Using Known Long-Period Comets to Constrain the Inner Oort Cloud and Comet Shower Bombardment Nathan Kaib & Tom Quinn University of Washington

Outline Long-Period Comet Production Inner Oort Cloud Comet Production Constraints from Known LPCs Comet Shower Significance

Long-Period Comets

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Jupiter-Saturn Barrier Comets must have large perihelion shift to make it past Jupiter/Saturn in one orbital period Only weakly bound comets will have large perihelion changes Jupiter/Saturn shield inner solar system from inner 20,000 AU of Oort Cloud AU

Simulations Initial cloud orbits (~10 6 ) drawn from recent OC formation simulation results (Kaib & Quinn, 2008) Oort Cloud model has 1.5:1 inner-to-outer population ratio Modify SWIFT (Levison & Duncan, 1994) with time-reversible adaptive timestepping routine (Kaib & Quinn, 2008) Evolved under influence of Sun, 4 giant planets, Milky Way tide and passing stars for 1.2 Gyrs Analyze LPCs from last 200 Myrs

OC Objects Fates a = 500,000 AU Start x N OC a (AU) Constrained Unconstrained

Inner OC LPCs Similar evolution in SDO simulations (Levison et al., 2006) Start q = 1 AU a = 28,000 AU

Inner OC LPCs N OC a (AU) Constrained Unconstrained

Original Orbits q LPC < 5 AU

Incoming Orbits q LPC < 5 AU

Start Inner OC LPCs N OC a (AU)

Population Constraints Predicted LPC rate: 1/116,000 per Myr Max Observed rate: 10 per yr (everhart, 1967) Predicted population: ~10 12 km-sized bodies between 3,000 and 20,000 AU (assuming n OC ~ r -3.2 ) Not much larger than current outer Oort Cloud population estimates (3-5 x ) Modest inner Oort Cloud can produce observed LPC flux

Comet Showers AU Rare close stellar encounters (< 5000 AU) are able to perturb more tightly bound orbits The Earth is temporarily exposed to the entire Oort Cloud Possible source of mass extinctions seen in fossil record (Hut et al., 1987; Farley et al., 1998)

Comet Shower LPCs N OC a (AU)

M * = M Sun v  = 20 km/s, D min = 3000 AU  t = 10 5 yrs 25,000 AU4 AU

 v Sun = (2GM * )/(bv  ) Relative Shower Strength

1/  ~ (  v Sun ) -2 (Rickman, 2008) 1.5:13:110:1 Relative Shower Strength

Constrained Shower Curve most powerful shower yields 2-3 km-sized impactors (Weissman, 2007)

Rohde & Muller (2005) 3 impacts 2 Myr He 3 spike (Farley et al., 1998)

Conclusions Inner Oort Cloud is a significant and perhaps dominant source of LPCs Current LPC flux gives an estimate of the total Oort Cloud population, not just outer Implies comet showers are not responsible for more than ~1 extinction event

Oort Cloud Mass Problem Outer Oort Cloud traps 1-2% of planetesimals during formation Previous outer Oort Cloud mass estimates implied > 200 M Earth planetesimal disk between 4 and 40 AU (Dones et al., 2004) Inner Oort Cloud can trap more efficiently (5-15%) and still produce observed LPCs (Brasser et al., 2006; Kaib & Quinn, 2008)

Disk Mass Requirements Minimum mass solar nebula  40 M Earth (Dones et al., 2004)

LPC Inclinations 55% Retrograde

LPC semimajor axes Inner a med = 26,000 AU; Outer a med = 35,000 AU Observed a med = 27,000 AU

Compact Inner Clouds ~10 3 M Sun /pc 3 produces Sedna and LPCs (Brasser et al., 2006)

Showers from Alternative Clouds A < 3,000 AU case requires 1200 M Earth disk

Quantifying Shower Strength LPC defined as q < 5 AU M * = 0.8 M Sun v  = 20 km/s D min = 1300 AU

Jupiter-Saturn Barrier Edge Duncan et al. (1987) log  (1/a) q (AU)