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Assessing the Threat of Oort Cloud Comet Showers. Nathan Kaib & Tom Quinn University of Washington. Outline. Long-Period Comet Production Jupiter-Saturn Barrier Simulation Results Comet Shower Probability. Long-Period Comets. X. 25000 AU. Jupiter-Saturn Barrier.
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Assessing the Threat of Oort Cloud Comet Showers Nathan Kaib & Tom Quinn University of Washington
Outline Long-Period Comet Production Jupiter-Saturn Barrier Simulation Results Comet Shower Probability
25000 AU 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
25000 AU Comet Showers • Rare close stellar encounters (< 5000 AU) are able to perturb more tightly bound orbits • The Earth is temporarily exposed to the entire Oort Cloud
Simulations • Initial cloud orbits (~106) drawn from recent OC formation simulation results (Kaib & Quinn, accepted) • Modify SWIFT (Levison & Duncan, 1994) with time-reversible adaptive timestepping routine (Kaib & Quinn, accepted) • Evolved under influence of Sun, 4 giant planets, Milky Way tide and passing stars • Vary stellar mass, impact parameter, and encounter velocity
25,000 AU 4 AU M* = MSun v = 20 km/s, Dmin = 3000 AU Dt = 105 yrs
Calculate years for normal LPC flux to produce comets Quantifying Shower Strength M* = 0.8 MSun v = 20 km/s Dmin = 1300 AU LPC defined as q < 5 AU
Simulation Results v = 20 km/s
Use impulse approximation to calculate DvSun for each stellar passage: DvSun = (2GM*)/(bv)
Finding Shower Frequency • Use Rickman et al. (accepted) stellar encounter code to generate ~106 passages • Find dN(DvSun)/dt
Conclusions • Comet shower intensity for a given Oort Cloud is basically governed by one parameter: DvSun • Showers releasing at least 10 Myrs of LPCs occur every 100 Myrs • Showers releasing at least 40 Myrs occur every Gyr
Conclusions II • Transforming results to an impact rate is HARD: Need to know real current flux of LPCs • Weissman (2007) estimates 1 background LPC impact every 38 Myrs • Implies 1 shower-induced impact every Gyr • Conservative estimate: we use a minimum inner Oort Cloud population
Divide LPC distribution by Oort Cloud distribution Probability of LPC as a function of a
Effects of Solar Formation Setting Inner Oort Cloud population very sensitive to formation environment of Sun (Fernadez & Brunini, 2000; Brasser et al., 2006; Kaib & Quinn, accepted)
Dt = 50 yrs Dt = 10 yrs Dt = 200 days r = 300 AU r = 400 AU Not Symplectic: q drifts by ~0.4 AU over sim
The tide of the Milky Way also perturbs the Oort Cloud (COBE, NASA)
About 2x as powerful as stellar passages (Heisler & Tremaine 1986) Galactic tide causes perihelion and inclination to oscillate