The Nucleosynthetic Signature of Population III

Abstract

Growing evidence suggests that the first generation of stars may have been quite massive (~100-300 M_sun). Could these stars have left a distinct nucleosynthetic signature? We explore the nucleosynthesis of helium cores in the mass range M_He=64 to 133 Msun, corresponding to main-sequence star masses of approximately 140 to 260 M_sun. Above M_He=133 M_sun, without rotation and using current reaction rates, a black hole is formed and no nucleosynthesis is ejected. For lighter helium core masses, ~40 to 63 M_sun, violent pulsations occur, induced by the pair instability and accompanied by supernova-like mass ejection, but the star eventually produces a large iron core in hydrostatic equilibrium. It is likely that this core, too, collapses to a black hole, thus cleanly separating the heavy element nucleosynthesis of pair instability supernovae from those of other masses, both above and below. Indeed, black hole formation is a likely outcome for all Population III stars with main sequence masses between about 25 M_sun and 140 M_sun (M_He = 9 to 63 M_sun) as well as those above 260 M_sun. Nucleosynthesis in pair-instability supernovae varies greatly with the mass of the helium core which determines the maximum temperature reached during the bounce. At the upper range of exploding core masses, a maximum of 57 M_sun of Ni56 is produced making these the most energetic and the brightest thermonuclear explosions in the universe. Integrating over a distribution of masses, we find that pair instability supernovae produce a roughly solar distribution of nuclei having even nuclear charge, but are remarkably deficient in producing elements with odd nuclear charge. Also, essentially no elements heavier than zinc are produced due to a lack of s- and r-processes.