Magnetic field and convection in the cool supergiant Betelgeuse

TL;DR

This study detects and analyzes magnetic fields in Betelgeuse, revealing a variable longitudinal magnetic component correlated with stellar convection and radial velocity fluctuations, indicating magnetic activity in the star's convective zones.

Contribution

First detection of magnetic fields in Betelgeuse using spectropolarimetry, showing their variability and association with convective motions.

Findings

Magnetic field strength varies from 0.5 to 1.5 G

Magnetic signatures are asymmetrical and red-shifted

Magnetic elements may be concentrated in sinking convective flows

Abstract

We present the outcome of a highly-sensitive search for magnetic fields on the cool supergiant Betelgeuse. A time-series of six circularly-polarized spectra was obtained using the NARVAL spectropolarimeter at T\'elescope Bernard Lyot (Pic du Midi Observatory), between 2010 March and April. Zeeman signatures were repeatedly detected in cross-correlation profiles, corresponding to a longitudinal component of about 1 G. The time-series unveils a smooth increase of the longitudinal field from 0.5 to 1.5 G, correlated with radial velocity fluctuations. We observe a strong asymmetry of Stokes V signatures, also varying in correlation with the radial velocity. The Stokes V line profiles are red-shifted by about 9 km/s with respect to the Stokes I profiles, suggesting that the observed magnetic elements may be concentrated in the sinking components of the convective flows.