1. 16 th Dec 2009 Hilke Schlichting (CITA) Planetesimal Accretion & Collisions in the Kuiper Belt KIAA 16 th December 2009 Hilke E. Schlichting Canadian Institute for Theoretical Astrophysics

2. 16 th Dec 2009 Hilke Schlichting (CITA) Mass & Angular Momentum is delivered in 3 different phases: Planetesimal Accretion Giant Impacts Post Giant Impact Accretion Final Planet Part1: Planetesimal Accretion & its effect on Planetary Spins

3. 16 th Dec 2009 Hilke Schlichting (CITA) Collisionless accretion (e.g. Lissauer & Kary 91, Dones & Tremaine 93 ) Hill Radius: Earth-Moon distance=0.25 R H @ 1AU - Collisionless accretion for planetesimal sizes > 10m - Semi-collisional accretion for planetesimal sizes < 10m Schlichting & Sari 2007

4. 16 th Dec 2009 Hilke Schlichting (CITA) Prograde Retrograde Mean specific angular momentum accreted Schlichting & Sari 2007

5. 16 th Dec 2009 Hilke Schlichting (CITA) Implications for Planetary Spins Planetesimal Accretion Giant Impacts Post Giant Impact Accretion Random Component of AM: Systematic Component of AM: prograde Maximally spinning protoplanets, prograde rotation LzLz Significant amount of mass accreted after giant impacts Semi-collisional accretion of a small fraction of the planet’s mass sufficient to alter spin properties

6. 16 th Dec 2009 Hilke Schlichting (CITA) Semi-collisional accretion might dominate growth of protoplanets and post giant impact accretion. Preference for prograde rotation Solar System Comparison - Terrestrial planets: - consistent with spin properties of terrestrial planets - Asteroid belt: -Two most massive Asteroids, Ceres (P~9.1h, (90-β) ~ 12º, Thomas et al. 05) and Vesta (P~5.3h, (90-β) ~ 40º, Drummond et al. 98), display prograde rotation. -Even smaller main-belt asteroids (r>150km) display a preference for prograde rotation Johansen & Lacerda 2009

7. 16 th Dec 2009 Hilke Schlichting (CITA) Part2: The Kuiper Belt Size Distribution ~40 AU from the Sun Source of the short period comets ~1000 detected objects Solar System debris disk Size estimates based on brightness – Typical radius R=100km

8. 16 th Dec 2009 Hilke Schlichting (CITA) The Size Distribution give Information about: - formation process (power law index above the break, q 1 ) - collisional history (location of break radius, r break ) - material properties of KBOs (power law index below the break, q 2 ) (Pan & Sari 2005) (e.g. Stern 1996, Davis & Farinella 1997, Kenyon & Luu 1999, O’Brien & Greenberg 2003, Pan & Sari 2005) r break ~ 45km (Fuentes et al. 2008, Fraser & Kavelaars 2008)

9. 16 th Dec 2009 Hilke Schlichting (CITA) Transiting Kuiper Belt Objects KBOs < 10km too small to be detected directly Observe indirectly using stellar occultations (Dyson 1992, Axelrod et al. 1992) Occultation events produced by KBOs are rare & of short duration Large number of star hours & high frequency FGS observations are ideal because of 1) high sampling frequency: 40 Hz 2) long baseline: 14 years 3) space based 4) good control sample Expected number of events 0 (q~3.0) to > 130 (q>4.5) The Hubble Space Telescope has 3 Fine Guidance Sensors (FGSs)

10. 16 th Dec 2009 Hilke Schlichting (CITA) Transiting Kuiper Belt Objects (Roques) for a=40 AU, λ=500nm Fraunhofer Regime: Shape of diffraction pattern does NOT depend on KBO shape and KBO size, Amplitude ~ r 2

11. 16 th Dec 2009 Hilke Schlichting (CITA) Opposition 4 km/s 30 km/s 7 km/s Detection Method 1)Search for events by fitting Fraunhofer Light curve template (3 free parameters, velocity, amplitude, mean) 2)For significant events we check - guide stars properties (position & subtend angular size) - compare velocity of best fit with velocity for observation geometry (assuming e<<1) 3)Compare with event rate in control sample, check for instrumental artifacts

12. 16 th Dec 2009 Hilke Schlichting (CITA) From the observations we know: θ Star /θ Fresnel ~ 0.3 Ecliptic latitude +14 deg Best fit values: r ~ 520 m a ~ 45 AU Chi-squared fit: χ 2 /dof=20.1/21 Compared PMT readings Checked second FGS No correlated noise Examined the engineering telemetry for HST Less than 2% chance to be false- positive (Schlichting et al. 2009, Nature)

13. 16 th Dec 2009 Hilke Schlichting (CITA) Cumulative KBO size distribution Rule out inferred KBO surface densities from previous claimed detections (Roques 2006, Chang 2006, 2007) by more than 5σ!