The circumstellar medium around a rapidly rotating, chemically homogeneously evolving, possible gamma-ray burst progenitor

Allard Jan van Marle(1,2),
Norbert Langer(1),
Sung-Chul Yoon(3), and
Guillermo Garcia-Segura(4)

1- Astronomical institute, University of Utrecht;
2- Bartol Research Institute, University of Delaware;
3- Astronomical Institute "Anton Pannekoek",University of Amsterdam;
4- Instituto de Astronomia-UNAM

Rapidly rotating, chemically homogeneously evolving massive stars are considered to be progenitors of long gamma-ray bursts.
We present numerical simulations of the evolution of the circumstellar medium around a rapidly rotating 20 Msol star at a metallicity of Z=0.001. Its rotation is fast enough to produce quasi-chemically homogeneous evolution. While conventionally, a star of 20 Msol would not evolve into a Wolf-Rayet stage, the considered model evolves from the main sequence directly to the helium main sequence.
We use the time-dependent wind parameters, such as mass loss rate, wind velocity and rotation-induced wind anisotropy from the evolution model as input for a 2D hydrodynamical simulation.
While the outer edge of the pressure-driven circumstellar bubble is spherical, the
circumstellar medium close to the star shows strong non-spherical features during and after the periods of near-critical rotation.
We conclude that the circumstellar medium around rapidly rotating massive stars differs considerably from the surrounding material of non-rotating stars of similar mass. Multiple blue-shifted high velocity absorption components in gamma-ray burst afterglow spectra are predicted. As a consequence of near critical rotation and short stellar evolution time scales during the last few thousand years of the star's life, we find a strong deviation of the circumstellar density profile in the polar direction from the 1/R^2 density profile normally associated with stellar winds close to the star.

Reference: Accepted for publication in Astronomy & Astrophysics
Status: Manuscript has been accepted