Rotating Wolf-Rayet stars in a post RSG/LBV phase

An evolutionary channel towards long-duration GRBs?^⋆

¹ Armagh Observatory, College Hill, Armagh BT61 9DG, UK
² School of Physics and Astronomy, University of Exeter, Stocker Rd, Exeter EX4 4QL, UK
³ Argelander-Institut für Astronomie der Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany

Received: 16 December 2011
Accepted: 3 October 2012

Abstract

Context. Wolf-Rayet (WR) stars with fast rotating cores are thought to be the direct progenitors of long-duration gamma-ray bursts (LGRBs). A well accepted evolutionary channel towards LGRBs is chemically-homogeneous evolution at low metallicities, which completely avoids a red supergiant (RSG), or luminous blue variable (LBV) phase. On the other hand, strong absorption features with velocities of several hundred km s^-1 have been found in some LGRB afterglow spectra (GRB 020813 and GRB 021004), which have been attributed to dense circumstellar (CS) material that has been ejected in a previous RSG or LBV phase, and is interacting with a fast WR-type stellar wind.

Aims. Here we investigate the properties of Galactic WR stars and their environment to identify similar evolutionary channels that may lead to the formation of LGRBs.

Methods. We compile available information on the spectropolarimetric properties of 29 WR stars, the presence of CS ejecta for 172 WR stars, and the CS velocities in the environment of 34 WR stars in the Galaxy. We use linear line-depolarization as an indicator of rotation, nebular morphology as an indicator of stellar ejecta, and velocity patterns in UV absorption features as an indicator of increased velocities in the CS environment.

Results. Based on previous nebular classifications, we determine an incidence rate of  ~23% of WR stars with “possible ejecta nebulae” in the Galaxy. We find that this group of objects dominates the population of WR stars with spectropolarimetric signatures of rotation, while WR stars without such nebulae only rarely show indications of rotation. This confirms the correlation between rotation and CS ejecta from our previous work. The corresponding objects are most likely in an early stage after a preceding RSG or LBV phase, and have not yet lost their angular momenta due to the strong mass-loss in the WR phase. From their photometric periods we estimate rotation parameters in the range ω = ν_rot/ν_crit = 0.04...0.25, corresponding to moderate rotation speeds of 36...120 km s^-1. These values are very uncertain, but comply with the specific surface angular momentum requirement for LGRB progenitors. From UV absorption profiles we only find weak evidence for a correlation between rotation and increased CS velocities. In particular, the CS velocities of Galactic WR stars are much lower than what is observed for GRB 020813 and GRB 021004.

Conclusions. Our results indicate that, in the Galaxy, “young” WR stars shortly after a RSG/LBV phase, show spectropolarimetric signatures of rotation. Their rotation rates are likely to be enhanced with respect to the majority of Galactic WR stars. According to their estimated specific surface angular momenta, a subgroup of stars exploding in this phase may represent an evolutionary channel towards LGRBs at high metallicities, comparable to the Galaxy. Although the UV absorption features in our sample turn out to be different from those observed in GRB 020813 and GRB 021004, it is interesting that for three WR stars with signatures of rotation, UV absorptions have previously been attributed to extended CS structures. The large size of these structures (r ~ 100 pc) can account for the observed stability of the absorbing material in LGRB afterglows against ionizing radiation from the GRB itself. This may resolve a fundamental problem with the interpretation of the afterglow features as CS material.