Capture of field stars by globular clusters in dense bulge regions

Abstract

The recent detection of a double red giant branch (RGB) in the optical color-magnitude diagram of the bulge globular cluster HP 1—a more populated, metal-poor, steep giant branch corresponding to the cluster itself and another that is metal rich and curved—led us to explore in the present Letter the possibility that field stars are captured by a globular cluster orbiting in dense bulge regions over several gigayears. Analytical arguments, as well as N-body calculations for a cluster model of 105 Mʘ in a bulgelike environment, suggest that a significant fraction of cluster stars may consist of captures. Metal-poor globular clusters in the inner bulge, like HP 1, contrasting in Δ[Fe/H] by at least 1.0 dex with respect to the surrounding metal-rich stars, are ideal probes to further test the capture scenario. In turn, if this scenario is confirmed, the double RGB of HP 1 could provide direct estimates of blanketing amounts, which is fundamental for the photometric calibration of metal-rich stellar populations.