  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 and
exterior wakes raised by the body (Goldreich & Tremaine 1980; Ward 1986). This
presents a challenge for standard oligarchic approaches to forming the
terrestrial planets (Kokubo & Ida 1998) as the timescale to grow the progenitor
objects near 1 AU is longer than that for them to decay into the Sun. In this
paper we investigate the middle and late stages of oligarchic growth using both
semi-analytic methods (based upon Thommes et al. 2003) and N-body integrations,
and vary gas properties such as dissipation timescale in different models of
the protoplanetary disc. We conclude that even for near-nominal migration
efficiencies and gas dissipation timescales of ~1 Myr it is possible to
maintain sufficient mass in the terrestrial region to form Earth and Venus if
the disc mass is enhanced by factors of ~2-4 over the minimum mass model. The
resulting configurations differ in several ways from the initial conditions
used in previous simulations of the final stages of terrestrial accretion (e.g.
Chambers 2001), chiefly in (1) larger inter-embryo spacings, (2) larger embryo
masses, and (3) up to ~0.4 Earth masses of material left in the form of
planetesimals when the gas vanishes. The systems we produce are reasonably
stable for ~100 Myr and therefore require an external source to stir up the
embryos sufficiently to produce final systems resembling the terrestrial
planets.
