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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.
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