Radial velocity variations in red giant stars : pulsations, spots and planets

Abstract

Radial velocity variations in K giants are observed on long (order hundreds of days) and short (order hours) time scales. Short term variations are most likely caused by solar-like oscillations, stochastically excited in the turbulent atmosphere of the stars. Long term variations can be caused by either sub-stellar companions, pulsations or other mechanisms intrinsic to the star. The possible cause of observed long radial velocity variations in K giants is investigated with some different diagnostics. First, a strong correlation between surface gravity and the radial velocity amplitude is found. This indicates that it is likely that the radial velocity variations are induced by a mechanism intrinsic to the star. In addition, for a number of stars the radial velocity variations can be fit with a Keplerian orbit, indicating the possible presence of a sub-stellar companion. The orbital parameters of these inferred sub-stellar companions would be considerably different from those orbiting main sequence stars. But, there is growing evidence that more massive stars harbour more and more massive sub-stellar companions. In order to reveal the mechanism causing the radial velocity variations in individual stars, temperature and line depth variations at different epochs for a sub-sample of K giants are measured.