Formation and survival of Population III stellar systems

TL;DR

This paper uses hydrodynamic simulations to study the formation, evolution, and survival of Population III stellar systems, focusing on fragmentation, viscous transport, and the fate of secondary protostars in the early universe.

Contribution

It introduces a novel simulation approach with a stiff equation of state to better understand viscous transport and protostar merging in primordial star formation.

Findings

Inside-out fragmentation occurs due to faster central collapse.

Fragments migrate inward on viscous timescales, potentially merging with the primary protostar.

Disc destruction by radiative feedback halts viscous transport and influences final stellar mass and multiplicity.

Abstract

The initial mass function of the first, Population III (Pop III), stars plays a vital role in shaping galaxy formation and evolution in the early Universe. One key remaining issue is the final fate of secondary protostars formed in the accretion disc, specifically whether they merge or survive. We perform a suite of hydrodynamic simulations of the complex interplay between fragmentation, protostellar accretion, and merging inside dark matter minihaloes. Instead of the traditional sink particle method, we employ a stiff equation of state approach, so that we can more robustly ascertain the viscous transport inside the disc. The simulations show inside-out fragmentation because the gas collapses faster in the central region. Fragments migrate on the viscous timescale, over which angular momentum is lost, enabling them to move towards the disc centre, where merging with the primary protostar can occur. This process depends on the fragmentation scale, such that there is a maximum scale of $(1 - 5) \times 10^4$ au, inside which fragments can migrate to the primary protostar. Viscous transport is active until radiative feedback from the primary protostar destroys the accretion disc. The final mass spectrum and multiplicity thus crucially depends on the effect of viscosity in the disc. The entire disc is subjected to efficient viscous transport in the primordial case with viscous parameter $\alpha \le 1$. An important aspect of this question is the survival probability of Pop III binary systems, possible gravitational wave sources to be probed with the Advanced LIGO detectors.