On the Minimum Metallicity and Mass of the Population II Stars

Within collapsing protogalaxies, thermal instability leads to the formation of a population of cool fragments which are confined by the pressure of a residual hot background medium. The critical mass required for the cold clouds to become gravitationally unstable and to form stars is determined by both their internal temperature and external pressure. We perform a systematic study of the cooling properties of low-metallicity clouds, and we determine, for appropriate ranges of external pressure and metallicity, the minimum temperature clouds can attain prior to the formation of nearby massive stars. The intense UV radiation of massive stars would photoionize the clouds and quench star formation. We also determine the minimum metallicity these clouds must attain in order to form presently observable Population II stars. Based on our conclusion that low-mass stars cannot be formed efficiently in a metal deficient environment, we argue that brown dwarfs are unlikely to be the main contributors to mass in the Galactic halo.