Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Planetesimal Accretion & Collisions in the Kuiper Belt Hilke E. Schlichting Canadian Institute for Theoretical Astrophysics KIAA 16th December 2009 16th Dec 2009 Hilke Schlichting (CITA) Part1: Planetesimal Accretion & its effect on Planetary Spins Mass & Angular Momentum is delivered in 3 different phases: Planetesimal Accretion 16th Dec 2009 Giant Impacts Hilke Schlichting (CITA) Post Giant Impact Accretion Final Planet Collisionless accretion (e.g. Lissauer & Kary 91, Dones & Tremaine 93 ) Hill Radius: 1/ 3 MP RH a 3M Sun 200 RP @ 1AU Earth-Moon distance=0.25 RH Schlichting & Sari 2007 - Collisionless accretion for planetesimal sizes > 10m - Semi-collisional accretion for planetesimal sizes < 10m 16th Dec 2009 Hilke Schlichting (CITA) Mean specific angular momentum accreted Retrograde Prograde Schlichting & Sari 2007 16th Dec 2009 Hilke Schlichting (CITA) Implications for Planetary Spins Planetesimal Accretion Giant Impacts Random Component of AM: Maximally spinning protoplanets, prograde rotation 1 1/ 2 Lzrms N crit MR 2 7 Significant amount of mass accreted after giant impacts Systematic Component of AM: 2 LzSpin N 2 / 3crit MR 2 prograde 5 Lz 16th Dec 2009 Post Giant Impact Accretion Hilke Schlichting (CITA) Semi-collisional accretion of a small fraction of the planet’s mass sufficient to alter spin properties 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 16th Dec 2009 Hilke Schlichting (CITA) Johansen & Lacerda 2009 Part2: The Kuiper Belt Size Distribution • ~40 AU from the Sun • Source of the short period comets • ~1000 detected objects 16th Dec 2009 • Solar System debris disk • Size estimates based on brightness – Typical radius R=100km Hilke Schlichting (CITA) The Size Distribution give Information about: - formation process (power law index above the break, q1) - collisional history (location of break radius, rbreak) - material properties of KBOs (power law index below the break, q2) (e.g. Stern 1996, Davis & Farinella 1997, Kenyon & Luu 1999, O’Brien & Greenberg 2003, Pan & Sari 2005) 16th Dec 2009 q1 ~ 4.7 rbreak ~ 45km (Fuentes et al. 2008, Fraser & Kavelaars 2008) (Pan & Sari 2005) Hilke Schlichting (CITA) Transiting Kuiper Belt Objects KBOs < 10km too small to be detected directly Observe indirectly using stellar occultations Occultation events produced by KBOs are rare & of short duration (Dyson 1992, Axelrod et al. 1992) Large number of star hours & high frequency • The Hubble Space Telescope has 3 Fine Guidance Sensors (FGSs) • 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) 16th Dec 2009 Hilke Schlichting (CITA) Transiting Kuiper Belt Objects Fraunhofer Regime: Shape of diffraction pattern does NOT depend on KBO shape and KBO size, Amplitude ~ r2 Fresnel scale a 2 1.3km for a=40 AU, λ=500nm (Roques) 16th Dec 2009 Hilke Schlichting (CITA) Detection Method 30 km/s 4 km/s 1) Search for events by fitting Fraunhofer Light curve template (3 free parameters, velocity, amplitude, mean) 2) For significant events we check 7 km/s - guide stars properties (position & subtend angular size) Opposition - 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 16th Dec 2009 Hilke Schlichting (CITA) Best fit values: r ~ 520 m r amplitude a ~ 45 AU Chi-squared fit: χ2 /dof=20.1/21 From the observations we know: • θStar /θFresnel~ 0.3 • Ecliptic latitude +14 deg • • • • • (Schlichting et al. 2009, Nature) 16th Dec 2009 Hilke Schlichting (CITA) Compared PMT readings Checked second FGS No correlated noise Examined the engineering telemetry for HST Less than 2% chance to be falsepositive Cumulative KBO size distribution q 3.9 0.3 N ( r 250m) 2.114..78 107 deg 2 Rule out inferred KBO surface densities from previous claimed detections (Roques 2006, Chang 2006, 2007) by more than 5σ! 16th Dec 2009 Hilke Schlichting (CITA)