Pair-instability supernovae, gravity waves, and gamma-ray transients

Growing evidence suggests that the first generation of stars may have been quite massive (similar to 100-300 M-.). If they retain their high mass unti l death, such stars will, after about 3 Myr, make pair-instability supernov ae. Models for these explosions have been discussed in the literature for f our decades, but very few included the effects of rotation and none employe d a realistic model for neutrino trapping and transport. Both turn out to b e very important, especially for those stars whose cores collapse into blac k holes (helium cores above about 133 M-.). We consider the complete evolut ion of two zero-metallicity stars of 250 and 300 M-.. Despite their large m asses, we argue that the low metallicities of these stars imply negligible mass loss. Evolving the stars with no mass loss, but including angular mome ntum transport and rotationally induced mixing, the two stars produce heliu m cores of 130 and 180 M-.. Products of central helium burning (e.g., prima ry nitrogen) are mixed into the hydrogen envelope with dramatic effects on the radius, especially in the case of the 300 M-. model. Explosive oxygen a nd silicon burning cause the 130 helium core (250 M-. star) to explode, but explosive burning is unable to drive an explosion in the 180 M-. helium co re, and it collapses to a black hole. For this star, the calculated angular momentum in the presupernova model is sufficient to delay black hole forma tion, and the star initially forms an similar to 50 M-. 1000 km core within which neutrinos are trapped. The calculated growth time for secular rotati onal instabilities in this core is shorter than the black hole formation ti me, and they may develop. If so, the estimated gravitational wave energy an d wave amplitude are E-GW approximate to 10(-3) M-. c(2) and h(+) approxima te to 10(-21)/d(Gpc), but these estimates are very rough and depend sensiti vely on the nonlinear nature of the instabilities. After the black hole for ms, accretion continues through a disk. The mass of the disk depends on the adopted viscosity but may be quite large, up to 30 M-. when the black hole mass is 140 M-.. The accretion rate through the disk can be as large as 1- 10 M-. s(-1). Although the disk is far too large and cool to transport ener gy efficiently to the rotational axis by neutrino annihilation, it has ampl e potential energy to produce a 10(54) erg jet driven by magnetic fields. T he interaction of this jet with surrounding circumstellar gas may produce a n energetic gamma-ray transient, but given the probable redshift and the co nsequent timescale and spectrum, this model may have difficulty explaining typical gamma-ray bursts.