Evolution of a protobinary: Accretion rates of the primary and secondary

We reexamine accretion onto a protobinary based on two-dimensional numerical simulations with high spatial resolution. We focus our attention on the ratio of the primary and secondary accretion rates. Fifty-eight models are made for studying the dependence of the accretion rates on the specific angular momentum of infalling gas j(inf), the mass ratio of the binary q, and the sound speed c(s). When j(inf) is small, the binary accretes the gas mainly through two channels ( type I): one through the Lagrange point L2 and the other through L3. When j(inf) is large, the binary accretes the gas only through the L2 point ( type II). The primary accretes more than the secondary in both the cases, although the L2 point is closer to the secondary. After flowing through the L2 point, the gas flows halfway around the secondary and through the L1 point to the primary. Only a small amount of gas flows back to the secondary, and the rest forms a circumstellar ring around the primary. The boundary between types I and II depends on q. When j(inf) is very large, the accretion begins after several rotations ( type III). The beginning of the accretion is later when j(inf) is larger and c(s) is smaller. Our result that the primary accretion rate is higher for a large j(inf) is qualitatively different from results of earlier simulations. The difference is mainly due to limited spatial resolution and large numerical viscosity in the numerical simulations thus far.