Formation of an Infalling Disklike Envelope around a Protostar

We examined the gravitational contraction of isothermal molecular cloud cores with slow rotation by means of two-dimensional numerical simulations. Applying a sink-cell method, we followed the evolution of the cloud cores up to the stages at which most of the matter accretes onto the central region (i.e., a protostar and a rotationally-supported circumstellar disk). We show that both an infalling disklike envelope and a rotationally-supported disk around the central star are natural outcome of the gravitational contraction of a {\it prolate} cloud core with slow rotation. The early evolution of the infalling envelopes resembles sheet models recently proposed by Hartmann and coworkers. In the infalling disklike envelope, the radial profiles of the density, radial velocity, and azimuthal velocity can be approximated by $\rho\propto r^{-1.5}$, $v_r\propto r^{-0.5}$, and $v_\phi\propto r^{-1}$, respectively. The fate of the infalling envelopes is also discussed.