Models for the circumstellar medium of long gamma-ray burst progenitor candidates

Abstract

Long gamma-ray bursts are highly energetic events that are thought to occur when certain massive stars, that end their lives as Wolf-Rayet stars, collapse at the end of their evolution. We present models of the circumstellar medium around those massive stars that are thought to be possible progenitors of long gamma-ray bursts. During their evolution such stars lose a significant amount of mass in the form of stellar wind, a flow of low density material that leaves the star at high velocity. This stellar wind creates a bubble around the star. We make computational hydrodynamical models of the interactions between the stellar wind and the surrounding medium and between the different phases of the stellar wind. The results of our simulations can be compared to the observations of gamma-ray bursts in order to investigate which massive stars are in fact the progenitors of long gamma-ray bursts. In the afterglows of some gamma-ray bursts, absorption features appear that are blue-shifted relative to the source, indicating that matter is moving away from the gamma-ray burst progenitor at discrete velocities. We conclude that this is the result of hydrodynamical interactions in the circumstellar medium that result from changes in the stellar wind. This leads us to the conclusion that these stars had only a short final Wolf-Rayet phase since otherwise the blue-shifted absorption features would have disappeared. Observations of other gamma-ray burst afterglows indicate that the surrounding medium has a constant density, rather than a profile corresponding to a stellar wind. Our models show hat this can be explained by several external influences, combined with a comparatively weak stellar wind. This indicates that these stars most likely had a low initial metallicity.