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Assessing the Threat of Oort Cloud Comet Showers

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

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  1. Assessing the Threat of Oort Cloud Comet Showers Nathan Kaib & Tom Quinn University of Washington

  2. Outline Long-Period Comet Production Jupiter-Saturn Barrier Simulation Results Comet Shower Probability

  3. Long-Period Comets

  4. X

  5. 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

  6. 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

  7. 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

  8. 25,000 AU 4 AU M* = MSun v = 20 km/s, Dmin = 3000 AU Dt = 105 yrs

  9. 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

  10. Simulation Results v = 20 km/s

  11. Use impulse approximation to calculate DvSun for each stellar passage: DvSun = (2GM*)/(bv)

  12. One parameter controls shower strength

  13. Finding Shower Frequency • Use Rickman et al. (accepted) stellar encounter code to generate ~106 passages • Find dN(DvSun)/dt

  14. 1/t ~ (DvSun)-2(Rickman et al., accepted)

  15. 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

  16. 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

  17. Shower Semimajor Axes

  18. Divide LPC distribution by Oort Cloud distribution  Probability of LPC as a function of a

  19. Regions Sampled by LPCs

  20. 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)

  21. 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

  22. Building an Oort Cloud

  23. The tide of the Milky Way also perturbs the Oort Cloud (COBE, NASA)

  24. About 2x as powerful as stellar passages (Heisler & Tremaine 1986) Galactic tide causes perihelion and inclination to oscillate

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