Star Formation at Zero and Very Low Metallicities

TL;DR

This paper reviews star formation processes in the early universe, focusing on primordial stars, their evolution, feedback effects, and how metallicity influences subsequent star formation generations.

Contribution

It provides a comprehensive overview of star formation at zero and very low metallicities, highlighting the effects of feedback, magnetic fields, and metallicity on star mass and formation processes.

Findings

Pop III.1 stars are massive (~100-200Msun) and influence their environment significantly.

Metal-enriched gas leads to lower mass stars due to enhanced cooling mechanisms.

Star formation processes evolve with increasing metallicity and feedback effects.

Abstract

We describe how star formation is expected to proceed in the early metal-free Universe, focusing on the very first generations of stars. We then discuss how the star formation process may change as the effects of metallicity, external radiative feedback, and magnetic and turbulent support of the gas become more important. The very first stars (Pop III.1) have relatively simple initial conditions set by cosmology and the cooling properties of primordial gas. We describe the evolution of these stars as they grow in mass by accretion from their surrounding gas cores and how the accretion process is affected and eventually terminated by radiative feedback processes, especially HII region expansion and disk photoevaporation. The ability of the protostar and its disk to generate dynamically important magnetic fields is reviewed and their effects discussed. Pop III.1 star formation is likely to produce massive (~100-200Msun) stars that then influence their surroundings via ionization, stellar winds, and supernovae. These processes heat, ionize and metal-enrich the gas, thus altering the initial conditions for the next generation of star formation. Stars formed from gas that has been altered significantly by radiative and/or mechanical feedback, but not by metal enrichment (Pop III.2) are expected to have significantly smaller masses than Pop III.1 stars because of more efficient cooling from enhanced HD production. Stars formed from gas that is metal-enriched to levels that affect the dynamics of the collapse (the first Pop II stars) are also expected to have relatively low masses. We briefly compare the above star formation scenarios to what is known about present-day star formation.