Primordial Star Formation under the Influence of Far Ultraviolet Radiation: 1540 Cosmological Halos and the Stellar Mass Distribution

TL;DR

This study uses cosmological simulations to analyze the formation and mass distribution of primordial stars, revealing how their characteristics evolve over cosmic time and under the influence of FUV radiation.

Contribution

It provides a detailed model linking the physical properties of primordial gas clouds and FUV radiation to the resulting stellar masses, incorporating feedback effects from radiation hydrodynamics.

Findings

Pop III stars have characteristic masses of a few hundred solar masses at z ~ 25.

The fraction of massive Pop III.2D stars decreases over time as FUV radiation weakens.

Pop III.1 stars dominate at z < 20, with masses of a few times 10 solar masses.

Abstract

We perform a large set of cosmological simulations of early structure formation and follow the formation and evolution of 1540 star-forming gas clouds to derive the mass distribution of primordial stars. The star formation in our cosmological simulations is characterized by two distinct populations, the so-called Population III.1 stars and primordial stars formed under the influence of far ultraviolet (FUV) radiation (Population III.2D stars). In this work, we determine the stellar masses by using the dependences on the physical properties of star-forming cloud and/or the external photodissociating intensity from nearby primordial stars, which are derived from the results of two-dimensional radiation hydrodynamic simulations of protostellar feedback. The characteristic mass of the Pop III stars is found to be a few hundred solar masses at z ~ 25, and it gradually shifts to lower masses with decreasing redshift. At high redshifts z > 20, about half of the star-forming gas clouds are exposed to intense FUV radiation and thus give birth to massive Pop III.2D stars. However, the local FUV radiation by nearby Pop III stars becomes weaker at lower redshifts, when typical Pop III stars have smaller masses and the mean physical separation between the stars becomes large owing to cosmic expansion. Therefore, at z < 20, a large fraction of the primordial gas clouds host Pop III.1 stars. At z =< 15, the Pop III.1 stars are formed in relatively cool gas clouds due to efficient radiative cooling by H_2 and HD molecules; such stars have masses of a few x 10 Msun. Since the stellar evolution and the final fate are determined by the stellar mass, Pop III stars formed at different epochs play different roles in the early universe.