Population III star formation in an X-ray background – II. Protostellar discs, multiplicity, and mass function of the stars

ABSTRACT Disc fragmentation plays an important role in determining the number of primordial stars (Pop III stars), their masses, and hence initial mass function. In this second paper a series, we explore effect uniform far-ultraviolet H2-photodissociating X-ray radiation backgrounds on formation Pop using grid high-resolution zoom-in simulations. We find that, background, protostellar discs have lower surface density higher Toomre Q parameter, so they are more stable. For reason, irradiated undergo fewer fragmentations typically produce either binary systems or low-multiplicity systems. contrast, cases with weak no irradiation typical multiplicity 6 ± 3. addition, most massive protostar each system is smaller by roughly factor 2 when disc X-rays, due to accretion rate. With these two effects combined, function fragments becomes top-heavy strong background well described power law slope 1.53 high-mass cutoff 61 M⊙. Without 0.49 229 Finally, protostars migrate outward after likely high-angular momentum gas from outside migration frequent significant absence irradiation.