ACCRETION OF MASS AND SPIN ANGULAR-MOMENTUM BY A PLANET ON AN ECCENTRIC ORBIT

We have extended our previous calculations of planetary accretion rate s (Y. Greenzweig and J. J. Lissauer 1990, Icarus 87, 40-77, and 1992, Icarus 100, 440-463) to the case of a planet on an eccentric orbit. We have found that the dependence of accretion rates on planetary and pl anetesimal eccentricities can be reduced to a single parameter and tha t the extent of a planet's single-pass feeding zones depends (in the l imit of small planetary mass) only upon the sum of the eccentricities of the planet and the planetesimals and their mutual inclination, We h ave also addressed the problem of the origin of planetary rotation by calculating the spin angular momentum contributed by all particles whi ch are accreted by a planet in a uniform particle disk, Previous autho rs (J. J. Lissauer and D. IM. Kary 1991, Icarus 94, 126-159; L. Dones and S. Tremaine 1993, Icarus 103, 67-92) have obtained analytic soluti ons for nongravitating planets on circular orbits in two-dimensional d isks, We have developed new techniques which allow us to extend these calculations to the cases in which the planet's orbit is eccentric and the disk is three-dimensional. Nongravitating planets on eccentric or bits accrete the same specific rotational angular momentum as planets on circular orbits in disks of planetesimals on eccentric orbits, Plan etesimal inclinations reduce the specific angular momentum accreted by nongravitating planets by 25-65%. By performing a series of numerical three-body simulations, we have found that a gravitating planet on an orbit with eccentricity e(0) much less than 1 in a disk of planetesim als on circular orbits obtains the same rotation rate as a planet on a circular orbit in a disk of planetesimals which have e = e(0). When t he planet and the planetesimals are both on eccentric orbits with e mu ch less than 1, the spin angular momentum is determined by the distrib ution of relative eccentricity vectors. If the eccentricities of the p lanet and planetesimals differ significantly, inclination typically ac ts to reduce the magnitude of the rotation rate by a few tens of perce nt, However, in certain cases where the eccentricities of the planet a nd planetesimals are comparable, inclination can change slow retrograd e rotation to slow prograde rotation, A planet which accretes material primarily from the extremities of its feeding zone during the latter stage of its growth attains rapid systematic prograde rotation only if the amplitude of its epicyclic motion is comparable to or smaller tha n the radius of its Hill sphere. (C) 1997 Academic Press.