An estimate of the mass of zero metal stars

The temperature of a cloud of hydrogen-helium gas uncontaminated by metals is followed from post recombination conditions to hydrogen ionization, assuming the collapse takes place on a free-fall time-scale. Four characteristic masses are identifiable: 106 solar masses for a sufficiently hot cloud to allow H2 to form, 20 solar masses at H2 dissociation, 0.5 solar mass at optical depth unity, and 0.06 solar mass at H ionization. The initial cloud is of low enough density, approximately 104 cm-3, that stellar winds will prevent the formation of stars heavier than 100 solar masses. The characteristic masses 0.06 solar mass and 0.5 solar mass occur in environments with such high density, 10-2 and 10-8 g cm-3 respectively, that they are identified with a protostellar inner and outer core, and will continue to accrete material for a minimum stellar mass of a few solar masses. A metal production yield of 10 per cent combined with the minimum metal abundance for ordinary stars implies that the zero metal stars comprised at most 10-4 of the gas mass of the Universe. This small fraction of heavy stars would have been capable of re-ionizing the Universe at a z of 50 to 100. (author)