Globular cluster chemistry in fast rotating dwarf stars belonging to intermediate age open clusters

(shortened) The peculiar chemistry observed in the multiple populations of Galactic globular clusters is not generally found in other systems like dwarf galaxies and open clusters, and at the moment no model can fully explain its presence. Exploring the boundaries of the multiple population phenomenon and the variation of its extent in the space of cluster mass, age, metallicity, and compactness has proven to be a fruitful line of investigation. In the framework of a larger project to search for multiple population in open clusters, based on literature and survey data, I found peculiar chemical abundance patterns in a sample of intermediate age open clusters with publicly available data. More specifically, fast rotating dwarf stars (v sini > 50km/s) belonging to four clusters display a bimodality in either [Na/Fe], [O/Fe] or both. Additionally, two clusters show a Na-O anticorrelation in the fast rotating stars and one cluster shows a large [Mg/Fe] variation among the stars with high [Na/Fe], reaching the extreme Mg depletion observed in NGC 2808. Even considering that the sample sizes are small, these patterns call for attention in the light of a possible connection with the multiple population phenomenon of globular clusters. The specific chemistry observed in these fast rotating dwarf stars is thought to be produced by a complex interplay of different diffusion and mixing mechanisms, like rotational mixing and mass loss, in turn influenced by metallicity, binarity, mass, age, variability, and so on. However, with the sample in hand, it was not possible to identify which stellar parameters cause the observed Na and O bimodality and Na-O anticorrelation, suggesting that other stellar properties might be important besides stellar rotation.Comment: 7 pages, 3 figures, accepted for publication in A&

Gaia: the Galaxy in six (and more) dimensions

The ESA cornerstone mission Gaia was successfully launched in 2013, and is now scanning the sky to accurately measure the positions and motions of about two billion point-like sources of 3<V<20.5 mag, with the main goal of reconstructing the 6D phase space structure of the Milky Way. The typical uncertainties in the astrometry will be in the range 30-500 muas. The sky will be repeatedly scanned (70 times on average) for five years or more, adding the time dimension, and the Gaia data are complemented by mmag photometry in three broad bands, plus line-of-sight velocities from medium resolution spectroscopy for brighter stars. This impressive dataset is having a large impact on various areas of astrophysics, from solar system objects to distant quasars, from nearby stars to unresolved galaxies, from binaries and extrasolar planets to light bending experiments. This invited review paper presents an overview of the Gaia mission and describes why, to reach the goal performances in astrometry and to adequately map the Milky Way kinematics, Gaia was also equipped with state-of-the-art photometers and spectrographs, enabling us to explore much more than the 6D phase-space of positions and velocities. Scientific highlights of the first two Gaia data releases are briefly presented.Comment: 23 pages, 5 figures, invited review accepted for publication in ASR, based on the interdisciplinary lecture given at the COSPAR 2018 aseembly in Pasadena, US

Mining SDSS in search of Multiple Populations in Globular Clusters

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 Stromgren u) are considered. This anomalous spread in color indexes such as U-B or c_{y} 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 10^{8} 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 (0.5r_{h} < r < 5r_{h}), 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.Comment: 11 pages, 5 figures, typos adde