this telescope than in Chile. Near-infrared JHK observations were made at the UH 2.2m with QUIRC, a 1024K infrared array detector. The field of view on QUIRC was 3 arcminutes. The plate scale for QUIRC was 0.189" /pixel, read noise was 5e- (rms), and gain was 1.85e-/ AD U. Flatfielding, bias and dark-frame correction, and cosmic ray rejection were applied to both the CTIO and MKO data using standard IRAF routines. Dome flats were not used due to significant differences in dust features between these and the twilight flats. When flattening tests were performed between dome flats and twilight flats, the latter were always 3-4 times more successful at removing dust features and brightness gradients. 71 Landolt standard stars (1992) were used for atmospheric extinction, color correction, and zero point offset. Calibration of asteroid frames was performed using aperture photometry routines in IRAF. Aperture photometry for the nucleus 49PIArend-Rigaux required measurement of sky away from the coma and tail for sky subtraction. Extinction coefficients for both CTIO and MKO are listed in Table 2.4. There was evident time variable extinction on the nights of June 10 and June 12, which caused lower than average extinction coefficients in the V and R-filters. Calibration fields were used for nights that were non-photometric. UKIRT JHK Faint Standards were used for calibration of QUIRC data. Groups of 10-20 field stars were measured in each image for differential photometry. Cloud correction through differential photometry was quite useful in Chile as there were frequent bands of high cirrus that would pass over the summit. Weather was mostly clear at Mauna Kea. Due to the motion of the asteroid and the finite size of the chip, sequential sets of field stars were required for differential photometry, with at least one or two overlapping stars between sets. All stars' measurements were then corrected to their weighted average brightnesses. The average correction applied was subsequently also applied to the asteroid. Different sets were combined via overlapping stars between sets. Photometric apertures for all objects were twice the FWHM of the asteroid's (or comet's) point-spread function (PSF). Table 2.5 gives the average integration times and average SIN for each object observed in 2004. The SIN was high enough on (142) Polana, 1999 JD6' 1997 SEs, and 49PIArend Rigaux for lightcurve and color analysis to be performed. The SIN was unfortunately too low for (1566) Icarus. (944) Hidalgo, at V"-'12.5, was easy to observe for color measurements, although its position in the sky conflicted with observing time for (3200) Phaethon; therefore no lightcurves were constructed for Hidalgo. 72 Table 2.2 Observing Instruments Date(UT 2004) Telescope Instrument Filter(s) Apa Seeb Conditions June 03 UR 2.2 m QUIRC 1024 J 1.9 0.9 Photometric June 05 UR 2.2 m QUIRC 1024 JR 1.5 1.4 Photometric June 06 UR 2.2 m QUIRC 1024 JRK 1.5 0.8 Photometric June 07 UR 2.2 m QUIRC 1024 RK 1.9 1.2 Cirrus June 09 UR 2.2 m Tek 2048 B 2.2 1.0 Photometric June 10 UR 2.2 m Tek 2048 V 2.2 0.7 Photometric June 11 UR 2.2 m Tek 2048 V 2.6 0.8 Photometric June 12 UR 2.2 m Tek 2048 VR 2.2 1.2 Photometric June 18 CTIO 1.0 m AP7512K BVRl 3.8 1.6 Photometric June 19 CTIO 1.0 m AP7512K VRl 3.8 1.2 Light Cirrus/Clear June 21 CTIO 1.0 m AP7512K VI 8.4 3.0 Photometric June 22 CTIO 1.0 m AP7512K VI 3.8 1.6 Thick Cirrus Nov. 06 CTIO 1.0 m AP7512K Rl 3.3 1.5 Cirrus Nov. 11 CTIO 1.0 m AP7512K Rl 2.8 1.0 Photometric Nov. 14 CTIO 1.0 m AP7512K RI 2.8 1.2 Cirrus Nov. 15 CTIO 1.0 m AP7512K RI 4.2 1.3 Photometric Nov. 19 UR 2.2 m Tek2048 VR 2.2 0.7 Photometric Nov. 21 UR 2.2 m Tek2048 VR 2.6 0.7 Light Cirrus Nov. 22 UR 2.2 m Tek2048 VR 2.2 0.8 Photometric a Aperture for photometry [arcsec] b Seeing [arcsec,FWHM] 73 Table 2.3 Observing Conditions Date(UT 2004) RA Dec. r(AU) L'l(AU) aa Eb Moon(%) (944) Hidalgo Nov. 19 0049 +4109 2.050 1.206 18.8 69.8 49.5 Nov. 21 0045 +4129 2.044 1.215 19.7 49.4 70.8 Nov. 22 0043 +4138 2.041 1.220 20.0 41.3 79.8 1999 JD6 June 03 1609 +2713 1.383 0.495 34.4 49.2 98.4 June 05 1602 +2627 1.390 0.503 34.3 59.8 98.3 June 06 1558 +2603 1.393 0.509 34.4 68.5 93.8 June 07 1554 +2538 1.396 0.513 34.5 79.2 86.5 June 09 1548 +2444 1.401 0.524 34.6 100.4 67.1 June 10 1544 +2418 1.404 0.530 34.7 110.0 56.9 June 11 1542 +2350 1.407 0.535 34.9 119.3 46.3 June 12 15 38 +2324 1.409 0.541 35.1 127.4 36.2 1997 SE5 June 18 15 17 -21 20 2.261 1.350 14.8 142.9 0.2 June 19 15 15 -21 00 2.245 1.349 15.3 131.0 1.5 June 21 15 15 -2055 2.237 1.349 16.3 119.0 4.6 June 22 15 14 -2050 2.229 1.348 16.8 107.5 9.1 (142) Polana June 05 21 24 -14 57 2.278 1.630 23.4 34.2 93.2 June 06 21 25 -1452 2.279 1.621 23.2 19.5 85.7 June 18 21 26 -1434 2.295 1.527 20.4 120.6 0.4 June 19 21 26 -14 33 2.297 1.518 20.1 132.9 0.3 June 21 21 26 -1433 2.300 1.504 19.5 155.3 5.5 (1566) Icarus June 09 2109 -2824 1.652 0.832 29.6 17.7 66.8 June 11 2105 -29 15 1.665 0.824 28.1 45.1 45.0 June 12 2102 -2942 1.671 0.820 27.3 58.4 34.9 June 18 2046 -32 22 1.710 0.802 22.4 132.9 0.4 June 21 2036 -3340 1.73 0.796 21.6 167.5 5.3 49p/Arend-Rigaux Nov. 06 0045 -30 19 1.826 1.065 26.3 137.8 41.0 Nov. 11 0043 -30 01 1.793 1.067 28.2 135.6 2.6 Nov. 14 0042 -2943 1.774 1.069 29.2 99.4 3.6 Nov. 15 0041 -29 37 1.767 1.070 29.6 86.4 9.4 a Phase angle (Sun-asteroid-earth angle) [degrees] b Lunar separation [degree] from asteroid 74 Table 2.4 Extinction Coefficients [mag/airmass] Date (UT 2004) kB kv kR k[ kJ kH June 03 0.105+/-0.009 June 05 0.052+/-0.007 June 06 0.142+/-0.008 0.064+/-0.009 June 07 N/A N/A N/A N/A N/A N/A June 09 0.257+/-0.053 0.070+/-0.001 0.025+/ -0.001 June 10 0.247+/-0.044 0.066+/-0.002 0.080+/-0.002 0.040+/-0.002 June 11 0.212+/-0.026 0.199+/-0.002 0.124+/-0.002 0.078+/-0.003 June 12 0.055+/ -0.004 0.055+/-0.005 June 18 O.404+/-o.QlO 0.103+/-0.005 0.090+/-0.005 0.063+/-0.007 June 19 0.419+/-0.009 0.090+/-0.006 0.065+/ -0.007 0.034+/ -0.008 June 21 0.241+/-0.004 0.170+/-0.003 0.162+/-0.003 0.074+/-0.004 June 22 N/A N/A N/A N/A N/A N/A Nov. 06 N/A N/A N/A N/A N/A N/A Nov. 11 0.103+/-0.005 0.074+/-0.002 0.105+/-0.003 Nov. 14 N/A N/A N/A N/A N/A N/A Nov. 15 0.150+/-0.004 0.104+/-0.004 0.069+/-0.008 Nov. 19 0.241+/-0.062 0.105+/-0.001 0.042+/ -0.001 Nov. 21 N/A N/A N/A N/A N/A N/A Nov. 22 0.137+/-0.001 0.084+/-0.001 "N/ A" refers to nights that were non-photometric. Table 2.5 Integration Times, SIN ==~=:======:====c~==:::=~=::= OJ beet Int. time(s) S/N(avg) (944) Hidlago 25 300 1999 JD6 50 200 1997 SE5 100 60 (142) Polana 80 500 (1566) Icarus 150 15 49P/ Arend-Rigaux 120 65 75 2.3 Results Lightcurve data were first corrected for phase angle, geometric and heliocentric distance, and light-travel time. The reduced magnitude m(l,l,a) at 1AU geocentric and heliocentric distances was computed using the relation m(l, 1, a) = m - 5tog(rb.) (2.2) where m is the observed magnitude, a is the phase angle, r is the heliocentric distance [AU], and b:. is the geocentric distance [AU). The reduced magnitude is then normalized to zero phase angle (absolute magnitude H), by the H-G relation (Bowell et at. 1989): 63 22 m(l, 1, a) = H - 2.5Iog(1 - G)exp[-3.33tanO. (a/2)) + Gexp[-1.87tan1. (a/2)) (2.3) where H is the reduced magnitude at phase angle a=O°, and G is the slope parameter. Phase Dispersion Minimization (PDM; Stellingwerf 1978) was utilized where possible to confirm the rotational period of each object. PDM systematically tests lightcurve data for different periods, and subsequently displays the deviation ("theta-statistic") for each determined period (Bauer et at. 2002). Lower theta-values correspond to lower amounts of variance to each fit. Consequently, the lowest theta-statistic corresponds to the most probable period (Stellingwerf 1978; Bevington 1969). Fourier analysis also determines periods by using sine and cosine terms to fit a sinusoidal function to the data. While Fourier analysis was used to confirm the period of each object, lightcurves were ultimately phased with PDM. Because multi-filter photometry was obtained in this observing program, time-resolved colors could be calculated by interpolating one filter lightcurve (i. e., V) onto another filter lightcurve (R), and then subtracting (V-R). Another possible method involves fitting a Fourier function to each filter lightcurve, and then subtracting. Unfortunately there 76 can be fitting noise when using Fourier fits, due to the often unusual shapes of asteroid lightcurves. Interpolation was the method of choice in this analysis. 2.3.1 1999 JD6 Both PDM and Fourier calculated a period of 3.834hr for 1999 JD6; this period is half of the value found by Szabo et at. (2001). This 3.834hr periodicity is produced by brightness (or albedo) variations over the asteroid, and may reflect the rotation of the object. A symmetrically shaped ellipsoid, with uniform albedo on all parts ofits surface, while rotating on a principal axis, will produce a periodic brightness variation through only half of its rotation. In order to determine the true rotational period of the object, the cyclic period measured from the brightness variation must be doubled. There are many small NEAs that could be ellipsoidal-shaped collisional fragments from the main belt. They would have to be symmetric enough to produce this effect. With its small diameter, 1999 JD6 most probably has a symmetric ellipsoidal shape. By doubling the period of 3.834hr, we obtain a period of 7.668hr +/- 0.005hr. This period agrees with the result of Szabo et at. (2001), who obtained a period of 7.68hr +/-0.01hr. Fig. 2.1 shows the phased lightcurve of 1999 JD6 from both the QUIRC and Tek2048 observations. The ~m=l.O amplitude of the lightcurve is slightly less than the results of of SzabO et at. (2001), who obtained ~m=0.75. Fitting a period in either PDM or Fourier requires all data to be taken in a single filter. If multiple filters are used (as was the case for 1999 JD6), the colors of the object must be known as a function of rotation so that offsets can be applied and all data can be normalized to a single filter. Instantaneous B-V, V-R, and R-I measurements were made on 10 and 11 June, so that colors could be calculated. All data were then offset to the V-filter. J & H filter photometry had to be manually offset (aligning maxima of J & H lightcurves with V lightcurve) to match the optical datasets, as no V-J measurements were made. K-filter measurements from QUIRC had too low SIN to make reliable colors, or to be used in the composite lightcurve. Table 2.6 shows the instantaneous optical and near-IR colors. The large lightcurve amplitude of 1999 JD6 77 Table 2.6 1999 JD6 Colors Color B- V 0.673+/-0.013 V-R 0.414+/-0.011 R-I 0.389+/-0.010 J-H 0.354+/-0.033 makes instantaneous colors difficult, but by interleaving R-B-R-V-R measurements (also mimicked in the near-IR), lightcurve effects can be removed through interpolation. There is an apparent discrepancy in the lightcurve fit from -10 to 10% rotational phase, where the June 03 and 10 data do not match with the June 11 and 12 data. Images were investigated for possible field stars contaminating the observations on June 03 and 10, as this small dip in the curve is unexpected, based on the results in Szabo et ai. (2001). The asteroid did not pass over any field stars, and conditions were listed as photometric for both nights. The weather was identically clear on June 11 and 12. The variation of the lightcurve here does not seem dependent on color, as the dip that occurred in the June 03 and 10 data affected both the J and V data, respectively. There was a possibility that this variation was evidence of a binary asteroid, as shown by the shape of its lightcurve (see Section 2.3.4 for a discussion on this issue). The data were sent to P. Pravec of the Ondrejov Observatory, who regularly uses Fourier fitting to determine periodicity in binary lightcurves. No period beyond the 7.668hr period was successfully fitted to the data for 1999 JD6' which negated the possibility of a regular binary NEA. The cause of this lightcurve deviation in the J and V data remains a mystery. There may be a contribution from aspect angle variation that caused the inconsistency seen between the June 3 and June 11 & 12 data. From the composite lightcurve given in Fig. 2.1, one can discern which parts of lightcurve were obtained by multiple filters. Those areas that were covered by multiple filters could be used for time-resolved color measurements. Unfortunately, 1999 JD6 was not continuously observed with interleaved filters like the rest of the comparison objects (which would allow for even more accurate color interpolation). From the lightcurve we attempted, where 78 possible, to measure V-J, R-J, J-H, B-V and V-R over fractions of the asteroid's rotation. In some cases overlap was so tenuous that any variation seen in the colors could not be interpreted with confidence. In other cases, such as V-J and R-J, there was no obvious trend in changing colors. However, the B-V and V-R measurements represent fairly well sampled results. Both B-V and V-R coverage occurred from 10-45% rotation, and each displayed a decrease in reddening of the asteroid (see Fig. 2.2). The V-R measurements had longer rotational coverage, and show a gradual reddening following the decreasing color. Each color curve has a net L).m rv 0.10. Q) "0 I B 17 '2 b.O <'il ~ "0 ,,03Jun(J) Q) +> --. 6"!l (.11 ;) ....<'il ;g 17.5 ,,07Jun(H) <'il u .10Jun(V) .1lJun(V) .12Jun(R) 18 '-----L.----L_...L.---l-_'-----L.----L_...L.---l-_'-----L.----'_--1.-----' o 0.5 1 Rotational Phase (0815UT.03June2004) Figure 2.1 Phased lightcurve for 1999 JD6, phased to a period=7.668hr, at zero-phase of 08:15UT, 03 June 2004. The deviation in data from 10-20% rotation remains unexplained. A broadband spectrum of 1999 JD6 was constructed from the BVRI data (Fig. 2.3). According to Bus et al. (2002a), a K-type asteroid such as 1999 JD6 should have a moderately steep red slope shortward of 0.75j.Lffi, and then a flat, bluish slope longward of 0.75j.Lffi. While the BVR reddening can be seen in Fig. 2.3, the I-filter data point 79 1999 JD6 B-V June 2004 1999 JD6 V- R June 2004 I I I I 0.55 I I I I 0.7 - II~ 0.5 0.65 - ~\ > 0: \ I I 045 r- eq > fI~1~~j 0.6 \1~ - i\, 0.4 J-- d~ 0.55 - 0.35 a 0.1 0.2 0.3 0.4 0.5 a 0.1 0.2 0.3 0.4 0.5 Rotational Phase (0815UT.03June2004) Rotational Phase (0815UT.03June2004) Figure 2.2 B-V(left) and V-R(right) rotationally phased colors for 1999 JD6 from June 2004. The B-V colors represent data taken on June 9 & 11, and the V-R colors represent data taken on June 11 & 12. The decreasing color here correlates with the first maximum of the lightcurve (10-15& rotational phase) shown in Fig. 2.1. (centered at 0.83{lm) should have been slightly bluer than the R-filter data point (0.65{lm) to mimic the K-type spectral behavior presented in Bus et ai. (2002a). 1999 JD6 appears more like a T-type asteroid, which maintains a reddened slope past 0.75{lm. 2.3.2 (142) Polana Subsequent observations of the other comparison objects were performed with interleaving filters, which allowed for easier rotationally-resolved color measurements. The primary filters used were V& I, so that color variations, if present, could be more easily detected. Unfortunately higher sky background exists in the I-filter than the R-filter. (142) Polana was the largest and brightest member of the sample. Differential photometry proved to be uniquely deleterious to the quality of Polana's final lightcurve, primarily because the only stars in the field were much fainter than this bright asteroid. For most of the data on Polana, differential photometry was not required. Fortunately, most observations of Polana were obtained in clear weather. There was an appreciable amount of scatter in the J and H 80 1999 JD6 Broadband Spectrum 1 Q) () t:: cO +-' () Q) ...... --< Q) D:: 0.9 '"0 Q) .....N co ....6 o Z 0.8 0.4 0.6 0.8 Wavelength (microns) Figure 2.3 Broadband spectrum for 1999 JD6 , taken from its BVRI data points.The data is normalized to the I-filter (0.826f,Lm) measurement. lightcurve data for (142) Polana, such that the net lightcurve (Fig. 2.4) only utilizes optical lightcurve data. BVRIJH colors for (142) Polana are given in Table 2.7. Observations covered 85% of Polana's 9.764+/-0.005hr period (Barucci et al. 1994). Even with limited coverage, changes are evident from the lightcurve taken in 1994. The 2004 lightcurve presents three maxima, as opposed to the double maxima shown in the original 1994 lightcurve. The June 21 data had problems with transient clouds and bad seeing fluctuations. Unfortunately this caused a poor wrap of the phased lightcurve. The lightcurve amplitude of Polana, measured here at Llm=0.19+/-0.01, is ,,-,80% larger than that measured by Barucci et al. (1994), who measured Llm=0.1l+/-0.01. The cause of these differences is probably due to changes in aspect angle (changes in the angle of the rotational axis of the asteroid, relative to an observer on Earth). Since there is no pole solution yet for Polana, it is impossible to discern the exact difference in aspect 81 Table 2.7 (142) Polana Colors Color B- V 0.679+/-0.011 V-R 0.344+/-0.008 R-I 0.320+/-0.010 J-H 0.355+/-0.018 angles. Attempts were made at fitting the data to other periods near 9.764hr, but were not successful. Unfortunately, the first peak between 0-20% rotation has data with scatter, due to variable and increasing clouds on June 21. Differential photometry was not as successful for this dataset because Polana was significantly brighter than the surrounding field stars; therefore, variations detected in the field stars due to cirrus did not improve the photometry for (142) Polana. 142 Polana June 2004 9.9 • June 18 10.05 • June 19 • June 21 1O. 1 L....-I----I.----l..---l...-.l--L----L.----L.--L-.L-L----'----I.----'----'----'---'--L---'--I o 0.2 0.4 0.6 0.8 1 Rotational Phase Figure 2.4 V-filter lightcurve of (142) Polana, phased with the determined period of 9.764hr by Barucci et al. (1994). Zero-phase is at OO:OOUT, 18 June 2004. The triple-peaked lightcurve differs from Barucci et al. 's due to aspect angle differences. 82 A broadband spectrum was constructed from Polana's BVRI measurements, and is displayed in Fig. 2.5. The BVRI spectrum is typical of Band F-type asteroids like (3200) Phaethon or (704) Interamnia. (142) Polana Broadband Spectrum 1.1 Q) () t::: ....,co () ~ 1.05 'H Q) ~ '0 Q) .S::! 1 ~ 6.... o Z 0.95 0.4 0.6 0.8 Wavelength (microns) Figure 2.5 Broadband spectrum for (142) Polana, taken from its BVRI data points. The data is normalized to the I-filter (0.826j.l.m) measurement. Interpolation was also performed on (142) Polana in the V and I filters for time-resolved color measurements. Fig. 2.6 shows that there are no variations in color for this object. 2.3.3 1997 SE5 1997 SE5 was also observed in Chile. The V-filter lightcurve is shown in Fig. 2.7, and confirms the period of 9.058hr +/- 0.005hr by Pravec et ai. (1998). Interleaving I-filter measurements were made for a significant fraction of the data, for time-resolved V-I colors. The resultant V-I over rotation (Fig. 2.8) reveals fairly continuous color, with a marked exception in the June 21 data. The V-I jumped from an average V-I=0.85 to an average 83 142 Polana V-I June 2004 0.7 I :> 0.65 o.6 LL-.--'-----'---'---'---'--L-L-'---J---l---l---l-----l--'---'---'---'----'----'---' o 0.2 0.4 0.6 0.8 1 Rotational Phase Figure 2.6 V-I colors for (142) Polana, phased to period=9.764hr. No color variation could be detected within the error bars. of V-I=l.l. There were no field stars near the object during this time, although seeing fluctuated between 2.5 and 3.0". The photometric SIN for 1997 SE5 was similar between this night and the other three nights. The scatter that resulted in the June 21 data for Polana was due to clouds that rolled in during the last couple hours of the night. While there was no detectible cloud cover while observing 1997 SE5 (observed earlier in the night), a few images did betray the presence of stray cirrus, but should not have altered the V-I colors for the entire night. As with all other observations, apertures twice the size of the object's FWHM were used on June 21. Because of the poor seeing the aperture for photometry was also twice as large as the other three nights. Because the SIN was similar to the other nights, it seems that the aperture for photometry was appropriate as well. The individual V and I lightcurves produced for 1997 SE5 on June 21 appeared to be of similar quality to the other three nights' lightcurves. Each night utilized 15-20 field stars for differential 84 photometry, none of which ran into the asteroid on June 21. Because of 1997 SEs's slow motion, only one set of field stars was needed for the entire night. Ultimately, the cause for the sharp reddening that occurs between 10 and 40% rotation phase remains a mystery. Colors for 1997 SEs are given in Table 2.8. This is a significantly red object, which supports its D-type spectral classification (Hicks et al. 2000). No JH observations were made for this object. A BVRl broadband spectrum is displayed in Fig. 2.9. Bus et al. (2002a) describes featureless, red slopes on D-type asteroids. This description somewhat fits the D-type classification of 1997 SEs, although there is a bit more curvature to the spectrum than is expected for D-types. 1997 SE5 June 2004 .June 21 .June 22 o 0.5 1 Rotational Phase (OOOOUT 18June2004) Figure 2.7 Binned V-filter lightcurve for 1997 SEs, phased with the 9.058hr period ofPravec et al. (1998). Zero-phase was OO:OOUT, 18 June 2004. 85 2r--"'T'"'"'"',.....;;...,---"-F--"'-r-....,....:'---T=r:-~:....:r-~-.----,-----,1997 SE5 V-I June 2004 1.5 ~ ~ .. 1 • +-'- "'.t I -> 1 ~ II t~t ! t 0.5 .June 18 .June 19 .June 21 .June 22 0 0 0.5 1 Rotational Phase Figure 2.8 V-I colors for 1997 SEs, phased to period=9.058hr. Since the June 21 da.ta is probably due to instrumental effects (see text), no net color variation is detected here. 2.3.4 49P/ Arend-Rigaux Based on the 13.56hr period determined by Millis et al. (1988), roughly 70% of rotational coverage was obtained for low-activity comet 49P/ Arend-Rigaux. The phased lightcurve (Fig. 2.1O) presents a better fit at 13.47hr +/-o.03hr. The double-peaked lightcurve obtained by Millis et al. (1988) cannot be confirmed by the lightcurve in this work, but it does appear that a second maximum exists apart from the data already present in Fig. 2.10. Table 2.8 1997 SEs Colors Color B- V 0.818+/-0.044 V-R 0.399+/-0.023 R-I 0.428+/-0.029 86 1 () ~ ro +' () .....~ 0.9 0:: '0 ~ 0.8 ro E l-. o Z 0.7 0.4 0.6 0.8 Wavelength (microns) Figure 2.9 BVRl broadband spectrum for 1997 SEs. The data is normalized to the I-filter (0.826fLm) measurement. Interpolation was again applied to the R and I data for color measurements (Fig. 2.11). The thick cirrus present on November 11 led to low SIN data with much scatter. Unfortunately, these data could not be used in the R-I calculations. There is vague evidence of an R-I decrease in the first half of rotation, but not beyond the errors. The general conclusion for this object is that there are no obvious color variations detected in 49PIArend-Rigaux. The presence of a faint dust coma, evident in the images, may have clouded the true colors of the surface. While R and I filtered observations are much less affected by a gas coma, the presence of dust surrounding the nucleus of the object will still hinder nuclear observations. 49PIArend-Rigaux's measured color is R-1=0.468 +1- 0.025, confirming a reddened nuclear surface. Comets and other D-type asteroids are typically red, caused by organic surface materials and by UV radiation (Strazzula et at. 1991). 87 13.2 49 / Arend- Hi aux H-filter Nov.'04 13.3 • Nov 06 • Nov 11 13.6 • Nov 14 • Nov 15 13.7 '---''------'------'------'------L-----i--'---'---'--.L-'---J'------'--L---'---'--'---1-~ o 0.2 0.4 0.6 0.8 1 Rotational Phase (OOOOUT 06Nov.04) Figure 2.10 R-filter lightcurve of 49P/ Arend-Rigaux, phased to a 13.47hr period, which is 0.11 hr different from the period derived by Mills et al. (1988). Zero phase OO:OOUT, 06 • November 2004. 2.3.5 (1566) Icarus While lightcurves could not be constructed for (1566) Icarus, BVRI colors were made (Table 2.9) at the University of Hawaii 2.2m with Tek2048. The red colors of this object confirm its S-type taxonomic classification. Figure 2.12 presents a broadband spectrum constructed from the BVRI colors, which is similar to most S-type spectra (Bus et al. 2002a). 2.3.6 (944) Hidalgo D-type asteroid (944) Hidalgo was observed from Mauna Kea, albeit only briefly. Consequently, only a V-R color measurement could be made of this object with Tek2048 on the UH2.2m, V-R=0.680 +/- 0.003. This color is much redder than that measured for the red D-type asteroid 1997 SEs. However, there were only a few V-R measurements made for 88 49p/Arend- Rigaux R-I Nov. '04 o.6 ,--,---.-----r--'-r----,--r----.---,----'=-.---..---.---.-----r--.---r----,--r---,---, 0.5 -I 0:: 0.4 A Nov 06 A Nov 14 A Nov 15 o.3 L-.l----L----'----'----'--'--'----'---..L-.L-L-.l----L----'----'----'----'--'--'---' o 0.2 0.4 0.6 0.8 1 Rotational Phase (OOOOUT 06Nov.04) Figure 2.11 Binned V-I colors for 49PIArend-Rigaux, phased to the 13.47hr period. Of the measurements that have high enough SIN, no color variation is detectible. Hidalgo before observing (3200) Phaethon each night. The extremely red optical V-R color of this asteroid is supported by the very reddened near-IR spectral slope displayed in the Spectroscopy section. Table 2.9 (1566) Icarus Colors Color B- V 0.764+/~.024 V-R 0.407+/~.018 R-I 0.275+/~.021 89 (1566) Icarus Broadband Spectrum 1.1 v g 1.05 .....(\j () ~ ...... ~ 1 "d v N .~ ~ 0.95 >-. o Z 0.9 0.4 0.6 0.8 Wavelength (microns) Figure 2.12 (1566) Icarus broadband spectrum. Typical to most S-types, there is steep reddening until 0.75p,m, followed by a slight decrease, which is usually evidence of a 1.0p,m pyroxene absorption feature. The data is normalized to the I-filter (O.826p,m) measurement. 2.3.7 Imaging Analysis The Llm=l.O amplitude of 1999 JD6 implies an axial ratio of 2.5 . The period of this 1.2km object has been constrained to 7.668hr +j- 0.005hr. Sheppard & Jewitt (2004) discussed such characteristics to be representative of contact binary KBOs. They ascertained that contact binaries have amplitudes of Llm=0.9-1.2. The binaries have different types of curvature between maxima and minima, which in turn should be sharp and discontinuous. Their rotational periods should average between 6-24hr, with diameters larger than 1km. These characteristics all match with the properties of NEA 1999 JD6. However, the characteristics of contact binary KBOs may be quite different from the properties of binary (or even contact binary) NEAs. Pravec & Harris (2000) discerned that binary NEAs (non contact) usually have lightcurve amplitudes of Llm=0.11-0.15 and rotational periods of 90 2.2-2.7hr. Curiously, most known binary NEAs have perihelia inside lAD and primary object diameters of 1.0km (Pravec et at. 2005, in press). The largest monolithic asteroid that can rotate with a period faster than 2.7hr would have an estimated diameter of 0.15km. 1999 JD6 shares most of these qualities, except for its significantly larger rotation period and lightcurve amplitude. To date, no binary NEAs have been positively identified as contact binaries. Radar observations of NEAs are required to confirm contact binaries. 1999 JD6 was analyzed by P. Pravec with Fourier analysis for other possible periods that would indicate the presence of a secondary object, but none were found. This asteroid is most probably a highly ellipsoidal, collisionally-derived fragment from the main belt (Szabo et al. 2001). (1566) Icarus may be a prime binary NEA candidate. Its rotation period of 2.273hr, diameter of 1.3km, and lightcurve amplitude of ~m=0.05-0.22 (e.g., Gehrels 1970) are also characteristic properties of binary NEAs (Pravec & Harris 2000). Radar observations by Mahapatra et at. (1999) show Icarus has a very rough surface, possibly the result of a thin-layered, coarse-grained regolith. The B-V and V-R variation of ~m=0.10 is unique for 1999 JD6. While this asteroid does not bear a spectral resemblance to (3200) Phaethon, its perihelion of q=0.324AD may have caused thermal alteration of its surface features in a way similar to Phaethon. The broadband spectrum of 1999 JD6 displays a lack of an expected absorption at 1.0f.£m. As evidenced by the I-filter datapoint, the reddening continues to increase. Heating of objects to high enough temperatures can cause removal of absorption features and decrease in reflectance and/or reddening (Carvano et at. 2003). The color variation may also be related to the thermal processing of the asteroid. 1999 JD6 becomes bluer at its maximum brightness. (3200) Phaethon becomes conversely redder at its maximum brightness, but nonetheless shares a correlation of color variation with the rotational lightcurve. 1997 SE5 has similar dynamical properties to 49P/ Arend-Rigaux. Its broadband spectra, which depicts reddening through 0.826f.Lm, is characteristic of D-type asteroids (Bus et at. 2002a). The time-resolved V-I measurements display no periodic trend of color variation, 91 except for the unique color variation that occurs on June 21. The filters on the CTIO 1.0m were reported to be parfocal; however, the seeing variation on this night may probably have caused wavelength-dependent flux loss in one filter more than the other. All images of 1997 SE5 on June 21 (as was done on each night for consistency) were measured with the same photometric aperture, regardless of filter. If there were wavelength-dependent losses due to seeing (in this case, loss in the V-filter relative to the I-filter), then this would have produced the extreme reddening that occurred on June 21. As it is not observed on any of the other three nights, we attribute this color variation to instrumental effects, and conclude that 1997 SE5 has no rotationally dependent color variation. However, the extreme red (B V=0.818) color of this object cannot be ignored. This red color may be characteristic of the mantles of extinct comets, which would be apparently identical to other D-type asteroids. Strazzulla et at. (1991) argued that many cometary bodies irradiated by the Sun would display red mantles, perhaps similar to that of this near-Earth asteroid. The search for a link of color variation with F-type asteroids such as (142) Polana proved futile, though color variation may be more indicative of asteroids' individual histories. Apparently (3200) Phaethon's color variation may not be directly linked to its taxonomic class. The broadband spectrum constructed for Polana shows its negative spectral gradient from 0.545p,m to O.826p,m, which is typical for F-type asteroids. However, its 0.438p,m data point suggests a possible UV-absorption feature, which is more diagnostic of B-type asteroids. B-type asteroids are distinguished from F-types by the presence of this UV absorption feature, as discussed by Tholen (1984). Because (142) Polana is more probably a B-type than an F-type, the lack of color variation on this object does not rule out the possibility that other F-type asteroids (such as 704 Interamnia) might have color variation over their surfaces. 49P/ Arend-Rigaux contained no detectible color variation. The data may have been contaminated by the presence of a coma, as the comet was close to perihelion in November 2004. Observations of this object closer to aphelion (5.67AU), if it is bright enough, could 92 Table 2.10 Comparison Object Colors Object B-V(published) B-V(this work) Taxonomic Class 1999 JD6 0.67+ / -0.02a 0.673+/-0.013 K 1997 SEs N/A 0.818+/-0.044 D (142) Polana 0.62+/-0.01b 0.679+/-0.011' F/B (1566) Icarus 0.77+/-0.03c 0.764+/-0.024 S a Bus et at. (2002b), Binzel et at. (2001) b Cellino et at. (2001) c Tedesco (1989) be more fruitful for surface analysis of the nucleus. To date, no attempts have been made at time-resolved color measurements of 49P/ Arend-Rigaux (e.g., Luu 1993, Wisniewski 1990). Table 1.10 combines all measured B-V colors for the sample of comparison objects and compares them to previously published B-V colors and assigned taxonomic classes. None are as blue as (3200) Phaethon (B-V=0.587), though some have fairly solar colors (B-V sun=0.674; Tedesco et at. 1982). Except for 1997 SEs, which has no published colors, the other three objects' B-V measurements from this work match closely with the previously published colors, and correlate well with taxonomic class. The only object with unique and interesting results that correlated with the results of (3200) Phaethon was 1999 JD6. 1999 JD6 , like Phaethon, has color variation correlated with its lightcurve. The color variation displayed on (3200) Phaethon, if not directly related to its cometary candidacy, may be related to the thermal processing and UV irradiation on the surface. Surface patches on the surface that have been "gardened" from impacts with small particles may appear both bluer and brighter in ground-based observations. This space weathering could have produced the bluer patches seen on 1999 JD6. Craters may also produce these blue patches on the surface of 1999 JD6. The V-I measurements of the comparison objects revealed no definitive color variation. The background noise contributed from the I-filter may have removed small-scale color variations that were present on these objects. However, due to the inherent dynamical 93 assocation of 1997 SEs with cometary nuclei and 1999 JD6 's evident color variation, surface brightness profiles were constructed for both objects in the same manner as was performed for (3200) Phaethon (Chapter 1). Trailed background stars and the asteroids (if trailed, as was the case for 1997 SEs) were aligned perpendicular with the vertical axis, whereby vertical crosscuts were made for the SB profiles. Fig. 2.13 displays the composite images for both 1997 SEs and 1999 JD6. Fig. 2.14 shows the SB profile for 1997 SEs (V-filter) and Fig. 2.15 shows the SB profile for 1999 JD6 (R-filter). While no coma is evident on 1999 JD6, there was an interesting inflation on the left wing of 1997 SEs's profile, compared to the stellar profile. While nearby stars were carefully removed in the composite image, the lack of such an inflation in the profile on the right-hand side may indicate that this feature is an anomaly. Further analysis will be required to constrain this feature on 1997 SEs. Figure 2.13 Composite images for 1997 SEs(left) and 1999 JD6 (right). The image of 1997 SEs (North-right, East-up) represents ",,4500 seconds integration and is 94" square. The image of 1999 JD6 (North-up, East-left) represents ",,3500 seconds integration and is 43" square. 94 1997 SE5 S-B June18(V) 26 28 32 34 -4 -2 0 2 4 Radius (arcsec) Figure 2.14 Surface-Brightness profile (V-filter) of 1997 SEs. While nearby stars were removed from the composite image of 1997 SE5, the anomalous bump on the left side is probably due to improper removal of nearby stars. 2.4 Spectroscopy In order to study possible absorption features and overall spectral shapes of selected comparison objects, (944) Hidalgo, 1998 XB, and (757) Portland were observed with spectrographs at NASA IRTF, UKIRT, and Keck II Telescopes in November and December 2004. Tables 2.11 and 2.12 give the observing conditions of these separate runs. Spectral observations were made for (944) Hidalgo on Mauna Kea on 20 November and 24 Decemb r 2004 with the AA Infrar d el cop acility (ffiTF) and Keck II Telescope. The spectrograph SpeX (Rayner et al. 2003), was used in November for 0.8-2.5~m observations. The resolution was R"'lOO. The objects were observed a.t 7.5" separations in an ABBA sequence. The plate cale was 0.15 /pix, read noise was 50e- (rms) , 95 Table 2.11 Observing Instruments Date (UT 2004) Telescope Instrument Filter(s) Res.a Timeb SlitC Seed Conditions Nov. 20 IRTF SpeX JHK 100 120/1 0.8 0.5 Light Cirrus Dec. 17 UKIRT mST HK 800 120/2 0.84 0.5 Photometric Dec. 18 UKIRT mST HK 800 180/2 0.84 1.2 Thick Cirrus Dec. 19 UKIRT mST HK 800 150/2 0.84 0.7 Light Cirrus Dec. 20 UKIRT mST HK 800 150/2 0.84 1.2 Overcast Dec. 24 Keck II NIRSPEC KL 1300 1.0/120 0.76 0.7 Light Cirrus a Spectral resolution, !:::'>"/ >.. b Integration time [s] & Number of coadds c Slit width [arcsec] d Seeing [arcsec, FWHM] Table 2.12 Observing Conditions Date (UT 2004) RA Dec. r(AU) !:::.(AU) aa Eb Moon(%) (944) Hidalgo Nov. 20 0047 +4120 2.047 1.211 19.3 57.9 61.8 Dec. 24 00 15 +4526 1.971 1.445 14.6 53.1 95.0 (757) Portland Dec. 17 11 50 +0909 2.309 2.058 25.2 166.3 37.1 Dec. 18 11 51 +0904 2.310 2.047 25.2 175.5 48.4 Dec. 19 11 52 +0900 2.312 2.036 25.1 166.1 58.3 Dec. 20 11 53 +0855 2.313 2.024 25.1 153.0 68.2 1998 XB Dec. 17 1010 +3115 1.226 0.384 43.6 157.8 36.7 Dec. 18 1011 +31 53 1.226 0.379 43.1 146.3 47.3 Dec. 19 10 12 +3236 1.225 0.373 42.7 133.2 58.3 Dec. 20 10 13 +33 18 1.225 0.368 42.2 121.4 67.8 a Phase angle (Sun-asteroid-Earth) [degrees] b Lunar Separation from asteorid [degrees] 96 1999 JD6 S-B June12(R) 22 24 28 A 1999 JD6 'tdr" 30 "'------'------'------'-_'--.L----'------L------'------J'---L----'------'------'-_'--~"__'J -4 -2 0 2 4 Time Radius (arcsec) Figure 2.15 Surface-Brightness profile (R-filter) of 1999 JD6. While B-V and V-R color variations were detected on this asteroid, no coma is present. and gain was 15e-/ ADU. The slit was oriented to match with the parallactic angle, to account for differential refraction in the atmosphere. The standard star HIP 394 was used for Hidalgo, and is a G2V star so that it can be used for the dual purpose of telluric correction of atmospheric absorption as well as removal of the reflected solar spectrum. Reduction was applied using the Spextool package written by Cushing et al. (2004), which effectively removes telluric absorption features. The relative reflectance spectrum displays the spectral gradient and absorption features of the object, relative to the Sun. Fig. 2.16 displays the spectrum of (944) Hidalgo reduced using Spextool. A slightly different method of spectral reduction was also applied. This involved a combination of IRAF and IDL routines, which included flat-fielding, combining data frames to increase signal, and subsequent extraction of the spectrum using lRAF routines. After all spectra were extracted, they were then run through an IDL routine that performs 97 .. 1 1.5 2 2.5 Wavelength (microns) Figure 2.16 Spextool-reduced spectrum of (944) Hidalgo on 20 November 2005, at 08:00UT. Parts of the spectrum were removed due to residual atmospheric absorption still present at 1.4 and 1.9J.lm. Error bars (included with the data points) are smaller than the data points. subpixel shifts to maximize the wavelength match of the object and standard spectra, while simultaneously making telluric correction using the ATRAN model atmosphere (Lord 1992). This model depends on three parameters: elevation, precipitable water, and zenith angle. Elevation and zenith angle are independent variables, while precipitable water is varied until the best fit is obtained. From these values the atmospheric contribution is removed from each object and standard star frame. Consequently, all standard stars throughout the night can be utilized for calibration and removal of solar spectrum. Fig. 2.17 displays the ATRAN-reduced spectrum of (944) Hidalgo. As shown in the figure, this method more effectively removes atmospheric contribution at 1.4J.lm and 1.9J.lm. The spectrum of Hidalgo is unusually curved, as most D-type spectra are flat and slightly reddened (e.g., Emery & Brown 2004). 98 ~ 1.5 c 1.5 2 2.5 Wavelength (microns) Figure 2.17 ATRAN-reduced spectrum of (944) Hidalgo on 20 November 2004, at 08:00UT. Error bars (included with the data points) are smaller than the data points. (944) Hidalgo was additionally observed with NIRSPEC at the Keck II telescope atop Mauna Kea on 24 December 2004. NIRSPEC has a plate scale of 0.18" /pix, read noise of 25e-(rms), and gain of 5e-/ADU. Hidalgo was not the target object of this night, but was rather simply used as a test to ensure the capability of the tracking system for the telescope. The REDSPEC reduction routine written by L. Prato of UCLA was utilized to flatten, remove cosmic rays, and wavelength calibrate the spectra. Unfortunately, as shown in Fig. 2.18, Hidalgo:s low albedo of 0.06 and position at perihelion caused much thermal emission beginning at 3.05J.lm. Modeled thermal emission is overlaid in Fig. 2.18. Similarly, at UKIRT 1.4-2.5J.lm spectra were also obtained for comparison objects 1998 XB and (757) Portland on 17-20 December 2004, with the near-IR spectrograph UIST. UIST has a plate scale of 0.12" /pix, read noise of 40e-, and gain of 15e-/ ADU. The objects were observed in the last few hours of each night, after (3200) Phaethon had set. The spectra 99 4 '--''''>--'-r_H-,--id"'"T'"a_l¥-g_0-r-N_1,.-R_S.,-P_E,-C---.--2--,4D-----,€c_'-,'0_4-,--, I ~ 2 d «l ....o • _ ..... ~ """",•.'t..,.··~ • .. Q) ~ 0 Q) "N .-< «l -8 .g'"' -2 • (944) Hidalgo • CK Chondrite 3 3.2 3.4 3.6 Wavelength (micron) Figure 2.18 REDSPEC-reduced spectrum of(944) Hidalgo on 24 December 2004, normalized to 3.3j.£m. Located in the lower left-hand corner is a symbol displaying the average error in individual data points. Overplotted is a modeled thermal emission spectrum of the asteroid. Plotted in the upper left-hand corner is a symbol depicting the average error in the data points. were reduced with the same IRAF routines aforementioned in Chapter 1, as was done for (3200) Phaethon. 1998 XB is a reported S-type Aten asteroid by Binzel et aI. (2004)with a perihelion q=0.590AU. The resultant spectrum displayed in Fig. 2.19 show that the spectrum of 1998 XB has a 2.0j.£m pyroxene ab orption feature common of many S-type asteroids. (757) Portland is an M-type with a relatively short main belt period of 6.58hr. The spectrum obtained for this object (Fig. 2.20) is typical of most M-type asteroids in the near-IR (Harderson et aI. 2005). Atmospheric absorption at 2.0j.'m persisted due to transient cirrus, and was removed from the spectra for clarity. 100 1998 XB UIST 041217 1.4 I (l) () ~ ...., 1.2 () ~ ...... (l) 0:: 'd (l) N .-'@ S ~ 0.8 Z 0.6 1.6 1.8 2 2.2 2.4 Wavelength (microns) Figure 2.19 Spectrum of NEA 1998 XB, which displays a 2.0jtm pyroxene absorption feature characteristic of S-type asteroids. Spectrum is normalized to 1.6jtm. Persistent atmospheric contamination at 1.9jtm has been removed for clarity. Plotted in the upper left-hand corner is a symbol depicting the average error in the data points. 2.4.1 Spectroscopy Analysis The curvature from O.8-2.5jtm in (944) Hidalgo's spectrum is unusual for D-type asteroids. The red slope could be explained by multiple factors, such as carbon graphite. However, graphite has a linear slope from O.8-2.5jtm. Magnetite does have a curved and reddened slope from 1.2-2.5jtm, but it also has a distinct absorption band at 1.1jtm, and is blue in the optical. Hidalgo does not display this near-IR magnetite absorption feature. An attempt was made to remove Hidalgo's thermal emission using IDL software written by Y. Fernandez. The code utilizes the Near Earth Asteroid Thermal Model (NEATM) of Harris (1998). Fig. 2.21 shows the spectrum of Hidalgo with thermal component removed, along with a modeled reflectance spectrum, assuming that thermal properties 101 (757) Portland UIST 041217 1.4 I Q) u ~ ...., 1.2 u .....~ Q) 0:: "0 1~ Q) N .~ til 6 8 0.8 z 0.6 1.6 1.8 2 2.2 2.4 Wavelength (microns) Figure 2.20 Spectrum of main-belt asteroid (757) Portland. Spectrum is normalized to 1.6f.Lm, with atmospheric absorption again removed at 1.9 ""m. Plotted in the upper left hand corner is a symbol depicting the average error in the data points. of the asteroid mimic that of other main belt asteroids (i.e., beaming parameter 1]=0.756). The discontinuity between the modeled reflectance and the thermal-subtracted spectrum is different from the matched results in Chapter 1 (Fig. 1.27) for (3200) Phaethon. Fig. 1.28 revealed that the modeled reflectance fit quite well to the actual data. In the case for Hidalgo, the discrepancy may be derived from the fact that we used the NEATM as opposed to the Standard Thermal Model (STM). The STM is better suited for main-belt asteroids whose surface properties are dominated by the presence of extensive regoliths of small particle size, which decrease the overall thermal inertia and can increase the thermal emission. Near-Earth asteroids have relatively less regolith, and consequently have higher thermal inertias. The NEATM is better suited to these bodies with higher thermal inertia. 102 (944) Hidalgo NIRSPEC 24Dec. '04 4r--,-'----,---+----.,r_-r--¥-__r__,~_r____.___r__,r__r__,__,_____, I ~ 2 c ...,~ o ....~ Q) 0:: 0 "0 Q) .~ ~ 6 '"'o z -2 A (944) Hidalgo • CK Chondrite 3 3.2 3.4 3.6 Wavelength (micron) Figure 2.21 Thermal-subtracted spectrum of (944) Hidalgo. Overlaid is a reflectance spectrum calculated via the NEATM software. The discontinuity between the two fits may be due to the attempt to use a near-Earth asteroid modeling routine for a main-belt asteroid. Plotted in the upper left-hand corner is a symbol depicting the average error in the data points. (944) Hidalgo's spectrum was compared to the spectrum of CK chondrites, shown in Fig. 2.22. There appears to be spectral similarity for the two spectra from 3.0-3.4jJ.m, although there is more curvature in Hidalgo's spectrum from 2.9-3.4jJ.m. The signal from 3.4-3.7jJ.m is too dominated by thermal emission for an analysis to be made. 1998 XB displays the only detected absorption feature in the sample of objects of this investigation. Its absorption at 2.0jJ.m may have been broadened and shallowed by thermal processing and space weathering in its orbit near the Sun. (757) Portland has no unusual or unexpected absorption features, although time-resolved spectra were not obtained for this fast main-belt rotator. The only known absorption feature for M-type asteroids exists at 0.9jJ.m and is due to orthopyroxenes (Hardersen et at. 2005). 103 (944) HidalgojCK Chondrite 4.-----.--r'---.---f-..--r---¥---T----,--..--r----r---.----.-.,.--, I .... . • (944) Hidalgo • CK Chondrite 3 3.2 3.4 3.6 Wavelength (micron) Figure 2.22 Thermal-subtracted spectrum of Hidalgo, overplotted with a CK chondrite spectrum. Although there appears to be similarity between the two spectra from 2.9-3.4~m, the curvature of Hidalgo's spectrum is more pronounced at all wavelengths. Plotted in the upper left-hand corner is a symbol depicting the average error in the data points. 2.5 Conclusions Analysis of these comparison objects have helped constrain the relationship between (3200) Phaethon and F-type asteroids, Aten NEAs that approach the Sun at small perihelion distances, dynamically cometary asteroids, and even low-activity comets such as 49P/ Arend-Rigaux: No direct correlations have yet been found between Phaethon's color variations and F-type asteroids, due to the null results for color variation on (142) Polana. The lack of color variation may simply be evidence of a different evolutionary history on the surface of (142) Polana. It does seem that (142) Polana is more akin to B-type asteroids than F-types. The spectrum shown in Cellino et al. (2001) also di plays evidence of UV ab orption although the authors continued to refer to Polana 104 as an F-type. 49P/ Arend-Rigaux, a low-activity comet (0.08% active fraction; A'Hearn 1988), may have exhibited sufficient coma to mask the true colors on its surface. Aten asteroid 1999 JD6 shows interesting color variation that is correlated with the bluer colors at the maximum of its lightcurve. Conversely, redder colors are associated with brightest fraction of (3200) Phaethon's surface. In both cases, optical color variation is correlated with the lightcurves of both asteroids. 1998 XB displayed a pyroxene absorption feature at 2.0j.tm, which is characteristic of many S-type asteroids. Neither (757) Portland nor 1997 SEs revealed any properties unique from their respective taxonomic classifications. (944) Hidalgo displayed near-IR and L-band spectra with unexpected curvature, as D type asteroids (e.g., Hicks 2000) usually display flat, reddened and featureless spectra. The cause of this curvature could be caused by magnetite that has mixed with a carbon rich substance. Carbon is effective at removing detectible absorption features, such as the 1.1j.tm feature of magnetite. Further analysis of other meteorite spectra may lead to other possible mixtures for (944) Hidalgo. The color difference between Hidalgo and Phaethon, besides compositional variation, may also be due to the locations of the two asteroids. While Phaethon's surface temperature approaches 750K at perihelion, Hidalgo's temperature barely reaches 200K. Higher temperatures evidently can remove mineralogical and volatile absorption features (e.g. Carvano et al. 2003) in the optical and near-infrared, and this may be represented in the spectrum of (3200) Phaethon. Temperature at different orbital distances may be quite an effective factor in preserving or altering the spectral properties of minor planets in the Solar System. 2.6 References A'Hearn, M.F. (1988), Annu. Rev. 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Tucson: Univ. of Arizona Press, p. 669-686. Wisniewski, W.Z. (1990), Icarus, 86, 52-57. Wisniewski, W.Z., Michalowki, T.M., Harris, A.W., McMillan, RS. (1997),Icarus, 126, 395-449. 110 Chapter 3 Conclusions 3.1 (3200) Phaethon: an unusual object This investigation reveals that (3200) Phaethon does indeed have unique surface properties, as evidenced by periodic color variation in both the optical and near-infrared. The data from this program display an anti-correlation between the optical and near-IR color changes, which may be due to a combination of particle size effects and compositional variation. If a regolith is present on the surface of Phaethon, it must be somehow cemented to the surface, due to the high rotation rate of this asteroid. Weidenschilling (1997) described how comets experienced radial migration in the solar nebula; consequently, their surfaces should exhibit diverse compositions. If the color variation on Phaethon is due to composition, then these results may be the clearest physical evidence of the association of Phaethon with cometary bodies. However, the small amount of color variation (~m=0.035+0.005) detected may be more indicative of space weathering or particle size variations than a heterogeneous surface. The average BVRI colors of (3200) Phaethon are predominantly unique, relative to the other comparison objects used in this investigation. Fig. 3.1 displays the B-V VS. V-R and V-R VS. R-I plots of (3200) Phaethon, the comparison objects, and other C,S,M and D-type asteroids taken from Zellner et ai. (1985) (data originally plotted in Kron-Cousins magnitudes in Yoshida et ai. 2004). The B-V VS. V-R plot shows that Phaethon's colors at shorter wavelengths are unique from all asteroids, even F-type (142) Polana, whose B-V III VB. V-R is comparable with other C-types. However, the V-R VB. R-I colors of Phaethon are almost identical to Polana's V-R VB. R-I. The sharp blue colors on (3200) Phaethon are more pronounced at shorter wavelengths, which may be due to particle size effects. Preliminary phase reddening measurements verify the presence of a regolith on Phaethon. At longer wavelengths, this blueness shallows and approaches a solar color at 1.8-2.0~m. The presence of phase reddening thus far detected on Phaethon confirms the presence of some form of a particulate regolith on the surface of Phaethon. 06 • 0,9 > Cl: 0.5 ~ 0,6 I > 04 07 .. 06 0.3 )I (3200) PhaelhOO 0,3 0.4 05 0.8 07 0.2 0,3 0.4 0.5 0.6 V-R R-I Figure 3.1 B-V VB. V-R and V-R VB. R-I colors of (3200) Phaethon, comparison objects, and other asteroids, from Yoshida et al. (2004), as adapted from results of the ECAS survey. Solid triangles refer to C-types, asterisks refer to S-types, open squares refer to D-types and open stars refer to M-types. Phaethon is shown in blue, while the other comparison objects analyzed in this project are shown in red. (142) Polana's colors fall within the realm of bluer C-type asteroids, which is natural, since F- and B-types are (bluer) sub-classes of C-type asteroids. While K-type asteroids are not explicitly displayed on these plots, they are closely related to S-types in their visible colors. However, 1999 JD6 (a K-type; Szabo et al. 2001) appears to share more of a resemblance with M-type asteroids. A previous scenario was proposed that 1999 JD6 may be a T-type, due to its steady reddening beyond 0.7~m. M-types similarly share a steady (though shallower) reddening beyond 0.7~m. Ultimately, 1999 JD6 may taxonomically 112 closer to a T- or M-type asteroid. The color variation detected on 1999 JD6 may be evidence of space weathering, cratering or thermal processing from the Sun. The relationship between (3200) Phaethon and cometary nuclei may be complicated by the thermal processing that has occurred on this object. Cometary nuclei are known to be spectrally neutral or slightly reddened, whereas Phaethon exhibits quite blue colors (Hartmann et at. 1987). Cometary nuclei usually also have low geometric albedos (Pv<0.05), while Phaethon has an albedo of 0.11 (Green et at. 1985). There are a couple known comets with comparable albedos, such as 59P/Kearns-Kwee (Pv=O.lO) and 75P/Kohoutek (Pv=0.09; Hartmann et al. 1987). The blueness of Phaethon, unique from cometary nuclei and asteroids in general, may be a consequence of its elliptical orbit and small perihelion distance that brings it through a large variation of temperatures. The blueness may also be a natural feature ofF-type asteroids that have blue, featureless spectral shapes. (704) Interamnia, another F-type main belt asteroid, also has a blue, featureless optical slope (Binzel 2004). Comets, on the other hand, are conversely slightly redder than solar colors. The cause of this color in comets is due to the organic compounds present on their surfaces. If Phaethon is an extinct nucleus, its color is unique from colors of cometary nuclei. The effects of color variation and bluer colors were similarly detected on Aten asteroid 1999 JD6. While the presence of water ice may make spectra bluer, it is quite improbable that asteroids close to the Sun would still have this ice on their surfaces. (3200) Phaethon's color variation, similar to the case of 1999 JD6 , is correlated with its rotational lightcurve. The anti-correlation seen in (3200) Phaethon's photometric V Rand near-IR spectral variations may be similar to the "spectral reversal" detected on (4) Vesta (Hendrix et at. 2003). A combination of time-resolved UV and visible spectra on Vesta revealed that bluer colors in the UV were registered as redder colors in the optical. These findings were interpreted as evidence of a regolith that displays particle-size or shape dependencies at shorter wavelengths, but conversely display colors diagnostic of composition at longer wavelengths. Space weathering via bombardment of micrometeorites and solar wind particles was invoked as a mechanism that darkens and reddens the surface 113 of Vesta. The micrometeorites were hypothesized to be particles primarily composed of iron. Addition of these small, iron-rich particles, while darkening and reddening the visible colors, can cause the UV colors to become bluer. While a converse trend in color also occurs between the optical and near-IR for Phaethon, optically reddened areas are actually the brighter areas of the asteroid. Due to a lack of absorption features, Phaethon likely does not have much of an iron-rich composition, but more ofa carbon-black amorphous surface (see Chapter 1, Discussion & Conclusions). This carbon-black, if contaminated with occasional hydrocarbons, could become redder than in pure form. However, it is more probable that the hydrogen on the surface has been driven off by heating at perihelion. Indeed, Lahaye & Prado (1978) describe how carbon-black can be formed by condensation and thermal processing of gaseous hydrocarbons. If (3200) Phaethon did at one time contain gaseous hydrocarbons, it most likely also had a coma. 'fransparent silicates may also offer a solution to the reddening and higher albedo of Phaethon. 'fransparent silicates such as aubrite meteorites contain low concentrations of pyroxenes and olivines (Cloutis & Gaffey 1993). Aubrite has a fairly neutral near-IR spectral slope, as well as one of its proposed constituents, troilite (FeS). 'froilite contributes to the high albedo of the aubrite mineral as well as its slightly reddened spectrum. 'froilite and aubrite are proposed constituents of high-albedo E-type asteroids, which also have fairly flat (solar) spectral slopes (Fornasier & Lazzarin 2001, Clark et at. 2004). Neither aubrite nor troilite exhibit absorption bands in their optical or near-IR spectra. The anti-correlation between the time-resolved optical and near-IR colors may be due to a combination of particle size variations mixed with aubrite and associated transparent, low-pyroxene compounds. If the near-IR color variation is more indicative of composition, then the optical color variation must be more indicative of particle size. The cause for reddened spectra of finer-grained regoliths arises from pyroxenes and olivines trapped within chondrule grains (Carvano et at. 2003, Clark et at. 1992, Gradie & Veverka 1986, Johnson & Fanale 1973). This assumption is contrary to Mie scattering, which would assume that smaller particles would scatter to bluer wavelengths. 114 While Mie scattering is an idealized version of scattering where there is no coherent backscattering from particles in a regolith, it may be useful to introduce this as a simplified theory for an analysis of Phaethon's surface. Mie scattering, as discussed in Chapter 1, causes particles smaller than an observed wavelength to reflect blue, while particles larger than an observed wavelength reflect red (Jewitt & Meech 1988). Given this tendency, we can surmise two scenarios that may be causing the reddened peak in the optical V-R colors and bluest trough in the near-IR colors, both at 35% rotation. One situation may be that chemically red (iron or silicate) particles of size 0.6-0.7j,Lm are present in a certain region of the surface, which causes the optical color to become redder, but near-IR color to become bluer. Conversely, another situation may present chemically blue (carbon-black) particles of size 1.0-1.5j,Lm, which would appear red in the optical (due to particle scattering) but blue in the near-IR. Another cause of the optical and near-IR anti-correlation may be a simple variation in composition between carbon black and graphite. Carbon-black is a disordered, amorphous form of graphite, although spectrally dissimilar. The formation temperatures of each are usually much different, since graphite requires much higher temperatures than carbon black to form. Laboratory production of graphite usually use temperatures of 2000 3000°C. However, work by Buseck and Bo-Jun (1985) reveal that the initial stages of graphite formation (from an amorphous carbon state) can occur at 300-500°C, or 500-750K. This "cooler" regime is similar to the approximate temperature of Phaethon's surface at perihelion (0.14AU). For every perihelion passage, carbon-black could be slowly crystallizing to make the surface of Phaethon appear redder with time. As shown by Fig. 3.2, graphite's overall redder color than carbon-black could help explain the existence of color variation on (3200) Phaethon. Also overplotted is the average near-IR spectrum of (3200) Phaethon, taken from the SpeX data, as well as the broadband photometric BVRI data points in the optical wavelength regime. Graphite has a redder spectral slope than carbon-black (and Phaethon) at all wavelengths from 0.4-2.5j,Lm, which could explain the reddening that occurs on Phaethon in either the optical or the near-IR. 115 ~ 1.5 s::: Cll ....o 1lI ;;:: 1lI ~ 1 "tl 1lI .!::l (ij e 1.0o z 0.5 .. Carbon- Black .. Graphite .. (3200) Phaethon 0.5 1 1.5 2 2.5 Wavelength (microns) Figure 3.2 Carbon and Graphite 0.4-2.5~m spectra, normalized to 1.0~m. Also overplotted are the near-IR spectrum of (3200) Phaethon and its broadband data points from the BVRl photometry. In the lower left-hand corner is a symbol displaying the average error of the Phaethon data. Color variation on (3200) Phaethon is most likely a result of space weathering, which in turn could produce compositional variation and particle size effects (particle scattering). While space weathering traditionally involves S-type asteroids becoming redder and darker with time due to UV irradiation and micrometeorite bombardment of iron-rich particles, C-type asteroids are actually found to become bluer as they age (Nesvorny et aI. 2005). If this trend is applicable to F-types (a sub-class of C-types), this may help to explain the correlation of reddest color seen on the brightest maximum of (3200) Phaethon's optical lightcurve. While S-types normally have brighter and/or bluer surfaces associated with younger, fresher material, it may be that C/F-types have younger, fresher material on the 116 reddest fractions of their surfaces. These redder patches on C- or F-types may correspond to the fresh "blue" craters detected on S-types such as Ida. Could the brightest, red patch on the surface of (3200) Phaethon be evidence of a crater from impact with another asteroid? Could the Geminids be the result of this impact? Dynamically this scenario is improbable, unless the impact occurred at perihelion (0.14AU), where all orbits of all the Geminids intersect. The Geminids are of significantly denser matter than typical meteoroid showers, and without a coma detected on Phaethon, it remains possible that this asteroid is simply the remnant body of a collision with another asteroid. However, as discussed in the Introduction, this is dynamically very unlikely. The objective of this thesis was to understand whether the unique object (3200) Phaethon had ever been a cometary object in its history. While a direct answer to this question was hoped for through this investigation, the mystery of Phaethon's true origin remains. New questions arose while some were answered in this program. Table 3.1 displays the pros and cons of Phaethon's properties that argue for or against its cometary origin. While its blue color (B-V=0.587) is unusual when compared against most slightly reddened cometary nuclei, the spectral similarity with cometary object 107P/Wilson-Harrington shows that blue color does not exclude Phaethon's cometary candidacy. While a collision appears unlikely for the production of the Geminids, the ejection of these objects may have been caused by a unique form of space weathering on Phaethon. Space weathering may be enhanced for objects with orbits closer to the Sun. Phaethon's parent body relationship with the Geminids, which appears strongly correlated with cometary ejection vice asteroidal collision, cannot be ignored. It is the conclusion of this thesis that (3200) Phaethon may have been a "transition" object between asteroids and comets, and once contained substantial amounts of both volatiles and rocky debris in its composition. The release of these volatiles would have allowed for the ejection of the Geminids, possibly from the fresh, currently reddened part of the surface detected in rotationally resolved colors. While the volatiles are now gone on Phaethon, its rocky core still remains. Whether there are other transition objects in the outer Solar System remains 117 Table 3.1 Asteroidal vs. Cometary Origin of (3200) Phaethon Asteroidal Properties TJ=4.5 High thermal inertiaa Lack of detectable comab Geminid meteors resistant to crumblingC Cometary Properties Parent body of meteor streamd Fanned distribution of Geminid apheliae High eccentricity (0.89) and moderate inclination (22.6°) Observed visible & near-IR color variation! Spectral similarity with carbon-black9 a Green et al. 1985. b Hsieh & Jewitt (2005), this work. C Halliday (1988) d Hunt et al. (1986) e This work. ! This work. 9 This work. unknown, but as evidenced by the results of main belt object 133PlEIst-Pizarro (Hsieh et al. 2004), it is worthwhile to continue the investigation. 3.2 References Binzel, R.P. (2004), NASA Planetary Data System, EAR-A-I0028-4-SBN0001/SMASSII V1.0: 704INTERAO1. Buseck, P.R., Bo-Jun, H. (1985), Geochimica et Cosmochimia, 49, 2003-1016. 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