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Growth of a Protostar and a Young Circumstellar Disk with High Mass Accretion Rate onto the Disk

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 $Σ$, azimuthal velocity $v_ϕ$, 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 $α_0 =1 {\rm and} 0.1$, respectively, where $α_0 $ is the constant part of viscous parameter. In the case with $α_0 \leq 10^{-2}$, $M_{\rm disk}/M_*$ is still larger than unity at $2 \rm Myr$ from the beginning of the accretion phase.