Mass Loss on the Red Giant Branch: Plasmoid-driven Winds above the RGB Bump

Abstract

Abstract The onset of cool massive winds in evolved giants is correlated with an evolutionary feature on the red giant branch (RGB) known as the “bump.” Also at the bump, shear instability in the star leads to magnetic fields that occur preferentially on small length-scales. Pneuman has suggested that the emergence of small-scale flux tubes in the Sun can give rise to enhanced acceleration of the solar wind as a result of plasmoid acceleration (the so-called “melon-seed mechanism”). In this paper, we examine Pneuman’s formalism to determine if it may shed some light on the process that drives mass loss in stars above the RGB bump. Because we do not currently have detailed information for some of the relevant physical parameters, we are not yet able to derive a detailed model: instead, our goal in this paper is to explore a “proof of concept.” Using parameters that are known to be plausible in cool giants, we find that the total mass-loss rate from such stars can be replicated. Moreover, we find that the radial profile of the wind speed in such stars can be steep or shallow depending on the fraction of the mass-loss rate that is contained in the plasmoids: this is consistent with empirical data that indicate that the velocity profiles of winds from cool giants span a range of steepnesses.