Accretion in Gravitationally Contracting Clouds

Accretion flow in a contracting magnetized isothermal cloud was studied using magnetohydrodynamical simulations and a nested grid technique. First, the interstellar magnetized cloud experiences a “runaway collapse” phase, in which the central density increases drastically within a finite time scale. Finally, it enters an accretion phase, in which inflowing matter accretes onto a central high-density disk or a new-born star. We found that the accretion rate reaches (4–40) × cs3/G , where cs and G represent the isothermal sound speed and the gravitational constant, respectively. This is much larger than the standard accretion rate of 0.975cs3/G for a hydrostatic isothermal spherical cloud (Shu 1977, AAA 19.065.044). Due to the effect of an extra infall velocity achieved in the runaway phase (∼ 2cs), the accretion rate is boosted. This rate declines with time in contrast to Shu's solution, but keeps ≳ 2.5cs3/G. The observed gas infall rate around proto-stars such as L1551 IRS 5 and HL Tau is also discussed.