  The formation and evolution of the circumstellar disk in unmagnetized
molecular clouds is investigated using three-dimensional hydrodynamic
simulations from the prestellar core until the end of the main accretion phase.
In collapsing clouds, the first (adiabatic) core with a size of ~10AU forms
prior to the formation of the protostar. At its formation, the first core has a
thick disk-like structure, and is mainly supported by the thermal pressure.
After the protostar formation, it decreases the thickness gradually, and
becomes supported by the centrifugal force. We found that the first core is a
precursor of the circumstellar disk. This indicates that the circumstellar disk
is formed before the protostar formation with a size of ~10AU, which means that
no protoplanetary disk smaller than <10AU exists. Reflecting the thermodynamics
of the collapsing gas, at the protostar formation epoch, the circumstellar disk
has a mass of ~0.01-0.1 solar mass, while the protostar has a mass of ~10^-3
solar mass. Thus, just after the protostar formation, the circumstellar disk is
about 10-100 times more massive than the protostar. Even in the main accretion
phase that lasts for ~10^5yr, the circumstellar disk mass dominates the
protostellar mass. Such a massive disk is unstable to gravitational
instability, and tends to show fragmentation. Our calculations indicate that
the planet or brown-dwarf mass object may form in the circumstellar disk in the
main accretion phase. In addition, the mass accretion rate onto the protostar
shows strong time variability that is caused by the perturbation of
proto-planets and/or the spiral arms in the circumstellar disk. Such
variability provides a useful signature for detecting the planet-sized
companion in the circumstellar disk around very young protostars.
