Wolf-Rayet stars and cosmic gamma-ray bursts

The observational properties of cosmic gamma-ray bursts and of Wolf-Rayet ( WR) stars and their CO cores at the end of their evolution are analyzed. WR stars do not have hydrogen envelopes, facilitating the transformation of t he energy of collapse into observable gamma rays. Of the similar to 90 well -localized gamma-ray bursts, 21 have optical identifications, of which 16 h ave measured redshifts (z = 0.4-4.5). The distribution of gamma-ray bursts in energy N(DeltaE) has a large scatter, from 3 x 10(51) to 2 x 10(54) erg. There is some evidence that the distribution N(AE) is bimodal if we includ e the gamma-ray burst GRB 980425, which is associated with the peculiar typ e Ic supernova SN 1998bw in the nearby elliptical galaxy ESO 184-G82, for w hich DeltaE(gamma) approximate to 10(48) erg. These characteristics of gamm a-ray bursts are reminiscent of the distribution of final masses for the CO cores of WR stars, which uniformly covers a broad range: M-CO = (1-2)M-.-( 20-44)M-.. The possible bimodality of the gamma-ray burst energy distributi on (E-1 = 10(48) erg; DeltaE(2) = 3 x 10(51)-2 x 10(54) erg) could be assoc iated with the bimodal mass distribution for stellar relativistic objects ( M-NS = (1.35 +/- 0.15)M-.; M-BH = (4-15)M-.). The fact that SN 1998bw is a "peculiar" type Ic supernova, not typical for the collapses of WR stars (wh ich usually give rise to type Ib/c supernovae), could be related to the rot ation of the collapsing CO core. This "drags out" the time for the collapse , leading to the formation of a neutron star, a decrease in the gamma-ray b urst energy, and an increase in the fraction of kinetic energy transferred to the supernova envelope. The expected rate of collapse of the CO cores of WR stars in the Galaxy is similar to 10(-3)/yr. This is at least three ord ers of magnitude higher than the mean frequency of gamma-ray bursts per gal axy (similar to 10(-6)-10(-7)/yr). Two models for gamma-ray bursts with WR stars as progenitors are considered: the hypernova model of Paczynski (1998 ) and the pulsation instability CO-core collapse model proposed by Gershtei n (2000). In both models, the rate of CO-core collapses can be brought into agreement with the observed rate of gamma-ray bursts by taking into accoun t the anisotropy of the gamma radiation, associated with either a relativis tic jet or the random character of the initial CO-core collapse due to inst abilities. It is concluded that WR stars could be the progenitors of gamma- ray bursts. This hypothesis predicts the existence of two types of gamma-ra y bursts, corresponding to the bimodal mass distribution for stellar relati vistic objects, and of three types of optical afterglow, associated with co llapses of the CO cores of WR stars that are single, in WR+O binaries, and in hypothetical WR+(A-M) systems. The paper also briefly examines a model o f gamma-ray bursts as transient phenomena in the early stages of the evolut ion of galaxies (z > 1), when very massive stars (M > 100M(,)) weak in heav y elements could form. Such massive stars should also lose their hydrogen e nvelopes and be transformed into massive WR stars, whose collapses could be accompanied by gamma-ray bursts. It is suggested that WR galaxies are the most probable candidates for the host galaxies of gamma-ray bursts. (C) 200 1 MAIK "Nauka/Interperiodica".