Primordial star formation under the influence of far ultraviolet radiation: 1540 cosmological haloes and the stellar mass distribution

Abstract

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