An HST/ACS Survey of the Kuiper Belt Gary Bernstein (University of Pennsylvania) #ith co-I%s Lynne A&en (UBC), Mike Brown (Cal Tech), Mat' Holman (CfA), Renu Malhotra (LPL), & David Tri&ing (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 A&en, 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 Fa01 Do 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 I&ustration 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 .