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Stellar-Encounter Driven Red-Giant Star Mass-Loss in Globular Clusters

Globular Cluster (GC) Color-Magnitude Diagrams (CMDs) are reasonably well understood in terms of standard stellar-evolution. However, there are still some open issues, such as fully accounting for the Horizontal Branch (HB) morphology in terms of chemical and dynamical parameters. Mass-loss on the Red Giant Branch (RGB) shapes the mass-distribution of the HB stars, and the color distribution in turn. The physical mechanisms driving mass-loss are still unclear, as direct observations fail to reveal a clear correlation between mass-loss rate and stellar properties. The horizontal-branch mass-distribution is further complicated by Helium-enhanced multiple stellar populations, because of differences in the evolving mass along the HB. We present a simple analytical mass-loss model, based on tidal stripping through Roche-Lobe OverFlow (RLOF) during stellar encounters. Our model naturally results in a non-gaussian mass-loss distribution, with high skewness, and contains only two free parameters. We fit it to the HB mass distribution of four Galactic GCs, as obtained from fitting the CMD with Zero Age HB (ZAHB) models. The best-fit model accurately reproduces the observed mass-distribution. If confirmed on a wider sample of GCs, our results would account for the effects of dynamics in RGB mass-loss processes and provide a physically motivated procedure for synthetic CMDs of GCs. Our physical modeling of mass-loss may result in the ability to disentangle the effects of dynamics and helium-enhanced multiple-populations on the HB morphology and is instrumental in making HB morphology a probe of the dynamical state of GCs, leading to an improved understanding of their evolution.