A&A 478, 769-778 (2008)
DOI: 10.1051/0004-6361:20078802

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

A. J. van Marle^{1, 2}, N. Langer¹, S.-C. Yoon³, and G. García-Segura<sup>4

1</sup>  Astronomical Institute, Utrecht University, PO Box 80000, 3508 TA, Utrecht, The Netherlands
    e-mail: [A.vanMarle;N.Langer]@astro.uu.nl
²  Bartol Research Institute, University of Delaware, 102 Sharp Laboratory, Newark, 19716 DE, Delaware, USA
    e-mail: marle@udel.edu
³  Astronomical Institute "Anton Pannekoek", University of Amsterdam, Kruislaan 403, 1098 SJ, Amsterdam, The Netherlands
    e-mail: scyoon@science.uva.nl
⁴  Instituto de Astronomía-UNAM, APDO Postal 877, Ensenada, 22800 Baja California, Mexico
    e-mail: ggs@astrosen.unam.mx

(Received 5 October 2007 / Accepted 30 October 2007)

Abstract
Context.Rapidly rotating, chemically homogeneously evolving massive stars are considered to be progenitors of long gamma-ray bursts.
Aims.We present numerical simulations of the evolution of the circumstellar medium around a rapidly rotating 20  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  would not evolve into a Wolf-Rayet stage, the considered model evolves from the main sequence directly to the helium main sequence.
Methods.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.
Results.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.
Conclusions.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/R² density profile normally associated with stellar winds close to the star.

Key words: gamma-rays: bursts -- hydrodynamics -- ISM: bubbles -- stars: winds, outflows -- stars: Wolf-Rayet

© ESO 2008

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