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

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.