The search for magnetic fields in two Wolf-Rayet stars and the discovery of a variable magnetic field in WR55

TL;DR

This study investigates magnetic fields in Wolf-Rayet stars, detecting a variable magnetic field in WR55 and suggesting possible variability in WR46 related to stellar rotation, advancing understanding of magnetic phenomena in evolved massive stars.

Contribution

First detection of a variable magnetic field in WR55, providing new insights into magnetic activity in Wolf-Rayet stars and their environments.

Findings

Variable magnetic field detected in WR55 of a few hundred Gauss.

No significant field detected in WR46, but variability suggests possible rotation-related changes.

Magnetic field detection linked to star formation environment in WR55.

Abstract

Magnetic fields in Wolf-Rayet (WR) stars are not well explored, although there is indirect evidence, e.g. from spectral variability and X-ray emission, that magnetic fields should be present in these stars. Being in an advanced stage of their evolution, WR stars have lost their hydrogen envelope, but their dense winds make the stellar core almost unobservable. To substantiate the expectations on the presence of magnetic fields in the most-evolved massive stars, we selected two WR stars, WR46 and WR55, for the search of the presence of magnetic fields using FORS2 spectropolarimetric observations. We achieve a formally definite detection of a variable mean longitudinal magnetic field of the order of a few hundred Gauss in WR55. The field detection in this star, which is associated with the ring nebula RCW78 and the molecular environment, is of exceptional importance for our understanding of star formation. No field detection at a significance level of 3sigma was achieved for WR46, but the variability of the measured field strengths can be rather well phased with the rotation period of 15.5h previously suggested by FUSE observations.