The Hubble Space Telescope UV Legacy Survey of Galactic globular clusters – XIV. Multiple stellar populations within M 15 and their radial distribution★

Abstract

In the context of the Hubble Space Telescope UV Survey of Galactic Globular Clusters (GCs), we derived high-precision, multi-band photometry to investigate the multiple stellar populations in the massive and metal-poor GC M 15. By creating for redgiant branch (RGB) stars of the cluster a 'chromosome map', which is a pseudo twocolour diagram made with appropriate combination of F275W, F336W, F438W, and F814W magnitudes, we revealed colour spreads around two of the three already known stellar populations. These spreads cannot be produced by photometric errors alone and could hide the existence of (two) additional populations. This discovery increases the complexity of the multiple-population phenomenon in M 15.Our analysis shows that M 15 exhibits a faint sub-giant branch (SGB), which is also detected in colour-magnitude diagrams (CMDs) made with optical magnitudes only. This poorly-populated SGB includes about 5% of the total number of SGB stars and evolves into a red RGB in the m F336W vs. m F336W − m F814W CMD, suggesting that M 15 belongs to the class of Type II GCs.We measured the relative number of stars in each population at various radial distances from the cluster centre, showing that all of these populations share the same radial distribution within statistic uncertainties. These new findings are discussed in the context of the formation and evolution scenarios of the multiple populations.

The Hubble Space Telescope UV Legacy Survey of Galactic Globular Clusters – XVII. Public Catalogue Release

“…For the data reduction we used an evolution of the software described in Anderson et al (2008). A detailed description of the adopted tools is given by Bellini et al (2017), Nardiello et al (2018), and Libralato et al (2018).…”

“…In the "multiple-pass" photometry stage, we used the images, the PSF arrays and the transformations obtained during the "first-pass" to simultaneously find and measure stars in all the individual exposures. The tool we used is the FORTRAN software kitchen_sync2 (KS2; Bellini et al 2017;Nardiello et al 2018;Libralato et al 2018). The software analyses all the images simultaneously to find and measure the sources; in this way it is able to also measure the stars that cannot be detected in individual exposures.…”

Multiple populations in globular clusters and their parent galaxies

“…The sample of all Type II GCs comprises NGC 362, NGC 1261, NGC 1851, NGC 6715 (M 54), NGC 6934, NGC 7078 (M 15), NGC 7089 (M 2) and Terzan 5 (e.g. Marino et al 2009Marino et al , 2015Marino et al , 2018bDa Costa et al 2009;Yong & Grundahl 2008;Yong et al 2014Yong et al , 2016Carretta et al 2010;Johnson et al 2015Johnson et al , 2017Nardiello et al 2018;Ferraro et al 2009;Massari et al 2014).…”

Spatial mixing of binary stars in multiple-population globular clusters

“…In other clusters (see e.g. Milone et al 2013;Dalessandro et al 2014;Lee 2017;Nardiello et al 2018), FG and SG stars are spatially mixed and any initial trend must have been erased by the effects of the clusters' long-term dynamical evolution.…”