Evolution and fate of very massive stars

Abstract

There is observational evidence that supports the existence of Very Massive Stars in the local universe. First, very massive stars (Mini<=320 M) have been observed in the Large Magellanic Cloud . Second, there are observed SNe that bear the characteristics of Pair Creation Supernovae which have very massive stars as progenitors. The most promising candidate to date is SN2007bi. In order to investigate the evolution and fate of nearby very massive stars, we calculated a new grid of models for such objects, for solar, LMC and SMC metallicities, which covers the initial mass range from 120 to 500M. Both rotating and non-rotating models were calculated using the Geneva stellar evolution code and evolved until at least the end of helium burning and for most models until oxygen burning. Since very massive stars have very large convective cores during the Main-Sequence phase, their evolution is not so much affected by rotational mixing, but more by mass loss through stellar winds. Their evolution is never far from a homogeneous evolution even without rotational mixing. All the VMS, at all the metallicities studied here, end their life as WC(WO) type stars. At solar metallicity, none of our models is expected to explode as a PCSN. At the metallicity of the LMC, only stars more massive than 300 M are expected to explode as PCSNe. At the SMC metallicity, the mass range for the PCSN progenitors is much larger and comprises stars with initial masses between about 100 and 290 M . All VMS stars in the metallicity range studied here produce either a type Ib or a type Ic SN. We estimate that the progenitor of SN2007bi, assuming a SMC metallicity, had an initial mass between 160 and 175 M . None of models presented in this grid produce GRBs or magnetars. They lose too much angular momentum by mass loss or avoid the formation of a BH by producing a completely disruptive PCSN.