DPS Pasadena Meeting 2000, 23-27 October 2000
Session 36. Comets: Orbital Dynamics, Nuclei
Oral, Chairs: H. Weaver, H. Levison, Wednesday, 2000/10/25, 4:00-6:00pm, Little Theater (C107)

## [36.02] Formation of the Oort Cloud Revisited

L. Dones, H. Levison (SwRI), M. Duncan (Queen's Univ.), P. Weissman (JPL)

We have completed our calculations for the standard model'' of the formation of the Oort cloud [OC] of comets. We integrated the orbits of 1775 test particles, initially in low-inclination, low-eccentricity orbits with semi-major axes, a, of 4--40 AU, for times up to 4 billion years. Our simulation included the gravitational effects of the Sun, the four giant planets, galactic tides, and passing stars. The population of the OC peaks at about 1~Gyr, with some 8% of the original 1775 test particles in the OC. At 4~Gyr, 5% of the 1775 particles remain in the OC. Our results suggest that after 1~Gyr, the inner and outer Oort clouds (2,000 < a < 15,000~AU and a > 15,000~AU, respectively) contain nearly equal numbers of comets. At 4 Gyr, 0.6% of the particles reside in the scattered disk (50~AU~< a < 2,000~AU); i.e., its population is {~10% that of the Oort cloud. Assuming an outer OC population of 1012 comets and an average cometary mass of 4 \times 1016~g ([1]), our model results imply that the original mass in planetesimals between 4 and 40~AU was {~250M\oplus, some 4 times the mass in solids in a minimum-mass solar nebula. While this mass is reasonable ([2]), the standard model makes predictions that are not borne out by observation. Specifically, (1) the inferred population of the scattered disk is much smaller than predicted ([3],[4]); (2) OC comets appear to form at colder temperatures than our results would suggest ([5]), and (3) models for the origin of Halley-type comets (HTCs) [Levison et al.,~this meeting] require a massive inner OC or scattered disk as a source region for the HTCs. We will discuss alternative scenarios for the formation of the OC that may help to alleviate these difficulties (see Stern & Weissman, this meeting). This work was supported by grants from the NASA PG&G and Origins programs. [1] Weissman, P. (1996). In Completing the Inventory of the Solar System (T.W. Rettig and J.M. Hahn, Eds.), pp. 265--288. [2] Lissauer, J. (1987). Icarus 69, 249--265. [3] Duncan, M.J., and Levison, H.F. (1997). Science 276, 1670--1672. [4] Trujillo, C.A., et al.~(2000). Astrophys. J. 529, L103-L106. [5] Stern, S.A., et al.~(2000). Astrophys. J. Lett., submitted.