Multiple stellar populations along the red Horizontal Branch and Red Clump of Globular Clusters

Abstract

We exploit multi-band Hubble Space Telescope photometry to investigate multiple populations (MPs) along the red horizontal branches (HBs) and red clumps of fourteen metal-rich Globular Clusters (GCs), including twelve Milky Way GCs and the Magellanic Cloud GCs NGC 1978 and NGC 416. Based on appropriate two-color diagrams we find that the fraction of 1G stars in Galactic GCs correlates with cluster mass, confirming previous results based on red-giant branch (RGB) stars. Magellanic-Cloud GCs show higher fractions of 1G stars than Galactic GCs with similar masses, thus suggesting that the environment affects the MP phenomenon. We compared and combined our population fractions based on HB with previous estimates from MS and RGB, and we used ground-based UBVI photometry (available for NGC 104, NGC 5927, NGC 6366, NGC 6838) to extend the investigation over a wide field of view. All studied GCs are consistent with flat distributions of 1G and 2G stars within ~1 arcmin from the cluster center except NGC 416, where the 2G is more centrally concentrated. 2G stars of NGC 104 and NGC 5927 are more centrally-concentrated than the 1G, whereas the distribution is flat for NGC 6366 and NGC 6838. We discover that most of the analyzed GCs exhibit extended sequences of 1G stars along the red HB, not consistent with a simple population. The comparison between appropriate synthetic and observed CMDs reveals that these extended distributions are consistent with either star-to-star variation in helium or with an internal metallicity spread, recalling the inhomogeneity of 1G stars along the ChMs.