Astronomy and Astrophysics, volume 547A, 83-83 (2012/11-1)
Rotating Wolf-Rayet stars in a post RSG/LBV phase. An evolutionary channel towards long-duration GRBs?
GRAEFENER G., VINK J.S., HARRIES T.J. and LANGER N.
Abstract (from CDS):
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 k/s have been found in some LGRB afterglow spectra (GRB020813 and GRB021004), 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. 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. 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. 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=νrot/νcrit=0.04...0.25, corresponding to moderate rotation speeds of 36...120k/s. 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 GRB020813 and GRB021004. 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 GRB020813 and GRB021004, 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∼100pc) 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.
stars: Wolf-Rayet - stars: rotation - stars: winds, outflows - stars: evolution - Gamma-ray burst: general - polarization
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<Available at CDS (J/A+A/547/A83): table4.dat table5.dat>
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