ACCRETION IN GRAVITATIONALLY CONTRACTING CLOUDS

Accretion flow in a contracting magnetized isothermal cloud was studie d using magnetohydrodynamical simulations and a nested grid technique. First, the interstellar magnetized cloud experiences a ''runaway coll apse'' phase, in which the central density increases drastically withi n a finite time scale. Finally, it enters an accretion phase, in which inflowing matter accretes onto a central high-density disk of a new-b orn star. We found that the accretion rate reaches (4-40) x c(s)(3)/G, where c(s) and G represent the isothermal sound speed and the gravita tional constant, respectively. This is much larger than the standard a ccretion rate of 0.975 c(s)(3)/G for a hydrostatic isothermal spherica l cloud (Shu 1977, AAA 19.065.044). Due to the effect of an extra infa ll velocity achieved in the runaway phase (similar to 2 c(s)), the acc retion rate is boosted. This rate declines with time in contrast to Sh u's solution, but keeps greater than or similar to 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.