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Accretion Phase of Star Formation in Clouds with Different Metallicities

The main accretion phase of star formation is investigated in clouds with different metallicities in the range of 0 \le Z \le Z_\odot, resolving the protostellar radius. Starting from a near-equilibrium prestellar cloud, we calculate the cloud evolution up to \sim100 yr after the first protostar formation. The star formation process considerably differs between clouds with lower (Z \le 10^-4 Z_\odot) and higher (Z > 10^-4 Z_\odot) metallicities. Fragmentation frequently occurs and many protostars appear without forming a stable circumstellar disc in lower-metallicity clouds. In these clouds, although protostars mutually interact and some are ejected from the cloud centre, many remain as a small stellar cluster. In contrast, higher-metallicity clouds produce a single protostar surrounded by a nearly stable rotation-supported disc. In these clouds, although fragmentation occasionally occurs in the disc, the fragments migrate inwards and finally fall onto the central protostar. The difference in cloud evolution is due to different thermal evolutions and mass accretion rates. The thermal evolution of the cloud determines the emergence and lifetime of the first core. The first core develops prior to the protostar formation in higher-metallicity clouds, whereas no (obvious) first core appears in lower-metallicity clouds. The first core evolves into a circumstellar disc with a spiral pattern, which effectively transfers the angular momentum outwards and suppresses frequent fragmentation. In lower-metallicity clouds, the higher mass accretion rate increases the disc surface density within a very short time, rendering the disc unstable to self-gravity and inducing vigorous fragmentation.