Impact of cosmic rays on Population III star formation

We explore the implications of a possible cosmic-ray (CR) background generated during the first supernova explosions that end the brief lives of massive Population III stars. We show that such a CR background could have significantly influenced the cooling and collapse of primordial gas clouds in minihaloes around redshifts of z similar to 15-20, provided the CR flux was sufficient to yield an ionization rate greater than about 10(-19) s(-1) near the centre of the minihalo. The presence of CRs with energies less than or similar to 10(7) eV would indirectly enhance the molecular cooling in these regions, and we estimate that the resulting lower temperatures in these minihaloes would yield a characteristic stellar mass as low as similar to 10M(circle dot). CRs have a less-pronounced effect on the cooling and collapse of primordial gas clouds inside more massive dark matter haloes with virial masses greater than or similar to 10(8)M(circle dot) at the later stages of cosmological structure formation around z similar to 10-15. In these clouds, even without CR flux the molecular abundance is already sufficient to allow cooling to the floor set by the temperature of the cosmic microwave background.