Dust Formation in Very Massive Primordial Supernovae

At redshift z ∼ > 5 Type II supernovae (SNII) are the only known dust sources with evolutionary timescales shorter than the Hubble time. We extend the model of dust formation in the ejecta of SNII by Todini & Ferrara (2001) to investigate the same process in pair-instability supernovae (SNγγ), which are though to arise from the explosion of the first, metal free, very massive (140-260 M⊙) stars in the Universe. We find that 7%-20% of the SNγγ progenitor mass is converted into dust, a value > 10 times higher than for SNII of zero metallicity; however, SNγγ dust depletion factors (fraction of produced metals locked into dust grains) are smaller (< 40%) than SNII ones. These conclusions depend very weakly on the thermodynamics of the ejecta, which instead affect considerably the grain formation epoch, composition and size distribution. For the fiducial adiabatic index γ = 1.25 grain condensation starts ≈ 50 yr after the explosion; silicate (magnetite) grains form in stars with mass M < 200M⊙ (M > 200M⊙); carbon grains never form as the available C is locked into CO molecules; typical grain sizes are very small (≈ 10 Å). We give the dust depletion factors for various elements of observational relevance and a brief discussion of the implications of dust formation for the IMF evolution of the first stars, cosmic reionization and the intergalactic medium.