Effects of dissipating gas drag on planetesimal accretion in binary systems

We numerically investigate the conditions for planetesimal accretion in the circumprimary disk under the perturbing presence of a companion star, with focus on the γ Cephei system. Gas drag is included with a dissipating time scale of 10⁵ years. We show at the beginning(within 10³ ∼ 10⁴ years), gas drag damps the ΔV between planetesimals of same sizes and increases ΔV between planetesimals of different sizes. However, after increasing to high values(300∼800m/s), we find the ΔV between bodies of different sizes decrease to very low values (below 10m/s) in a few 10⁵yrs (depending on the gas-dissipating time scale T_damp, radial size R_p and semi-major axis a_p of planetesimals). Hence, the high ΔV is somewhat short-lived, and runaway accretion can be turned on later. We conclude that the conditions for planetary formation in binary systems (even close binary systems) are much better than what we expected before.