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

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.Comment: 27 pages, 20 figures, 5 tables, accepted for pubblication in Ap

Hubble Space Telescope survey of Magellanic Cloud star clusters: photometry and astrometry of 113 clusters and early results

In the past few years, we have undertaken an extensive investigation of star clusters and their stellar populations in the Large and Small Magellanic Clouds (LMC and SMC) based on archival images collected with the Hubble Space Telescope. We present photometry and astrometry of stars in 101 fields observed with the Wide Field Channel of the Advanced Camera for Surveys and the Ultraviolet and Visual Channel and the Near-Infrared Channel of Wide Field Camera 3. These fields comprise 113 star clusters. We provide differential-reddening maps for those clusters with significant reddening variations across the field of view. We illustrate various scientific outcomes that arise from the early inspection of the photometric catalogs. In particular, we provide new insights into the extended main-sequence turnoff (eMSTO) phenomenon: (i) We detected eMSTOs in two clusters, KMHK 361 and NGC 265, which had no previous evidence of multiple populations. This finding corroborates the conclusion that the eMSTO is a widespread phenomenon among clusters younger than ~2 Gyr. (ii) The homogeneous color-magnitude diagrams (CMDs) of 19 LMC clusters reveal that the distribution of stars along the eMSTO depends on cluster age. (iii) We discovered a new feature along the eMSTO of NGC 1783, which consists of a distinct group of stars on the red side of the eMSTO in CMDs composed of UV filters. Furthermore, we derived the proper motions of stars in the fields of view of clusters with multi-epoch images. Proper motions allowed us to separate the bulk of bright field stars from cluster members and investigate the internal kinematics of stellar populations in various LMC and SMC fields. As an example, we analyze the field around NGC 346 to disentangle the motions of its stellar populations, including NGC 364 and BS 90, young and pre-main-sequence stars in the star-forming region associated with NGC 346, and young and old field stellar populations of the SMC. Based on these results and the fields around five additional clusters, we find that young SMC stars exhibit elongated proper-motion distributions that point toward the LMC, thus providing new evidence for a kinematic connection between the LMC and SMC.We thank the anonymous referee for various suggestions that improved the quality of the manuscript. This work has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research innovation programme (Grant Agreement ERC-StG 2016, No. 716082 ’GALFOR’, PI: Milone, http://progetti.dfa.unipd. it/GALFOR) and from the European Union’s Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie Grant Agreement No. 101034319 and from the European Union – NextGenerationEU, beneficiary: Ziliotto. A.P.M., M.T., and E.D. acknowledge support from MIUR through the FARE project R164RM93XW SEMPLICE (PI: Milone). A.P.M. and M.T. have been supported by MIUR under PRIN program 2017Z2HSMF (PI: Bedin). This research was supported in part by the Australian Research Council Centre of Excellence for All Sky Astrophysics in 3 Dimensions (ASTRO 3D) through project number CE170100013. This work is based on observations made with the NASA/ESA Hubble Space Telescope, obtained from the data archive at the Space Telescope Science Institute (STScI). STScI is operated by the Association of Universities for Research in Astronomy, Inc. under NASA contract NAS 5-26555.Peer ReviewedPostprint (published version