Effects of Type I Migration on Terrestrial Planet Formation

Planetary embryos embedded in a gas disc suffer a decay in semimajor axis --type I migration -- due to the asymmetric torques produced by the interior andexterior wakes raised by the body (Goldreich & Tremaine 1980; Ward 1986). Thispresents a challenge for standard oligarchic approaches to forming theterrestrial planets (Kokubo & Ida 1998) as the timescale to grow the progenitorobjects near 1 AU is longer than that for them to decay into the Sun. In thispaper we investigate the middle and late stages of oligarchic growth using bothsemi-analytic methods (based upon Thommes et al. 2003) and N-body integrations,and vary gas properties such as dissipation timescale in different models ofthe protoplanetary disc. We conclude that even for near-nominal migrationefficiencies and gas dissipation timescales of ~1 Myr it is possible tomaintain sufficient mass in the terrestrial region to form Earth and Venus ifthe disc mass is enhanced by factors of ~2-4 over the minimum mass model. Theresulting configurations differ in several ways from the initial conditionsused in previous simulations of the final stages of terrestrial accretion (e.g.Chambers 2001), chiefly in (1) larger inter-embryo spacings, (2) larger embryomasses, and (3) up to ~0.4 Earth masses of material left in the form ofplanetesimals when the gas vanishes. The systems we produce are reasonablystable for ~100 Myr and therefore require an external source to stir up theembryos sufficiently to produce final systems resembling the terrestrialplanets.Comment: 49 pages, 22 figures; accepted in AJ, expected Dec '0