Dust formation in very massive primordial supernovae

At redshift z greater than or similar to 5, Type II supernovae (SNeII) are the only known dust sources with evolutionary time-scales shorter than the Hubble time. We extend the model of dust formation in the ejecta of SNeII by Todini & Ferrara to investigate the same process in pair-instability supernovae (PISNe), which are though to arise from the explosion of the first, metal-free, very massive (140-260 M.) cosmic stars. We find that 15-30 per cent of the PISN progenitor mass is converted into dust, a value >10 times higher than for SNeII; PISN dust depletion factors (the fraction of produced metals locked into dust grains) range between 0.3 and 0.7. These conclusions depend very weakly on the mass of the PISN stellar progenitor, which in contrast affects considerably the composition and size distribution. For the assumed temperature evolution, grain condensation starts 150-200 d after the explosion; the dominant compounds for all progenitor masses are SiO2 and Mg2SiO4 while the contribution of amorphous carbon and magnetite grains grows with progenitor mass; typical grain sizes range between 10(-3) and a few times 0.1 mum and are always smaller than 1 mum. We give a brief discussion of the implications of dust formation for the initial mass function evolution of the first stars, cosmic reionization and the intergalactic medium.