2011MNRAS.416..591T

We performed hydrodynamical simulations to investigate the formation and evolution of protostars and circumstellar discs from the pre-stellar cloud. As the initial state, we adopted the molecular cloud core with two non-dimensional parameters representing the thermal and rotational energies. With these parameters, we derived 17 models and calculated the cloud evolution ∼ 10⁴ yr after the protostar formation. We found that early evolution of the star–disc system can be qualitatively classified into four modes: the massive-disc, early-fragmentation, late-fragmentation, and protostar-dominant modes. In the `massive-disc mode', to which the majority of models belong, the disc mass is greater than the protostellar mass for over 10<sup>4</sup> yr and no fragmentation occurs in the circumstellar disc. The collapsing cloud shows fragmentation before the protostar formation in the `early-fragmentation mode'. The circumstellar disc shows fragmentation after the protostar formation in the `late-fragmentation mode', in which the secondary star gains most of its mass from the circumstellar disc after fragmentation and has a mass comparable to that of the primary star. The protostellar mass rapidly increases and exceeds the circumstellar disc mass in the `protostar-dominant mode'. This mode appears only when the initial molecular cloud core has a very small rotational energy. Comparison of our results with observations indicates that the majority of protostars have a fairly massive disc during the main accretion phase: the circumstellar disc mass is comparable to or more massive than the protostar mass. It is expected that such a massive disc promotes gas-giant formation by gravitational instability in a subsequent evolutionary stage.