Revealing the Asymmetry of the Wind of the Variable Wolf-Rayet Star WR1 (HD4004) Through Spectropolarization

TL;DR

This study uses spectropolarimetric data to analyze the wind asymmetry of Wolf-Rayet star WR1, suggesting the presence of Corotating Interaction Regions (CIRs) as the cause of observed polarization features, indicating recent evolutionary transition.

Contribution

It provides the first spectropolarimetric evidence supporting CIRs in WR1's wind and links polarization features to recent WR phase onset.

Findings

Intrinsic polarization of WR1 estimated at 0.443%

Depolarization observed across spectral lines

CIRs likely cause of wind asymmetry

Abstract

In this paper, high quality spectropolarimetric observations of the Wolf-Rayet (WR) Star WR1 (HD4004) obtained with ESPaDOnS at CFHT are presented. All major emission lines present in the spectrum show depolarization in the relative Stokes parameters Q/I and U/I. From the behaviour of the amount of line depolarization as a function of line strength, the intrinsic continuum light polarization of WR1 is estimated to be P/I=0.443 \pm 0.028 % with an angle of \theta=-26.2^o. Although such a level of polarization could in principle be caused by a wind flattened by fast rotation, the scenario in which it is a consequence of the presence in the wind of Corotating Interaction Regions (CIRs) is preferred. This is supported by previous photometric and spectroscopic observations showing periodic variations with a period of 16.9 days. This is now the third WR star thought to exhibit CIRs in its wind that is found to have line depolarization. Previous authors have found a strong correlation between line depolarization and the presence of an ejected nebula, which they interpret as a sign that the star has reached the WR phase relatively recently since the nebula are thought to dissipate very fast. In the cases where the presence of CIRs in the wind is favoured to explain the depolarization across spectral lines, the above mentioned correlation may be indicating that those massive stars have only very recently transited from the previous evolutionary phase to the WR phase.