Intrinsic polarization of Wolf-Rayet stars due to the rotational modulation of the stellar wind

TL;DR

This study models the intrinsic polarization of Wolf-Rayet stars caused by rotational wind modulation, providing an indirect method to estimate their rotational velocities relevant for gamma-ray burst progenitors.

Contribution

It introduces hydrodynamical wind models to predict polarization levels, linking stellar rotation with observable polarization signals in Wolf-Rayet stars.

Findings

Hydrodynamical models predict prolate wind structures with low polarization levels.

Even high rotational velocities are consistent with observational constraints.

Wind structures are similar to those in optically thin wind models.

Abstract

Fast rotating Wolf-Rayet stars are expected to be progenitors of long duration gamma-ray bursts. However, the observational test of this model is problematic. Spectral lines of Wolf-Rayet stars originate in expanding stellar wind, therefore a reliable spectroscopical determination of their rotational velocities is difficult. Intrinsic polarization of Wolf-Rayet stars due to the rotational modulation of the stellar wind may provide an indirect way to determine the rotational velocities of these stars. However, detailed wind models are required for this purpose. We determine the intrinsic polarization of Wolf-Rayet stars from hydrodynamical wind models as a function of rotational velocity. We used 2.5D hydrodynamical simulations to calculate the structure of rotating winds of Wolf-Rayet stars. The simulations account for the deformation of the stellar surface due to rotation, gravity darkening, and nonradial forces. From the derived models, we calculated the intrinsic stellar polarization. The mass loss rate was scaled to take realistic wind densities of Wolf-Rayet stars into account. The hydrodynamical wind models predict a prolate wind structure, which leads to a relatively low level of polarization. Even relatively large rotational velocities are allowed by observational constrains. The obtained wind structure is similiar to that obtained previously for rotating optically thin winds. Derived upper limits of rotational velocities of studied Wolf-Rayet stars are not in conflict with the model of long duration gamma-ray bursts