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Angular Momentum in the Kuiper Belt Scott S. Sheppard Carnegie Institution of Washington Department of Terrestrial Magnetism
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Main Asteroid Belt 24 > 200 km Trojan Asteroids 2 ~ 200 km Kuiper Belt 10,000 > 200 km
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Gravitational Self Compression > Material Strength Primordial Distribution of Angular Momentum For Diameters > 200 km: - Early Collisional environment Size Comparison of Rocky/Icy Bodies in the Solar System
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Dynamical Classes in the Outer Solar System Dynamically Disturbed and Collisionally Processed.
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Sedna
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Plan View of the Kuiper Belt Brightest KBO is 19th magnitude Diameter > 200 km Mag < 22.5
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Overview of Data Sample of over 40 large KBOs 1) Light curves 2) Phase curves UH 2.2m -> Shapes -> Surface Characteristics -> Densities -> Binaries -> Angular Momenta -> Outgassing du Pont 2.5m Short and Long Term Variations
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Short-term Variability 2000 GN171 1.Albedo 2.Elongation 3. Binary 2000 GN171 period = 7.9 hours
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29% > 0.15 mags 18% > 0.40 mags 12% > 0.60 mags KBOs (40 in sample)
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1. Albedo effects are usually only 10 to 20% (Degewij et al. 1979)
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2. Elongation Rotational Triaxial Ellipsoids (Jacobi Ellipsoids) Fast Rotations < 7 hours For large objects (> 200 km) Spherical Gravitational Compression > Material Strength Triaxial elongation from rotational angular momentum (Leone et al. 1984) P = (3 Pi / G rho) 1/2 crit Centripetal acceleration = gravitational acceleration As angular momentum increases an object will go from being a sphere to biaxial to
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a/b = 10 0.4 x delta mag Axis Ratio from rotational light curve: Period and amplitude can be related to an objects density Varuna
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Jewitt and Sheppard 2002 Varuna Density ~ 1100 kg/m 3 Assume Rotationally distorted Strengthless Rubble Pile Cosmochemically Plausible Rock Fraction ~ 0.5 Porosity ~ 10 to 20% Chandrasekhar 1987 Leone et al. 1984
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1999 TC36 3. Eclipsing Binaries -Probability of eclipse events to our line of sight decreases as the separation increases -Tidal interactions distort close components Photometric Range Max ~ 0.75 mags Photometric Range Max ~ 1.2 mags (Leone et al. 1984) (Trujillo and Brown 2002)
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2001 QG298 Period = 13.7744 hours Range = 1.1 mags Diameter ~ 250 km
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2001 QG298Hektor Kleopatra 2001 QG298 is only the 3rd known minor planet with diameter > 50 km and a photometric range > 1 magnitude
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Trojan Asteroid 624 Hektor Main Belt Asteroid 216 Kleopatra KBO 2001 QG298
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Merline, Dumas and Menard 1999 CFHT Adaptive Optics images of Kleopatra
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Sheppard and Jewitt 2004 Comparison of Large Main Belt Asteroids and Kuiper Belt Objects
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Margot 2002 Comparison of typical binary systems within the Solar System. 100 km 1 km 20,000 km 1000 km 2.5 km
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Funato et al. 2004 KBO Binary Formation Mechanisms: Tidal Disruption Direct Collisions Three Body Interactions
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Known Binaries of Large Minor Planets in the Solar System Does a large angular momentum of the primary correspond to satellite formation? Current angular Momentum of Large objects hints At an earlier denser Kuiper Belt. Maybe 100 times more dense.
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Noll et al. (2002) found about 4% of KBOs were binary with separations > 0.15” We find 5 of 34 KBOs are in the close, similar component, eclipsing binary region (15%) Consistent with Goldreich et al. (2002) model of binary formation but not with the Weidenschilling model (2002) Collisionless interactions In a denser Kuiper Belt During the formation epoch. - Dynamical Friction would create more close in binaries (Because of projection effects, the fraction may be much larger)
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Conclusions - Many Kuiper Belt Objects have large amplitude light curves - Some may be rotationally deformed rubble piles - Many are probably contact or nearly contact binaries - Kuiper Belt must have been about 100 times more dense in the distant past to explain current amount of angular momentum we see. - Binary formation is still unclear, but direct collisions may have be an important factor.
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Short and Long Term Variability Consecutive Nights Multiple Months Absolute Photometry Mag = Msun – 2.5 log(albedo x radius x phase / heliocentric x geocentric ) 2 2 2
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Period = 8.08 hours Period = 4.04 hours 1995 SM55 V-R=0.38 Single-peak Double-peak Binary or Cometary or Complex Rotation? t = u Q / p K r w Damping time scale 23 u is rigidity Q is ratio energy in oscillation to that lost p is the density K is irregularity of body r is the radius w is angular frequency
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1. Nonuniform Surface Markings Iapetus Photometric Range ~ 0.3 mags Photometric Range ~ 2 mags B – V ~ 0.1 mags (Millis 1977) -synchronous rotation -atmosphere
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20000 Varuna Rotational Lightcurve (diameter ~ 900 km) Period = 6.3442 hours
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20000 Varuna: Found No Color Variation with Rotation
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Asteroid and KBO Limiting Densities Sheppard and Jewitt 2002
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5 KBOs can not be easily explained from albedo or rotational elongation
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