The Horizontal Branch of NGC 1851: constraints on the cluster subpopulations

We investigate the distribution of stars along the Horizontal Branch of the Galactic globular cluster NGC 1851, to shed light on the progeny of the two distinct Subgiant Branch populations harbored by this cluster. On the basis of detailed synthetic Horizontal Branch modelling, we conclude that the two subpopulations are distributed in different regions of the observed Horizontal Branch: the evolved stars belonging to the bright Subgiant branch component are confined in the red portion of the observed sequence, whereas the ones belonging to the faint Subgiant branch component are distributed from the blue to the red, populating also the RR Lyrae instability strip. Our simulations strongly suggest that it is not possible to reproduce the observations assuming that the two subpopulations lose the same amount of mass along the Red Giant Branch. We warmly encourage empirical estimates of mass loss rates in Red Giant stars belonging to this cluster. Subject headings: globular clusters: individual (NGC 1851) — Hertzsprung-Russell diagram – stars: horizontal branch – stars: mass loss 1.