Instabilities in the envelope of stars

Abstract

Stars shine, id est they release energy from their surfaces. This energy is mostly provided by thermonuclear fusion in their cores, and has thus to be transported through the whole star. Especially in the low density outer envelope where recombination of heavy elements leads to increased opacities, stars may struggle transporting their luminosity outwards. This can give rise to instabilities such as convection and pulsations. We use 1D hydrodynamic stellar structure calculations to investigate the conditions in the outer layers of stars, linking these to the appearance of observational phenomena such as surface motions, broadening of the absorption lines, and inhomogeneities in the outflows. Our results point to the importance of the effects of turbulent motion in the outer envelope, especially in stars close to their Eddington limit for which convection is not an efficient energy transport mechanism but rather an excitation mechanism for waves. As our knowledge of these phenomena and their origin increases, we can better derive stellar parameters such as the rotational velocities and the stellar wind mass-loss rates, and improve our understanding of the physics in the low density radiation pressure dominated layers close to the Eddington limit, which are key for the evolution of massive stars towards their final fate.