Discovery of a magnetic field in the O9 sub-giant star HD 57682 by the MiMeS Collaboration

TL;DR

This paper reports the discovery of a strong magnetic field in the O9IV star HD 57682, using spectropolarimetric data, and analyzes its stellar and magnetic properties, revealing a magnetically confined wind.

Contribution

First detection of a magnetic field in HD 57682, providing detailed measurements of its stellar parameters and magnetic field characteristics using spectropolarimetry and Bayesian analysis.

Findings

Magnetic field strength of approximately 1680 G

Star has a low mass-loss rate of about 1.4×10^{-9} M_sun/yr

Magnetic field strongly confines the stellar wind

Abstract

We report the detection of a strong, organised magnetic field in the O9IV star HD 57682, using spectropolarimetric observations obtained with ESPaDOnS at the 3.6-m Canada-France-Hawaii Telescope within the context of the Magnetism in Massive Stars (MiMeS) Large Program. From the fitting of our spectra using NLTE model atmospheres we determined that HD 57682 is a $17^{+19}_{-9}$ M$_{\odot}$ star with a radius of $7.0^{+2.4}_{-1.8}$ R$_\odot$, and a relatively low mass-loss rate of $1.4^{+3.1}_{-0.95}\times10^{-9}$ M$_{\odot}$ yr$^{-1}$. The photospheric absorption lines are narrow, and we use the Fourier transform technique to infer $v\sin i=15\pm3$ km s$^{-1}$. This $v\sin i$ implies a maximum rotational period of 31.5 d, a value qualitatively consistent with the observed variability of the optical absorption and emission lines, as well as the Stokes $V$ profiles and longitudinal field. Using a Bayesian analysis of the velocity-resolved Stokes $V$ profiles to infer the magnetic field characteristics, we tentatively derive a dipole field strength of $1680^{+134}_{-356}$ G. The derived field strength and wind characteristics imply a wind that is strongly confined by the magnetic field.