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

TL;DR

This study uses high-resolution hydrodynamics simulations to explore star formation in low-metallicity gas clouds, revealing that cloud fragmentation is suppressed while disc fragmentation persists, leading to oligarchic protostellar growth.

Contribution

It provides new insights into the conditions for cloud and disc fragmentation across a range of low metallicities, highlighting the dominance of disc fragmentation and oligarchic growth.

Findings

Cloud fragmentation is suppressed at metallicities above 10^{-5} Z_sun.

Disc fragmentation occurs regardless of metallicity.

Protostellar systems grow oligarchically with few surviving protostars after initial formation.

Abstract

We study low-metallicity star formation with a set of high-resolution hydrodynamics simulations for various gas metallicities over a wide range $0$--$10^{-3} \ {\rm Z}_{\bigodot}$. 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 $\gtrsim 10^{-5} \ {\rm Z}_{\bigodot}$. 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 overall 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 events. For a few hundred years after the first adiabatic core formation, only several protostars survive in the disc, and the total mass of protostars is $0.52$--$3.8 \ {\rm M}_{\bigodot}$.