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The formation and evolution of the circumstellar disk in unmagnetizedmolecular clouds is investigated using three-dimensional hydrodynamicsimulations from the prestellar core until the end of the main accretion phase.In collapsing clouds, the first (adiabatic) core with a size of ~10AU formsprior to the formation of the protostar. At its formation, the first core has athick disk-like structure, and is mainly supported by the thermal pressure.After the protostar formation, it decreases the thickness gradually, andbecomes supported by the centrifugal force. We found that the first core is aprecursor of the circumstellar disk. This indicates that the circumstellar diskis formed before the protostar formation with a size of ~10AU, which means thatno protoplanetary disk smaller than <10AU exists. Reflecting the thermodynamicsof the collapsing gas, at the protostar formation epoch, the circumstellar diskhas a mass of ~0.01-0.1 solar mass, while the protostar has a mass of ~10^-3solar mass. Thus, just after the protostar formation, the circumstellar disk isabout 10-100 times more massive than the protostar. Even in the main accretionphase that lasts for ~10^5yr, the circumstellar disk mass dominates theprotostellar mass. Such a massive disk is unstable to gravitationalinstability, and tends to show fragmentation. Our calculations indicate thatthe planet or brown-dwarf mass object may form in the circumstellar disk in themain accretion phase. In addition, the mass accretion rate onto the protostarshows strong time variability that is caused by the perturbation ofproto-planets and/or the spiral arms in the circumstellar disk. Suchvariability provides a useful signature for detecting the planet-sizedcompanion in the circumstellar disk around very young protostars.

Formation Process of the Circumstellar Disk: Long-term Simulations in the Main Accretion Phase of Star Formation