Searching for a magnetic field in Wolf-Rayet stars using FORS2 spectropolarimetry

TL;DR

This study used spectropolarimetry with ESO FORS2 to detect and analyze magnetic fields in Wolf-Rayet stars, revealing evidence of rotation-modulated magnetic variability and measuring fields up to a few hundred Gauss.

Contribution

First spectropolarimetric detection of magnetic fields in Wolf-Rayet stars, including monitoring of variability linked to stellar rotation.

Findings

Magnetic field detected in WR6 at 3.3 sigma significance.

LMC star BAT99 7 shows a magnetic field of 327+-141 G.

WR6 exhibits sinusoidal magnetic variability consistent with rotation.

Abstract

To investigate if magnetic fields are present in Wolf-Rayet stars, we selected a few stars in the Galaxy and one in the Large Magellanic Cloud (LMC). We acquired low-resolution spectropolarimetric observations with the ESO FORS2 instrument during two different observing runs. During the first run in visitor mode, we observed the LMC Wolf-Rayet star BAT99 7 and the stars WR6, WR7, WR18, and WR23 in our Galaxy. The second run in service mode was focused on monitoring the star WR6. Linear polarization was recorded immediately after the observations of circular polarization. During our visitor observing run, the magnetic field for the cyclically variable star WR6 was measured at a significance level of 3.3sigma (<B_z> = 258+-78G). Among the other targets, the highest value for the longitudinal magnetic field, <B_z> = 327+-141G, was measured in the LMC star BAT99 7. Spectropolarimetric monitoring of the star WR6 revealed a sinusoidal nature of the <B_z> variations with the known rotation period of 3.77d, significantly adding to the confidence in the detection. The presence of the rotation-modulated magnetic variability is also indicated in our frequency periodogram. The reported field magnitude suffers from significant systematic uncertainties at the factor 2 level, in addition to the quoted statistical uncertainties, owing to the theoretical approach used to characterize it. Linear polarization measurements showed no line effect in the stars, apart from WR6. BAT99 7, WR7, and WR,23 do not show variability of the linear polarization over two nights.