The evolution and fate of Very Massive Objects

The structure and evolution of Very Massive Objects (stars of 100-100,000 solar masses) are discussed in terms of simple semianalytic models. The helium enrichment due to mass loss is estimated and evidence is presented for a dynamical instability arising in the hydrogen shell burning phase of a 500 solar-mass Population I star. The fate of a VMO is decided in its oxygen core phase. Calculations of the effects of the pair instability, oxygen and silicon burning, and alpha-quenching on the global binding energy of initially isentropic polytropic cores lead to the prediction that the critical oxygen core mass above which complete collapse to a black hole occurs is about 100 solar masses corresponding to an initial star mass greater than 200 solar masses. Cores smaller than this explode; the kinetic energy liberated is estimated.