Mass motions in the atmospheres of red giants in the globular clusters M55 and M13

Abstract

We present Na D and Hα stellar line profiles for a sample of 19 stars in M55 and M13, in a search for spectral features indicative of mass motions in the atmospheres of red giants. We combine the observations of the stars in these two clusters with those made previously for stars in ω Cen, M22 and M4 to give a total sample of 63 globular cluster red giants. From this total sample we deduce that only stars with a luminosity above log(L/L⊙)≈2.8 show Hα emission. The luminosity limit for the onset of significant Na D coreshift is log(L/L⊙)≈2.9, while that for significant Hα coreshifts is ≈0.4 dex lower at log(L/L⊙)≈2.5, and this therefore represents the threshold for the first detectable mass-motion phenomena as the star evolves up the giant branch. By comparing our results with those in the literature, time variation of the strength of Hα emission is observed for two stars, while five stars display variations in Na D coreshifts over periods greater than six months.