Wolf-Rayet stars and GRB connection

Abstract

Observed properties of GRBs, WR stars and their CO-cores in the end of evolution are analyzed. A possible bimodality of the observed GRB energy distribution ($10^{48}$ erg for GRB9809425; $3\times 10^{51}÷2\times 10^{54}$ erg for others) is in accord with the bimodal mass distribution of relativistic objects ($M_{NS}=(1.35\pm 0.15) M_\odot$; $M_{BH}=(4÷15) M_\odot$). The peculiarity of SN1998bw can be related to the rotation of the collapsing CO-core. The expected galactic collapse rate of CO-cores of most compact WR stars of type WO is $\sim 10^{-5}$ per year, only by one and a half order of magnitude higher than the GRB rate. The allowance for a gamma-ray beaming or random outcome of the CO-core collapse due to some instabilities brings this rate in accordance GRB rate. We argue that WR stars (most probably, of type WO) can be considered as progenitors of cosmic gamma-ray bursts. Two types of GRBs are predicted in correspondence with the bimodal mass distribution of the relativistic objects. Three types of optical afterglows should appear depending on which CO-core is collapsing: of a single WR star, of a WR star in a WR+O or a hypothetic WR+(A-M) binary system. We briefly discuss a model of GRB as a transient phenomenon occurring at early stages of galactic evolution ($z>1$), when very massive ($M>100 M_\odot$) low-metallicity stars could form. WR-galaxies can be most probable candidates for GRB host galaxies.