An HST/ACS Survey of the Kuiper Belt

Gary Bernstein (University of Pennsylvania)

ith co-I’s Lynne Aen (UBC), Mike Brown (Cal Tech), Mat Holman (CfA), Renu Malhotra (LPL), & David Triing (Penn➙Steward)

and for Astrometry: N. Zacharias (USNO) & J. A. Smith (LANL)

☀ Background Image: section of summed Funded by GO-9433 grants ACS TNO search-field data. The Kuiper Belt as an accretion and dynamics laboratory

Design and Execution of the ACS Survey

TNO Discoveries & Followup

Implications for Solar System Evolution

Light Curves and Astrometry Planet Formation Scenario:

Theoretical Expectation: Observational Evidence:

Collapse of gas/dust cloud Nebular/dust phases are to form protostar and nebular observed around other stars disk.

Coagulation of grains, runaway gravitational growth of planetesimals Planetesimal phase Oligarchic growth of planetesimals. is observable only in our outer "fossilized" Solar System - the Kuiper Belt.

Gas accretion onto largest bodies, dissipation of gas, ejection of small bodies Giant planets have been Mature planetary system detected around other stars Known Outer Solar-System Bodies (as of 2003 April 19)

Key Plutino Scattered

Other TNO

Centaur

Comet Asteroid Uranus Giant Planet

Neptune Open symbols denote orbits uncertain due to short arcs. Pluto Outer Solar System Objects (as of 4/03)

0.6 Neptune-crossing

5:2 Resonant 2:1 0.4 3:2 y t i c i r t n e c c

E Pluto

0.2

1:1 Uranus (Trojan) Neptune 0 20 30 40 50 60 70 80 90 100 Semimajor Axis (AU) KBO Resonances and Planetary Migratio 3:2 Resonances inNeptune frame(Malhotra 1995)

Pluto and many KBOs (”Plutinos”) are locked in 3:2 resonance with Neptune, crossing Neptune’s orbit yet avoiding ejection.

Malhotra (1993) explains “accident” of Pluto’s resonance by positing an outward migration by ~8 AU of Neptune during the clearing of the planetesimal disk. Resonances “sweep” through the population, acting like phase-space traps. Orbit of 2001 QR322 in Neptune frame(Chiang et al 2003) Prediction is that many KBOs should be found in 3:2 and 2:1 resonances

KBOs have been found in following resonances (Chiang et al ‘03):1:1 (”Trojan”), 5:4, 4:3, 3:2 (”Plutino”), 5:3, 7:4, 2:1, 7:3, 5:2Resonance sweeping can only explain some of these, obviously not 1:1. Outer Solar System Objects (as of 4/03) Centaurs

0.6 Neptune-crossing

5:2 Resonant 2:1 0.4 3:2 y t i c i "Scattered r t

n Disk" e c c

E Pluto

0.2

CAUTION: "Classical" Belt Strong Selection Effects 1:1 Uranus (Trojan) Cold Primordial Disk? Neptune 0 20 30 40 50 60 70 80 90 100 Semimajor Axis (AU) Finding Faint TNOs

4 Recall that distant TNOs have f r− ∝ Traditional response to fainter target is longer integration, but S/N gain for moving objects stops when object trails across sky background. TNOs move few arcsec per hour.

Non-sidereal tracking of telescope only works for one preselected motion vector, not appropriate for a survey

< Ground-based exposures limited to ~10 minutes before trailing, or m 24 R ∼

Digital Tracking: Take short exposures, shift and add digitially to follow any candidate orbit, then run detection algorithm.

Used successfully from ground in 1992 by GMB, with Cochran et al WFPC2 data, many other times since. a) Original Single Exposure (c) Single Exposure, Fixed Objects Subtracted/Masked

R=25.5 R=25.0 R=24.5 R=24.0 R=23.5

R=23.0 C o r w T r r e r a o c c n t k l g y e

d t r r a

a a t c e t k e d

(b) Combined Image (d) Combined Image Track at Sidereal Rate Track at KBO rate Ground-Based Survey of 1.5 Square Degrees Aen, Bernstein, & Malhotra (2001)

Neptune E J S U u a p a ra rt i tu n Pluto h te r u r n s 300 6.5 ) m k (

t 7 c e j b 200 O f H o r e t e old 8 m sh a e i r th D n tio ec ) 100 et 26 d > 9 w (R lo e B

10 11

0 10 20 30 40 50 60 70 Heliocentric Distance (AU) Size Distribution of KBOs

Diameter at 35 AU (km, albedo=0.04) 100 10 105 CLSD/WFPC2 ABM01 ??? For fixed distance, mag GL98/01 104 distribution is the size LJ98 distribution. JLT98 TJL01

e 1000 CB99 e r

g Fa01 e

Faintest two points on this plot d La01

e 100 known to be contaminated by r a u

false positives! q Keck data s

r

e 10 p

)

Data seem consistent with a m < (

single power law, N 1

g o l

0.63(m 23) 0.1 N(< m) = 10 − Number vs Mag (~size) for Trans-Neptunian Objects dN q r− , q = 4.0 0.5 ⇒ dr ∝ ± 0.01 20 22 24 26 28 30 R Magnitude Theoretical Expectations for Size Distributio

Single power law must break down to avoid divergent mass, t = 0 Myr t = 5 Myr surface brightness. t = 25 Myr t = 50 Myr t = 100 Myr t = 130 Myr t = 265 Myr Modelling of collisional t = 1000 Myr accretion and erosion requires detailed numerical study, assumptions about fracturing, dynamics, etc. Model by Davis et al Dohnanyi shows that q=3.5 is a point of steady-state collisional cascade under scale-free Does this size break exist? Kuiper fracturing laws. Expect this Belt is perhaps our only opportunity below some disruption scale. to test an accretion model! Outstanding Kuiper Belt Questions

What is the size (magnitude) distribution of objects?

Are the dynamical classes physically distinct?

What event(s) or process(es) heated the planetesimal disk?

Is there truly a truncation? What caused it?

Are any of the dynamical classes viable source reservoir for the observed 1-10 km Jupiter family comets?

What can we learn about the history of the solar system, and planetary systems in general? Plan for the ACS Survey Six ACS fields, 0.02 square degrees (12x HDF) Observe each field 22 ksec over 5 days Sum flux over 30 exposures in 1014 candidate orbits≈ Wait 5 days; most TNOs pass stationary points and reverse Observe another 15 ksec to confirm discoveries Detection limits: m<29.2 in F606W Expect 85 detections under extrapolation. Diameter at 35 AU (km, albedo=0.04) 100 10 105

ABM01 GL98/01 104 LJ98 Expected ACS Result JLT98

TJL01 ? pe lo e 1000 s CB99 yi e n

r a hn g o Fa01 D e to k d La01 a

e br e 100 r a u q s r e 10 p

) m < (

N 1 g o l

0.1 Number vs Mag (~size) for Trans-Neptunian Objects

0.01 20 22 24 26 28 30 R Magnitude A it takes is

116 orbit Cycle 11 allocation (executed in 125 orbits, thanks Tony Roman!) 30,000 lines of C++ code New Fourier-domain PSF characterization method PSF interpolation across ACS Image registration & distortion (Anderson) Image combining with reduced PSF distortion Image subtraction Digital tracking addition & detection Orbit calculation and trailed-PSF fitting directly to images. Several CPU-years on P4 processors Schedul

Feb 2002: Phase II request

Aug 2002: Preparations begin in earnest

Jan 26 2003: First TNO observation

Feb 09 2003: Last observation

April 14 2003: All candidates identified

April 29 2003: Keck retrieval run (Magellan clouded out)

August 2003: Deep search complete Iustration of ACS Data Quality

180 X Position of 2000 FV53 170 in part of ACS Data 160 ) c

e 150 s c r 140 a (

n

o 2 i t i

s 1 o P 0 -1 ...linear trend removed -2 -3 -60-40 -20020 Time (Hours)

0.4

0.2

0.0

-0.2 predicted parallax from HST orbit at 39 AU -0.4

-70-65 -60-55 -50 TNO Discoveries in the ACS Survey

2000 FV53 2003 BF91 2003 BG91 2003 BH91 m=23.4 m=26.95 m=28.15 m=28.38

Single Exposure 400 seconds S/N=90

Summed Orbit 2000 seconds

Summed Visit 6000 seconds

Faintest solar system All Data objects ever detected! 38,000 seconds 1.5 arcsec 1 photon per 5 S/N=73 S/N=26 S/N=23 seconds at HST Distance: 40.26 AU 42.14 AU 42.55 AU Inclination: 2.5 deg 1.5 deg 2.0 deg Eccentricity: 0.07 ?? ?? Diameter: 44 km 28 km 25 km Analysis of a survey data

1000 TB )

Combine data from all large, well- 2 Larsen g 100 e characterized surveys within 3 D d TJL ( e

10 t a degrees of the invariable plane. 10 e e c

r ABM t i

A 1 o

Gladman n h s c 0.1 Deviation from single power law at r a ACS high confidence. e 0.01 CB S 1 0

) 100

Split the TNO sample into 2 - g

“Classical” KB and “Excited” e 10 d

populations. CKBs defined as: 1 - 1

g All TNO a 0.1 CKB 38AU d 55AU m (

≤ ≤ ) 0.01 Excited m

i < 5◦ ( Σ 0.001

LFs of dynamical classes distinct at 18 20 22 24 26 28 96% confidence. R Magnitude Double-power-law fits to the Magnitude Distributio

Excited 0.6 CKB 1 ) 2

- 0.4 g e ) d

g 1

- CKBO a g 2 a α

m 0.2 m

e ( r

p ) o 3 e l 2 - s

p m

d ( 6 n s . 0 e - s - 0. t 0 a n 1 i

a x

M F ) ( m ( Σ 0.1 -0.2 Excited

-0.4

18 20 22 24 26 28 R Magnitude

0.6 0.8 1.0 1.2 1.4

Bright-end slope α1 What does it mean?

Stern (1995) gives sizes below ~200 km as subject to disruption in current collisional environment: t = 0 Myr t = 5 Myr t = 25 Myr t = 50 Myr t = 100 Myr t = 130 Myr t = 265 Myr t = 1000 Myr

Crudely, this is size of potential break in size distribution. Resembles >1 Gyr phases in Davis simulation - Excited older than CKB Implications

Total mass of the CKB: (for 1 p=0.04, =42 AU, ρ=1000 kg/m3): 0.8 R <

0.010 M 15% ⊕ s ± O N 0.6 2000 KX14 T

Extrapolation of Excited y n A

bright end predicts largest f o

y

object in the Pluto-Quaoar t i Quaoar l i 0.4 b

range. Pluto is uniquely but a b o not anomalously large. r P

0.2 Pluto Note largest CKBO is predicted (and is, so far) 60x lower mass. 0 14 16 18 20 22 R magnitude Are KBOs a viable source reservoir for Jupiter-family comets? One astronomer’s noise is another’s signal Cycle 13 Observations

Additional 6 orbits granted and executed in May 2004 (GO-10268, PI Trilling)

Retrieve the 3 new objects to determine eccentricity - really CKB objects?

Additional astrometry will give sufficient orbital info to locate these objects with JWST in 2010. Recovery of 2003 BG91 Phased photometry of ACS TNOs

1.2 P=4.2 h P=9.1 h 0.6 ) )

c 1 c e e

s >1 mag variation! s / / n n

o 0.4 o r r

t 0.8 t c c e e l l e e ( (

x 0.6 0.2 x u u l l f 2003 BF f 0.4 2003 BG91 91 23 P=2.8 h P=3.79 h ) 22 ) c c

e 0.4 e s 0.07 mag variation! s / / n n

o 21 o r r t t c c e e l l

e 20 e ( (

0.2 x x u u l l f 2000 FV 19 f 2003 BH91 53 0 18 0 0.2 0.4 0.6 0.8 1 0.2 0.4 0.6 0.8 1 phase phase Trilling & Bernstein (submitted to AJ) A Model for 2003 BF91 Is 2000 FV53 a strengthless body?

4

P = photom. period 3.5 Too dense? P = twice photom. period P(θ) 3 Spots? Eros 2.5 )

3 m c / 2 Pluto

g

(

ρ Maclaurin Jacobian 1.5 e: Mathilde ve! Excluded: 1 (Varuna) Oblat Too light? Asymmetric oo 0.5 T no light cur a = b a >> b a ≈ b ≈ c b > c b ≈ c 0 0 0.04 0.08 0.12 0.16 0.2 0.24 0.28 τ ( = K / W ) Rubble-pile solutions

90

density = 0.5 75 density = 1.0 density = 2.0

) 60 s e e epose r g 1 0 e symmetric 45 . . d 0 1

(

= =

φ oo a ) ) θ θ

T ( ( 30 P P Angle of R

15 Acceptable! 0 0 0.2 0.4 0.6 0.8 1 b / a Astrometric Residuals for 2000 FV53

4 ) c e s c r

a 2 i l l i m (

r 0 o r r E

W -2 - E

-4

4 ) c e

s 2 c r a i l l i

m 0 (

r o r r

E -2

S - N -4

0. 0.5 1. 11. 11.5 3. 3.5 13. 13.5 Time (days)

Bernstein, Zacharias, & Smith (in prep) Is miiarcsecond KBO astrometry just a neat trick?

N. Zacharias (USNO): 5 mas tie to ICRS very practical, better over 1-degree patches?

1 mas at 40 AU = 30 km = 5 seconds of HST motion

Detectable photocenter motion on 300-km spotted body

Displacement from undiscovered solar system masses is

2 2 M T D − ∆x = 1 km M 1 yr 100 AU ! ⊕ " ! " ! " Future Developments

Sky to R=21 is mostly covered by Palomar/Quest

NOAO Deep Ecliptic Survey, CFH Legacy survey to expand sky coverage for R<24

Full sky to R=24 will be done by Pan-STARRS, LSST - 10,000’s of objects with full orbital dynamics

Could reach R=26 over limited regions of sky

Some further coverage from ACS archive (PI: Tamblyn) or another Large program.

SNAP could repeat ACS survey to get 10,000’s of R=28 objects, detail dynamics/size correlations.

Occultation surveys (TAOS) explore <10km regime

New Horizons probe flies by 2 (?) KBOs c. 2017 Summary ACS works flawlessly for TNO search. Background- limited detection for 29th mag moving objects.

Clear detection of a feature in the size distribution, with dependence upon dynamical state of subpopulations.

No sign of cold population beyond 50 AU, for D>40 km

Scattered disk a feasible JFC source, but still a stretch?

Suprising light curves, consistent with rubble pile in 200 km body 2000 FV53

Additional info forthcoming from followup & astrometry. Solar System Discovery Space 105 Jupiter Naked Eye (<1610) Saturn Telescopic (1610-1900) Sun Uranus Neptune Photo (1900-1990) 4 10 Venus (1781) (1846) CCD (1990-2002) Mercury New (2003+) Galilean Titan (1655) Mars Moon Moons (1610) Pluto (1930) g in 1000 Ceres (1801) g a

) Pallas(1802) Charon (1978) m Quaoar (2002) I Vesta (1807) e m Sedna c a k p ( Juno (1804) S d s l 1992 QB1 e u i i F 100 Kuiper Belt - d e d a i W R y d Voyager Moons o Phobos 2003 Bx91

B Eros (1978-1989) 10 Deimos (1877) NEA's Space Occultation Survey

1 Pan-STARRS LSST

0.1 0.1 1 10 100 1000 Distance at Discovery (AU)