Oort Cloud Formation — The Role of the Sun’s Birth Cluster

Hal LevisonSouthwest Research Institute

Boulder, COUSA

Collaborators: Martin Duncan, Ramon Brasser, and Dave Kaufmann

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Oort Cloud Formation — The Standard View

I Even in 1950, Oort realized that comets must have come from the

proto-planetary disk.

I Current thinking comes from Duncan, Quinn, & Tremaine (1987):

From Dones et al. (2004)

The Scattered Disk

I These models produce both the Oort cloud and Scattered Disk.

But, there is a major problem:

I Formation models say the OC/SD ratio should be 10:1.

I Long-period comets come from OC ⇒∼4×1011. (Francis 2008)

I Jupiter-family comets from SD ⇒∼6×108. (Duncan & Levison 1997)

I So, observed ratio is 700:1!

Capturing the Oort Cloud from Other Stars

I Need star clusters because densities are high and velocities small.

I Most (All?) stars form in (embedded) clusters.

I Clusters where stars form typically:I Contain ∼10 — ∼ 1000 stars.I Between 70% and 90% of mass is in gas.

I But, gas goes after a few Myr by supernovae and strong stellar winds.I Very fast once these processes turn on (i.e. when big stars get old).I Causes cluster to explode.

I Radii between ∼0.1 and ∼1 pc. Not that different from the size ofthe Oort cloud.

OortCloud

Capturing the Oort Cloud from Other Stars (cont)

The simulations:

I We performed N-body simulations of stars and comets in a cluster.I Cluster:

I 30 < N < 300 stars.I 0.3 < rc < 1 pc.I Star mass: 10% – 33% of total.

I We placed a scattered disk around each star.I 1000 < a < 5000AU.I q = 30AU.

I Comets get trapped mainly by Random Capture:I Since the Oort cloud is ∼ the size of the cluster, any comet can be

trapped by any star.I They just have to head off in the same direction.

Can this explain the Oort cloud?

I Calculations only tell us what fraction of a typical SD is captured.I We have no clue what this means.

I So, turn the problem around.I Use the observed SD to estimate the indigenous OC population,I =⇒ &90% of LPCs come from other stars.

Conclusions

I The standard model says both Oort cloud and Scattered disk are builtfrom same stuff.

I OC/SD should be ∼ 10.

I However, observations say OC/SD ∼ 700.I We can make up this difference if OC was captured from other stars

when Sun was in its cluster.I If true, then most famous comets formed in the disks of other stars!

I This includes Halley and Hale-Bopp.

This talk can be found at www.boulder.swri.edu/∼hal/talks.html.

We thank NASA’s CAN, and Origins and OPR programs for support.

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1. Title

2. Oort Cloud Formation — The Standard View

3. Scattered Disk

4. Capturing the Oort Cloud from Other Stars

5. Capturing the Oort Cloud from Other Stars (cont) - simulations

6. Conclusions