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

¹ 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 . 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  density profile normally associated with stellar winds close to the star.