Mass-loss on the red giant branch: the value and metallicity dependence of Reimers’ η in globular clusters

McDonald, I. and Zijlstra, A. A. (2015). Mass-loss on the red giant branch: the value and metallicity dependence of Reimers’ η in globular clusters. Monthly Notices of the Royal Astronomical Society, 448(1) pp. 502–521.

DOI: https://doi.org/10.1093/mnras/stv007

Abstract

The impact of metallicity on the mass-loss rate from red giant branch (RGB) stars is studied through its effect on the parameters of horizontal branch (HB) stars. The scaling factors from Reimers and Schröder and Cuntz are used to measure the efficiency of RGB mass-loss for typical stars in 56 well-studied Galactic globular clusters (GCs). The median values among clusters are, respectively, | $\eta _{\rm R} = 0.477 \pm 0.070 ^{+0.050}_{-0.062}$ | and | $\eta _{\rm SC} = 0.172 \pm 0.024 ^{+0.018}_{-0.023}$ | (standard deviation and systematic uncertainties, respectively). Over a factor of 200 in iron abundance, η varies by ≲30 per cent, thus mass-loss mechanisms on the RGB have very little metallicity dependence. Any remaining dependence is within the current systematic uncertainties on cluster ages and evolution models. The low standard deviation of η among clusters (≈14 per cent) contrasts with the variety of HB morphologies. Since η incorporates cluster age, this suggests that age accounts for the majority of the ‘second parameter problem’, and that a Reimers-like law provides a good mass-loss model. The remaining spread in η correlates with cluster mass and density, suggesting helium enrichment provides the third parameter explaining HB morphology of GCs. We close by discussing asymptotic giant branch (AGB) mass-loss, finding that the AGB tip luminosity is better reproduced and η has less metallicity dependence if GCs are more co-eval than generally thought.