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16th Dec 2009 Hilke Schlichting (CITA)
Planetesimal Accretion & Collisions in the Kuiper Belt
KIAA16th December 2009
Hilke E. Schlichting
Canadian Institute for Theoretical Astrophysics
16th Dec 2009 Hilke Schlichting (CITA)
Mass & Angular Momentum is delivered in 3 different phases:
Planetesimal
Accretion
Giant Impacts
Post Giant Impact
Accretion
Final Plane
t
Part1:
Planetesimal Accretion & its effect on Planetary Spins
16th Dec 2009 Hilke Schlichting (CITA)
Collisionless accretion (e.g. Lissauer & Kary 91, Dones &
Tremaine 93 )
Hill Radius:
Earth-Moon distance=0.25 RH
P
Sun
PH
R
M
MaR
200
3
3/1
@ 1AU
- Collisionless accretion for planetesimal sizes > 10m- Semi-collisional accretion for planetesimal sizes < 10m
Schlichting & Sari 2007
16th Dec 2009 Hilke Schlichting (CITA)
Pro
gra
de
Retr
og
rade
Mean specific angular momentum accreted
Schlichting & Sari 2007
16th Dec 2009 Hilke Schlichting (CITA)
Implications for Planetary Spins
Planetesimal Accretion
Giant Impacts
Post Giant Impact
Accretion22/1
7
1MRNL crit
zrms
23/2
5
2MRNL crit
zSpin
Random Component of AM:
Systematic Component of AM:
prograde
Maximally spinning
protoplanets, prograde rotation
Lz
Significant amount of
mass accreted after giant
impactsSemi-collisional accretion of a
small fraction of the planet’s
mass sufficient to alter spin properties
16th Dec 2009 Hilke Schlichting (CITA)
Semi-collisional accretion might dominate growth of protoplanets and post giant impact accretion.
Preference for prograde rotationSolar 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
16th 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
16th Dec 2009 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)
(Pan & Sari 2005)
(e.g. Stern 1996, Davis & Farinella 1997, Kenyon & Luu 1999, O’Brien & Greenberg 2003, Pan & Sari 2005)
7.4~1qrbreak ~ 45km
(Fuentes et al. 2008, Fraser & Kavelaars 2008)
16th 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 Hz2) long baseline: 14 years3) space based4) 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)
16th Dec 2009 Hilke Schlichting (CITA)
Transiting Kuiper Belt Objects
(Roques)
kma
3.12
scale Fresnel
for a=40 AU, λ=500nm
Fraunhofer Regime:
Shape of diffraction pattern does NOT depend on KBO shape and KBO size, Amplitude ~ r2
16th Dec 2009 Hilke Schlichting (CITA)
Opposition
4 km/s30 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
16th Dec 2009 Hilke Schlichting (CITA)
From the observations we know:
• θStar /θFresnel~ 0.3
• Ecliptic latitude +14 deg
Best fit values: r ~ 520 m a ~ 45 AU
amplituder
Chi-squared fit: χ2 /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)
16th 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!
278.47.1 deg 101.2)250(
3.09.3
mrN
q
