  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.
