Angular Momentum in the Kuiper Belt Scott S. Sheppard Carnegie Institution of Washington Department of Terrestrial Magnetism

Main Asteroid Belt 24 > 200 km Trojan Asteroids 2 ~ 200 km Kuiper Belt 10,000 > 200 km

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

Dynamical Classes in the Outer Solar System Dynamically Disturbed and Collisionally Processed.

Sedna

Plan View of the Kuiper Belt Brightest KBO is 19th magnitude Diameter > 200 km Mag < 22.5

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

Short-term Variability 2000 GN171 1.Albedo 2.Elongation 3. Binary 2000 GN171 period = 7.9 hours

29% > 0.15 mags 18% > 0.40 mags 12% > 0.60 mags KBOs (40 in sample)

1. Albedo effects are usually only 10 to 20% (Degewij et al. 1979)

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

a/b = x delta mag Axis Ratio from rotational light curve: Period and amplitude can be related to an objects density Varuna

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

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)

2001 QG298 Period = hours Range = 1.1 mags Diameter ~ 250 km

2001 QG298Hektor Kleopatra 2001 QG298 is only the 3rd known minor planet with diameter > 50 km and a photometric range > 1 magnitude

Trojan Asteroid 624 Hektor Main Belt Asteroid 216 Kleopatra KBO 2001 QG298

Merline, Dumas and Menard 1999 CFHT Adaptive Optics images of Kleopatra

Sheppard and Jewitt 2004 Comparison of Large Main Belt Asteroids and Kuiper Belt Objects

Margot 2002 Comparison of typical binary systems within the Solar System. 100 km 1 km 20,000 km 1000 km 2.5 km

Funato et al KBO Binary Formation Mechanisms: Tidal Disruption Direct Collisions Three Body Interactions

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.

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)

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.

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

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

1. Nonuniform Surface Markings Iapetus Photometric Range ~ 0.3 mags Photometric Range ~ 2 mags B – V ~ 0.1 mags (Millis 1977) -synchronous rotation -atmosphere

20000 Varuna Rotational Lightcurve (diameter ~ 900 km) Period = hours

20000 Varuna: Found No Color Variation with Rotation

Asteroid and KBO Limiting Densities Sheppard and Jewitt 2002

5 KBOs can not be easily explained from albedo or rotational elongation