Wolf-Rayet stars as gamma-ray burst progenitors

Abstract

It became clear in the last few years that long gamma-ray bursts are associated with the endpoints of massive star evolution. They occur in star forming regions at cosmological distances (Jakobsson et al., 2005), and are associated with supernova-type energies. The collapsar model explains gamma-ray burst formation via the collapse of a rapidly rotation massive iron core into a black hole (Woosley, 1993). The short time scale of gamma-ray emission requires a compact stellar size object, of the order of light seconds. This constraint leaves only massive Wolf–Rayet stars as possible progenitors. However, this poses a difficulty: Wolf–Rayet stars in the local universe are known to have strong stellar winds (Nugis et al., 1998 T. Nugis, P.A. Crowther and A.J. Willis, A&A 333 (1998), p. 956. View Record in Scopus | Cited By in Scopus (73)Nugis et al., 1998), which lead to a rapid spin-down (Langer, 1998) – in agreement with the absence of signatures of rapid rotation in the Galactic Wolf–Rayet sample. An additional obstacle for forming a rapidly rotating Wolf–Rayet star in the course of single star evolution is the shear between core and envelope generated by the former’s contraction and the latter’s expansion after the main sequence. Magnetic torques are expected to lead to a strong coupling and related core spin down ([Spruit, 2002], [Heger et al., 2005] and [Petrovic et al., 2005]). Indeed, such coupling is required to understand the slow rotation of young neutron stars (Ott et al., 2006) and white dwarfs (Berger et al., 2005). This implies that single stars which, during their evolution, become a supergiant, i.e. obtain a massive extended envelope, will not be suitable GRB progenitors, even if they end their life as Wolf–Rayet star.