1
THE SPATIAL DISTRIBUTION OF COOL DUST IN PLANETARY NEBULAE
1 2
J.L. Hora , L.K. Deutsch
1
Harvard/Smithsonian Center for Astrophysics
2
Boston University
ABSTRACT studies were limited by the low resolution of the IRAS
scans.
Wehave observed a sample of eleven PNe using the
PHT-C100 camera on ISO, obtaining maps at 60 and
2. OBSERVATIONS AND REDUCTION
100 m. The PHT-32 mapping mo de was used, re-
sulting in images of the PNe and nearby sky back-
ground. A preliminary reduction of the data has con-
Wehave observed a sample of eleven PNe see Table
rmed that the radial extent of the co ol dust in gen-
1 using the PHT-C100 camera on ISO, obtaining
eral is similar to the optical nebula. Two of the PN
maps at 60 and 100 m. The PHT-32 mapping mo de
in our sample have b een clearly resolved at 60 and
was used, resulting in images of the PNe and nearby
100 m and several other PN show extended emis-
sky background. Various raster sizes were used from
sion. For example, in the b est-resolved ob ject in our
4x5 to 6x6 , dep ending on the size of the PN. An
sample NGC 6853 { \The Dumbb ell" , the bright
oversampling of 1 was used in most cases, but in some
central emission is oriented at a p osition angle that
cases an oversampling of 2/3 was used due to time
matches the brightest optical emission in the H and
constraints. The sampling parameters were chosen
[O III] lines. The spatial distribution of the far-IR
based on the source and background sky brightness,
emitting dust is consistent with the optical morphol-
and On-Target Times OTTs varied from 1400 to
ogy in all of the PN that were resolved in our data
2000 seconds.
set. This implies that the gas and co ol dust are well-
mixed in these ob jects.
The ISOPHOT data presented in this pap er was re-
duced using PIA version 7.2.2e , which is a joint
developmentby the ESA Astrophysics Division and
the ISOPHOT consortium. The resp onsivities used
Key words: ISO; infrared astronomy; Planetary Neb-
in the PN image data reduction were based on in-
ulae.
terp olated values from the twoFCS1 measurements
done b efore and after the maps at eachwavelength.
Residual glitches present in the data were removed
1. INTRODUCTION interactively for each camera pixel. Final images for
four of the PN are shown b elow in Figures 1-4, as well
as optical and near-IR images of each of the nebulae
taken from the literature. For each gure, the 60 m
The infrared IR emission from most planetary neb-
image is on top and the 100 m image in the center.
ulae PNe is strongest in the far-IR, p eaking in the
The shorter wavelength images at the b ottom were
20 to 60 m region corresp onding to dust temp era-
rotated and scaled to match the ISOPHOT images.
tures of 70 to 150 K. This co ol dust comp onent is
The full dataset is summarized in Table 1 b elow. The
due to a di erent p opulation of grains than those re-
sky background has not b een subtracted from the im-
sp onsible for the near- and mid-IR emission, which
ages in the gures, but the uxes and temp eratures
are from small grains and molecules such as PAHs.
in Table 1 were calculated after subtracting a back-
Most of a PNs energy is emitted by this co ol dust, so
ground estimate based on regions of blank sky in each
knowledge of the co ol dust prop erties, including its
image. For NGC 6853 there is little background area
temp erature and spatial distribution, is essential to
outside the PN so this mayhave lead to an underes-
understand the energetics of the nebula. Also, one
timate of the total PN ux. The temp eratures given
can learn ab out the mass loss history of the star, and
are based on the uxes in the table and an emissivity
the pro cess of dust return to the interstellar medium
1
from the PN. Little is known ab out the prop erties
prop ortional to . Line emission in the 60 and 100
of this co ol dust comp onent, however, b ecause of the
m bands was not considered, whichmay b e adding
diculty asso ciated with observing at these wave-
a nonthermal comp onent to the uxes.
lengths. Observations p erformed prior to the ISO
mission, e.g. from IRAS, found that the FWHM sizes The PN uxes listed in Table 1 are based on the cali-
of the PNe in the far-IR were comparable to the PNs bration provided by the PIA software. The C100 de-
optical size, but showed some evidence for halo emis- tector is known to have several typ es of p eculiarities
sion Hawkins & Zuckerman 1991 . However, these Decin, Blomme, & Runacres 1999 that can a ect
2
the calibration accuracy, and it has b een estimated 3. RESULTS AND DISCUSSION
that the calibration accuracy from this detector is
40 p er cent Schulz et al. 1999 . The uxes in the
As seen in Figures 1{4, the spatial distribution of
table di er widely from the IRAS uxes rep orted for
the far-IR emission is similar to the optical and near-
these ob jects, although there may be di erences in
IR emission. The far-IR emission is present in b oth
the way the background was subtracted and p ossible
the bright ionized regions and in the areas of faint
confusion with other sources b ecause of the larger
nebulosity in these PN. The sections b elow discuss
b eam sizes in the IRAS data. Various improvements
the individual ob jects presented.
will likely b e made in the calibration accuracy in fu-
ture versions of the reduction software when the de-
tector b ehavior is b etter understo o d and mo deled;
therefore, the values in the table should not b e taken
3.1. NGC 6072
as the nal calibration of these data.
The PN NGC 6072 Figure 1 is more compact, but
the PN is slightly extended along the \bar" that ex-
tends from the upp er left to lower right in the opti-
Table 1. Summary of PN Data
cal image. Some evidence can also be seen for far-
IR emission along the axis p erp endicular to the bar
60 m Flux 100 m Flux Temp.
which is a fainter emission lob e seen in the optical
Ob ject Jy Jy K
image; this is most pronounced in the 100 m image.
NGC 40 25.0 11.2 88
There is a region directly to the rightof the source
NGC 6072 7.3 9.7 42
that is of lower brightness than the rest of the back-
NGC 6543 82.8 31.4 109
ground; this is an artifact caused by the detectors
NGC 6720 31.5 31.0 48
when a bright source is observed.
NGC 6826 14.3 6.7 84
NGC 6853 89.8 120 42
NGC 7008 28.8 18.3 64
NGC 7009 24.0 18.2 56
NGC 7026 9.5 15.0 40
NGC 7662 10.7 6.5 66
M 2-9 37.5 22.8 66
Figure 2. NGC 6720 { the top image is 60 m, the
midd le is 100 m, and the bottom is a near-IR image
taken at 2.12 m H , Latter & Hora 1997 .
2
3.2. NGC 6720
In the images of NGC 6720 the \Ring Nebula"; Fig-
ure 2 , the brightest emission in the ISO images is
Figure 1. NGC 6072 { the top image is 60 m, the
from the central region that corresp onds to the bright
midd le is 100 m, and the bottom is an optical image
ring seen in the H image. The far-IR images are
2
H from Zhang 1998 .
more extended in the direction that matches the ma- jor axis of the optical/near-IR ring roughly E-W, or
3
vertical in this image . There is some evidence for due to density enhancements rather than temp era-
far-IR emission from the molecular shells outside of ture changes. There is also some indication of the
the bright ring that extend approximately 3 arcmin fainter lob e emission that extends from the upp er left
in diameter, esp ecially in the 100 m image. This to lower right p ortion of this PN, although the S/N
would imply that this dust is at a lower temp erature is muchlower. The upp er right and lower left corner
than the dust detected in the 60 m image from the regions were used to estimate the background for the
inner ring. ux determination, whichmay not b e completely free
of PN ux.
Figure 3. NGC 6853 { the top image is 60 m, the
midd le is 100 m, and the bottom is an optical im-
Figure 4. NGC 7008 { the top image is 60 m, the
age H , [N II], and [O III] from Manchado et al.
midd le is 100 m, and the bottom is an optical image
1996 .
E plate from the digital sky survey Lasker et al.
1990 .
3.3. NGC 6853
In NGC 6853 Figure 3 , the detailed far-IR structure
also shows a close corresp ondence to the optical im-
3.4. NGC 7008
age. The brightest parts of the 60 and 100 m images
are along the bright \bar" in the optical image that
runs vertically in this orientation, with the bright-
est far-IR sp ot also matching the brightest lo cation In NGC 7008 Figure 4 , the 60 m image matches
in the optical. The \barrel" structure that stretches the optical image, with the brightest emission from
from the lower left to upp er right across the center of the outer parts of the shell. The lower edge of the
the PN in the optical image is also visible in the far- the nebula in the 60 m is brighter than the upp er
IR images. The barrel is the prominent structure edge, similar to the optical image. The 100 m image
in the temp erature image that is where the S/N has approximately the same spatial extent, but has a
was high enough to p erform the calculation . The central p eak rather than showing the full ring struc-
temp erature distribution through the center part of ture. This may b e due in part to the lower resolution
the nebula is fairly at see Section 3.5. b elow , so at 100 m whichwould cause the structure to merge
the intensity variations over this region seem to be into a broad p eak.
4
Figure5. NGC 6853 Color Temperature, determined
from the ISO 60 and 100 m images.
3.5. NGC 6853 Color Temp erature
Figure 5 shows a color temp erature image of NGC
6853, calculated from the 60 and 100 m images. The
calculation was done only on the higher S/N region in
the center of the nebula. An emissivity prop ortional
1
to was assumed for the dust. The temp erature
distribution is fairly uniform across this region of the
PN, varying from 41 to 45K, with a slight enhance-
ment on either side of the bright ridge of the nebula
seen in the optical and far-IR images. There is a hot
sp ot to the right of the PN that may be related to
a ghost image of the brightspotofthe nebula that
is near the center of the image. The background was
not subtracted from this image since it is non-uniform
and will havetobe mo deled carefully to remove its
e ects from the images. Also, the temp erature is un-
certain due to the ux calibration uncertainties dis-
cussed ab ove in Section 2.
ACKNOWLEDGMENTS
This research was funded in part by NASA Grant
NAG 5-3370.
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