Accretion in Gravitationally Contracting Clouds

Abstract

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) $\times c_s^3/G$, where $c_s$ and $G$ represent the isothermal sound speed and the gravitational constant, respectively. This is much larger than the standard accretion rate of $0.975c_s^3/G$ for a hydrostatic isothermal spherical cloud (Shu 1977, AAA19.065.044). Due to the effect of an extra infall velocity achieved in the runaway phase ($\sim 2 c_s$), the accretion rate is boosted. This rate declines with time in contrast to Shu's solution, but keeps $\gtsim 2.5 c_s^3/G$. The observed gas infall rate around proto-stars such as L1551 IRS 5 and HL Tau is also discussed.