The variability of Betelgeuse explained by surface convection

Abstract

Betelgeuse is a red supergiant (RSG) that is known to vary semi-regularly on both short and long timescales. The origin of the short period of Betelgeuse has often been associated with radial pulsations, but could also be due to the convection motions present at the surface of RSGs. We investigate the link between surface activity and the variability of the star. Linear polarization in Betelgeuse is a proxy of convection that is unrelated to pulsations. Using ten years of spectropolarimetric data of Betelgeuse, we looked for periodicities in the least-squares deconvolution profiles of Stokes $I$, $Q$, $U$ and the total linear polarization using Lomb-Scargle periodograms. We find periods in linear polarization signals that are similar to those in photometric variability. The 400 d period is too close to a peak of the window function of our data, but the two periods of 330 d and 200 d are present in the periodogram of Stokes $Q$ and $U$, showing that the variability of Betelgeuse can be interpreted as being due to surface convection. Since the linear polarization in the spectrum of Betelgeuse is not known to vary with pulsations, but is linked to surface convection, and since similar periods are found in the time series of photometric measurements and spectropolarimetry, we conclude that the photometric variability is due to the surface convective structures, and not to any pulsation phenomenon.