Disc fragmentation and oligarchic growth of protostellar systems in low-metallicity gas clouds

We study low-metallicity star formation with a set of high-resolution hydrodynamics simulations for various gas metallicities o v er a wide range 0-10 -3 Z ⊙ . Our simulations follow non-equilibrium chemistry and radiative cooling by adopting realistic elemental abundance and dust size distribution. We examine the condition for the fragmentation of collapsing clouds (cloud fragmentation; CF) and of accretion discs (disc fragmentation; DF). We find that CF is suppressed due to rapid gas heating accompanied with molecular hydrogen formation even with efficient dust cooling for metallicities ≳10 -5 Z ⊙. Instead, DF occurs in almost all runs regardless of metallicity. We also find that, in the accretion discs, the growth of the protostellar systems is o v erall oligarchic. The primary protostar grows through the accretion of gas, and secondary protostars form through the interaction of spiral arms or the break-up of a rapidly rotating protostar. Despite vigorous fragmentation, a large fraction of secondary protostars are destroyed through mergers or tidal disruption ev ents. F or a few hundred years after the first adiabatic core formation, only several protostars survive in the disc, and the total mass of protostars is 0.52-3 . 8 M ⊙ .