The First Stars: formation under X-ray feedback

We investigate the impact of an ionising X-ray background on metal-free Population III stars within a minihalo at z’ 25 starting from cosmological initial conditions. Using the smoothed particle hydrodynamics code gadget-2, we attain sucient numerical resolution to follow the gas collapsing into the centre of the minihalo up to densities of 10 12 cm 3 , at which point we form sink particles. This allows us to study how the presence of a cosmic X-ray background (CXB) aects the formation of H 2 and HD in the gas before it becomes fully molecular. These molecules play a major role in the cooling and fragmentation of the collapsing gas, which governs the nal mass of the stars formed. The primordial initial mass function, in turn, determines the impact the rst stars have on subsequent cosmic evolution. Using a suite of simulations for a range of possible CXB models, we follow each simulation for 5000 yr after the formation of the rst sink particle. The CXB provides two competing eects, with X-rays both heating the gas and enhancing its ability to cool by increasing the free electron fraction, allowing more H2 to form. We nd that X-ray heating dominates