Search for magnetic fields in particle-accelerating colliding-wind binaries

TL;DR

This study used spectropolarimetric data to search for stellar magnetic fields in particle-accelerating colliding-wind binaries, finding no strong fields but setting upper limits on possible weak magnetic fields.

Contribution

First quantitative constraints on stellar magnetic fields in PACWB, informing future models and ruling out strong or moderate fields in these systems.

Findings

No magnetic signatures detected in any of the 9 PACWB stars.

Upper limit of ~200 G for polar magnetic field strength in several stars.

Strong or moderate magnetic fields typical of magnetic massive stars are excluded.

Abstract

Some colliding-wind massive binaries, called particle-accelerating colliding-wind binaries (PACWB), exhibit synchrotron radio emission, which is assumed to be generated by a stellar magnetic field. However, no measurement of magnetic fields in these stars has ever been performed. We aim at quantifying the possible stellar magnetic fields present in PACWB to provide constraints for models. We gathered 21 high-resolution spectropolarimetric observations of 9 PACWB available in the ESPaDOnS, Narval and HarpsPol archives. We analysed these observations with the Least Squares Deconvolution method. We separated the binary spectral components when possible. No magnetic signature is detected in any of the 9 PACWB stars and all longitudinal field measurements are compatible with 0 G. We derived the upper field strength of a possible field that could have remained hidden in the noise of the data. While the data are not very constraining for some stars, for several stars we could derive an upper limit of the polar field strength of the order of 200 G. We can therefore exclude the presence of strong or moderate stellar magnetic fields in PACWB, typical of the ones present in magnetic massive stars. Weak magnetic fields could however be present in these objects. These observational results provide the first quantitative constraints for future models of PACWB.