Binary star formation: accretion-induced rotational fragmentation

Abstract

We describe in detail two numerical simulations which illustrate a promising mechanism for binary star formation. This mechanism relies on the likelihood that protostellar discs are fed by accretion flows in which the specific angular momentum increases with time. Consequently, protostellar discs may be spun up until they become rotationally unstable and fragment. This mechanism has two variants, accretion-induced bar-mode rotational fragmentation (ABRF) and accretion-induced spiral-mode rotational fragmentation (ASRF). ABRF. If the specific angular momentum of a protostellar disc increases rapidly, the disc becomes unstable to a bar mode and cleaves directly into two components of comparable size, which may then grow and move apart by ‘mopping up’ the remaining accretion flow. The components move apart because they repeatedly intercept the accretion flow and thereby convert some of the – by now – large angular momentum of the material in the accretion flow into orbital motion rather than spin. ASRF. If the specific angular momentum of the primary protostellar disc increases more slowly, the disc repeatedly forms spiral arms. Spiral arms formed in this manner can, if sufficiently massive, detach and condense to form initially small secondaries. Such secondaries are sometimes re-assimilated by the primary, but often they merge with one another and move away from the primary, again by ‘mopping up’ the remaining accretion flow. If the accretion flow persists for long enough, they may grow to be comparable in mass to the primary. In the simulations presented here, the initial conditions are dynamic, and the initial protostellar discs condense out of a shocked layer formed by a clump/clump collision. However, it seems likely that the basic ingredient of the mechanism, i.e. an accretion flow with increasing specific angular momentum, can arise also in more quasi-static star formation regions like Taurus.