739 research outputs found
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
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