Pair-Instability Supernovae, Gravity Waves, and Gamma-Ray Transients

Growing theoretical evidence suggests that the first generation of stars may have been quite massive (∼ 100 − 300 M ). If they retain their high mass until death, such stars will, after about 3Myr, make pair-instability supernovae. Theoretical models for these explosions have been studied in the literature for about four decades, but very few of these studies have included the effects of rotation and none ever employed a realistic model for neutrino trapping and transport. Both turn out to be very important, especially for those stars whose cores collapse into black holes (helium cores above about 140M ). We consider the complete evolution of two zerometallicity stars of 250 and 300M . Despite their large stellar masses, we argue that the lowmetallicities of these stars result in negligible mass-loss. Evolving the stars with no mass-loss and including angular momentum transport and rotationally induced mixing, these two stars produce helium cores of 130 and 180M respectively. Products of central helium burning (e.g. primary nitrogen) are mixed into the hydrogen envelope, which can dramatically change the expansion of the envelope, especially in the case of the 300M model. Explosive oxygen and silicon burning cause the 130M helium core (250M star) to explode, but explosive burning is unable to drive an explosion in the 180M helium core and it collapses to a black hole. For this star, the calculated angular momentum in the presupernova model is sufficient to delay black hole formation and the star initially forms a 50M , 1000km core within which neutrinos are trapped. Although the star does not become dynamically unstable, the calculated growth time of secular rotational instabilities is shorter than the black hole formation time, and such instabilities may develop. The estimated gravitational wave energy and wave amplitude would then be EGW ≈ 10−3 M c and h+ ≈ 10−21/d(Gpc), but these estimates are very rough and depend sensitively on the nonlinear nature of the instabilities. After the black hole forms, accretion continues through a disk. The mass of the disk depends on the adopted viscosity, but may be quite large, up to 30M when the black hole mass is 140M . The accretion rate through the disk can be as large as 1-10M s−1. Although the disk is far too large and cool to transport energy efficiently to the rotational axis by neutrino annihilation, it has ample potential energy to produce a 10 erg jet driven by magnetic fields. The interaction of this jet with surrounding circumstellar gas may produce an energetic gamma-ray transient, but given the redshift and time scale, this is probably not a model for typical gamma-ray bursts. Subject headings: gamma rays: bursts — stars: supernovae, nucleosynthesis