using known long-period comets to constrain the inner oort cloud and comet shower bombardment nathan...
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Using Known Long-Period Comets to Constrain the Inner Oort Cloud and Comet Shower Bombardment
Nathan Kaib & Tom QuinnUniversity of Washington
Outline
Long-Period Comet Production
Inner Oort Cloud Comet Production
Constraints from Known LPCs
Comet Shower Significance
Long-Period Comets
X
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
Simulations
• Initial cloud orbits (~106) 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 Fatesa = 500,000 AU
Start
Start
x
NOC
a (AU
)
Constrained
Unconstrained
Inner OC LPCs
Similar evolution in SDO simulations (Levison et al., 2006)
Start
q = 1 AUa = 28,000 AU
Inner OC LPCs
NOC
a (AU
)
Constrained
Unconstrained
Original Orbits
qLPC < 5 AU
Incoming Orbits
qLPC < 5 AU
Start
Inner OC LPCs
NOC
a (AU
)
Population Constraints
• Predicted LPC rate: 1/116,000 per Myr
• Max Observed rate: 10 per yr (everhart, 1967)
• Predicted population: ~1012 km-sized bodies between 3,000 and 20,000 AU (assuming nOC ~ r -3.2)
• Not much larger than current outer Oort Cloud population estimates (3-5 x 1011)
• Modest inner Oort Cloud can produce observed LPC flux
Comet Showers
25000 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
NOC
a (AU
)
M* = MSun v = 20 km/s,
Dmin = 3000 AU t = 105 yrs
25,000 AU 4 AU
vSun = (2GM*)/(bv)
Rel
ativ
e S
ho
wer
Str
eng
th
1/ ~ (vSun)-2 (Rickman, 2008)
1.5:1 3:1 10:1R
elat
ive
Sh
ow
er S
tren
gth
Constrained Shower Curve
most powerful shower yields 2-3 km-sized impactors
(We
issm
an
, 2
00
7)
Rohde & Muller (2005)
3 impacts2 Myr He3 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 MEarth 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 MEarth (Dones et al., 2004)
LPC Inclinations
55% Retrograde
LPC semimajor axes
Inner amed = 26,000 AU; Outer amed = 35,000 AUObserved amed = 27,000 AU
Compact Inner Clouds
~103 MSun/pc3 produces Sedna and LPCs
(Brasser et al., 2006)
Showers from Alternative Clouds
A < 3,000 AU case requires 1200 MEarth disk
Quantifying Shower Strength
LPC defined as q < 5 AU
M* = 0.8 MSun
v = 20 km/sDmin = 1300 AU
Jupiter-Saturn Barrier Edge
Duncan et al. (1987)
log
(1
/a)
q (AU)