2018ApJ...861...68T

We conduct a theoretical study of the formation of massive stars over a wide range of metallicities from 10^–5 to 1Z_☉ and evaluate the star formation efficiencies (SFEs) from prestellar cloud cores taking into account multiple feedback processes. Unlike for simple spherical accretion, feedback processes in the case of disk accretion do not set upper limits on stellar masses. At solar metallicity, launching of magneto-centrifugally driven outflows is the dominant feedback process to set SFEs, while radiation pressure, which has been regarded as pivotal, makes only a minor contribution even in the formation of stars over 100 M_☉. Photoevaporation becomes significant in the formation of stars over 20 M_☉ at low metallicities of ≲10^–2 Z_☉, where dust absorption of ionizing photons is inefficient. We conclude that if initial prestellar core properties are similar, then massive stars are rarer in extremely metal-poor environments of 10^–5-10^–3 Z_☉. Our results give new insight into the high-mass end of the initial mass function and its potential variation with galactic and cosmological environments.