Accretion onto binary YSOs through gap from their circum-binary disk

Abstract

We have reexamined gas accretion onto YSO binary from the circumbinary disk on the basis of 2D numerical simulations with very high spatial resolution. The binary was assumed to have either a circular or an elliptic orbit. At the initial stage, the circum-binary disk has an inner edge at rin = 1.75a, where a denotes the mean separation of the binary. The disk is assumed to rotate with the Keplerian velocity. We have confirmed that gas accretes from the circum-binary disk through the L2 point into the Roche lobe. The gas accretes mainly onto the primary after circulating half around the secondary. This means that the accretion decreases the mass ratio. The accretion through gap is due to some pairs of two-armed spiral shock waves. While a pair of them corotates with the binary, the other pairs rotate more slowly, e.g., at Ω = Ω*/4 in most models, where Ω* denotes the mean angular velocity. Whenever the slowly rotating shock waves get accross the L2 point, the gas flow increases. Thus the accretion rate of the binary changes with the frequency, v = (3/2) (Ω*/2π), in the case of a circular orbit. When the orbit is eccentric, the accretion rate changes mainly with the binary orbit but not purely periodically. © 2008 Springer-Verlag Berlin Heidelberg.