Mining SDSS in search of multiple populations in globular clusters

摘要

Several recent studies have reported the detection of an anomalous color spread along the red giant branch (RGB) of some globular clusters (GC) that appears only when color indices including a near ultraviolet band (such?as Johnson?U or Str?mgren?u) are considered. This anomalous spread in color indexes such as?U???B or?cy has been shown to correlate with variations in the abundances of light elements such as?C,?N, O, Na,?etc., which, in?turn, are generally believed to be associated with subsequent star formation episodes that occurred in the earliest few 108?yr of the cluster’s life. Here we use publicly available u, g, r Sloan Digital Sky Survey photometry to search for anomalous u???g?spreads in the RGBs of nine Galactic?GCs. In?seven of them (M?2, M?3, M?5, M?13, M?15, M?92 and M?53), we find evidence of a statistically significant spread in the u???g?color, not seen in?g???r and not accounted for by observational effects. In?the case of M?5, we demonstrate that the observed u???g?color spread correlates with the observed abundances of?Na, the?redder stars being richer in Na than the bluer ones. In?all the seven clusters displaying a significant u???g?color spread, we find that the stars on the red and blue sides of the RGB, in (g,?u???g)?color magnitude diagrams, have significantly different radial distributions. In?particular, the?red stars (generally identified with the second generation of cluster stars, in?the current scenario) are always more centrally concentrated than blue stars (generally identified with the first generation) over the range sampled by the data (0.5???rh???r???5???rh), in?qualitative agreement with the predictions of some recent models of the formation and chemical evolution of?GCs. Our results suggest that the difference in the radial distribution between first and second generation stars may be a general characteristic of?GCs.