Massive stars in low-metallicity environments: the progenitors of massive black holes

Stars with birth masses in the range of eight to several hundreds of solar masses end their lives as core-collapse supernovae, producing compact objects including black holes. Massive stars that resulted from primordial star formation are potential contributors to the re-ionization of the Universe and have likely played a crucial role in the formation of the first galaxies. The recent detection of gravitational wave signals from two ~30 Msun black holes strongly points to very massive progenitor stars that lost relatively little mass in stellar winds throughout their lives. Such weak winds are predicted for massive stars composed of almost pristine gas, i.e. gas with a very low metal content. We here propose to greatly further our understanding of massive stars in low-metallicity environments. To this end, we will study these stars in novels ways using ground-braking datasets of an unprecedented sample of extremely massive stars in Radcliffe 136 at a metallicity 1/2th of solar, including sources more massive than 100 Msun, and massive star populations in very metal-poor dwarf galaxies beyond the Local Group, notably SextansA at 1/15th Zsun. This will result in breakthrough insights in the physical processes that control the evolution of these stars, notably mass-loss, internal mixing & convective overshooting, and multiplicity. In turn, this will allow us to make tailored evolutionary predictions for these environments that are at the basis of constraining their mechanical and radiative-feedback properties, supernova characteristics and end products as compact objects, hence the frequency and properties of gravitational wave signals.