Mass loss on the red giant branch: the value and metallicity dependence of Reimers' {\eta} in globular clusters

TL;DR

This study investigates how metallicity influences mass loss in red giant stars within globular clusters, finding minimal dependence and highlighting the roles of age, cluster mass, and helium enrichment in stellar evolution.

Contribution

It provides new estimates of Reimers' eta parameter across multiple clusters and demonstrates its weak metallicity dependence, emphasizing age and helium enrichment as key factors.

Findings

Mass-loss rate parameter eta varies by less than 30% over a 200-fold metallicity range.

Cluster age largely explains the second parameter problem in HB morphology.

Helium enrichment correlates with residual variations in eta, influencing HB morphology.

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 (1975) and Schroder & Cuntz (2005) are determined for 56 well-studied Galactic globular clusters (GCs). The median values among clusters are, respectively, {\eta}_R = 0.477 +/- 0.070 (+0.050/-0.062) and {\eta}_SC = 0.172 +/- 0.024 (+0.018/-0.023) (standard deviation and systematic uncertainties, respectively). Mass-loss mechanisms on the RGB have very little metallicity dependence: over a factor of 200 in iron abundance, {\eta} varies by <~30 per cent, within the current systematic uncertainties on cluster ages and evolution models. Since {\eta} incorporates cluster age, the low standard deviation of {\eta} among clusters (~14 per cent) suggests that age can almost entirely account for the "second parameter problem". The remaining spread in {\eta} correlates with cluster mass and density, suggesting helium enrichment provides the third parameter explaining HB morphology of GCs. The metallicity variation is reduced further if globular clusters are more co-eval than generally thought. This would also better reproduce the observed AGB tip luminosities, which are not well modelled by extrapolating the RGB {\eta} to later evolutionary epochs.