Uncovering multiple populations in NGC 7099 (M 30) using Washington photometry

Abstract

Over the last decade, the classical definition of Globular Clusters (GCs) as simple stellar populations was revolutionized due to the discovery of "Multiple Populations" (MPs). However, our knowledge of this phenomenon and its characteristics is still lacking greatly observationally, and there is currently no scenario that adequately explains its origin. It is therefore important to study as many GCs as possible to characterize whether or not they have MPs, and determine their detailed behavior to enlighten formation scenarios, using a wide range of techniques. The Washington photometric system has proved to be useful to find MPs thanks mainly to the UV-sensitivity and high efficiency of the C filter. We search for MPs in the Galactic GC NGC 7099 (M30), the second GC being searched for MPs using this system. We obtained photometric data using the Swope 1m telescope at Las Campanas Observatory, as well as the 4m SOAR facility. Our reduction procedure included addstar experiments to properly assess photometric errors. We find a clear signal of MPs based on an intrinsically wide color spread on the RGB, in particular due to a relatively small fraction of stars significantly bluer than the main RGB locus. These stars should correspond to so-called first generation stars, which we estimate to be roughly 15\% of the total. However, we find these first-generation stars to be more spatially concentrated than their second generation counterparts, which is the opposite to the general trend found in other clusters. We briefly discuss possible explanations for this phenomenon.