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

TL;DR

This study models the circumstellar environment of a rapidly rotating, chemically homogeneous massive star, revealing unique non-spherical features and predicting observable signatures in gamma-ray burst afterglows.

Contribution

It provides the first detailed 2D hydrodynamical simulations of the circumstellar medium around such a star, highlighting significant deviations from standard wind profiles due to rapid rotation.

Findings

Circumstellar medium shows strong non-spherical features near the star.

Predicted multiple blue-shifted high velocity absorption components in GRB afterglows.

Deviations from the typical 1/R^2 density profile in polar directions.

Abstract

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