  The growing process of both a young protostar and a circumstellar disk is
investigated. Viscous evolution of a disk around a single star is considered
with a model where a disk increases its mass by dynamically accreting envelope
and simultaneously loses its mass via viscous accretion onto the central star.
We focus on the circumstellar disk with high mass accretion rate onto the disk
$\dot{M}=8.512c_{\rm s}^3/G$ as a result of dynamical collapse of rotating
molecular cloud core. We study the origin of the surface density distribution
and the origin of the disk-to-star mass ratio by means of numerical
calculations of unsteady viscous accretion disk in one-dimensional axisymmetric
model. It is shown that the radial profiles of the surface density $\Sigma$,
azimuthal velocity $v_{\phi}$, and mass accretion rate $\dot{M}$ in the inner
region approach to the quasi-steady state. Profile of the surface density
distribution in the quasi-steady state is determined as a result of angular
momentum transport rather than its original distribution of angular momentum in
the cloud core. It is also shown that the disk mass becomes larger than the
central star in the long time limit as long as temporary constant mass flux
onto the disk is assumed. After the mass infall rate onto the disk declines
owing to the depletion of the parent cloud core, the disk-to-star mass ratio
$M_{\rm disk}/M_*$ decreases. The disk-to-star mass ratio becomes smaller than
unity after $t> 10^5 \rm yr$ and $t>10^6 \rm yr$ from the beginning of the
accretion phase in the case with $\alpha_0 =1 {\rm and} 0.1$, respectively,
where $\alpha_0 $ is the constant part of viscous parameter. In the case with
$\alpha_0 \leq 10^{-2}$, $M_{\rm disk}/M_*$ is still larger than unity at $2
\rm Myr$ from the beginning of the accretion phase.

Earth and Planetary Astrophysics