Constraints on gamma-ray burst and supernova progenitors through circumstellar absorption lines

II. Post-LBV Wolf-Rayet stars

¹ Astronomical Institute, Utrecht University, PO Box 80 000, 3508 TA Utrecht, The Netherlands e-mail: marle@udel.edu
² Bartol Research Institute, University of Delaware, 102 Sharp Laboratory, Newark, 19716, Delaware, USA
³ Instituto de Astronomía-UNAM, APDO Postal 877, Ensenada, 22800 Baja California, Mexico

Received: 31 January 2007
Accepted: 8 April 2007

Abstract

Context.Wolf-Rayet stars are thought to be the progenitors of type Ib/c supernovae and of long gamma-ray bursts.

Aims.As shown by van Marle et al. (2005, A&A, 444, 837, Paper I), circumstellar absorption lines in early type Ib/c supernova and gamma-ray burst afterglow spectra may reveal the progenitor evolution of the exploding Wolf-Rayet star. While the quoted paper deals with Wolf-Rayet stars which evolved through a red supergiant stage, we investigate here the initially more massive Wolf-Rayet stars which are thought to evolve through a Luminous Blue Variable (LBV) stage.

Methods.We perform hydrodynamic simulations of the evolution of the circumstellar medium around a 60  star, from the main sequence through the LBV and Wolf-Rayet stages, up to core collapse. We then compute the column density of the circumstellar matter along rays from the central light source to an observer at infinity, as a function of radial velocity, time and angle. This allows a comparison with the number and velocities, or blue-shifts, of absorption components in the spectra of LBVs, Wolf-Rayet stars, type Ib/c supernovae and gamma-ray burst afterglows.

Results.Our simulation for the post-LBV stage shows the formation of various absorption components. In particular, shells with velocities in the range of 100 km s^-1 to 1200 km s^-1 are formed at the beginning of the Wolf-Rayet stage, which are, however, rather short lived; they dissipate on time scales shorter than 50 000 yr. As the LBV stage is thought to occur at the beginning of core helium burning, the remaining Wolf-Rayet life time is expected to be one order of magnitude larger.

Conclusions.When interpreting the absorption components in the afterglow spectrum of GRB 021004 as circumstellar, it can be concluded that the progenitor of this source did most likely not evolve through an LBV stage, as no intermediate velocity absorption components are predicted to prevail until core collapse. However, a close binary with a late common-envelope phase (Case C) may produce a circumstellar medium that closely resembles the LBV to Wolf-Rayet evolution, but with a much shorter Wolf-Rayet period. This scenario can not be ruled out.