1,244 research outputs found
On the radial distribution of stars of different stellar generations in the globular cluster NGC 3201
We study the radial distribution of stars of different stellar generations in
the globular cluster NGC 3201. From recently published multicolour photometry,
a radial dependence of the location of stars on the giant branch was found. We
coupled the photometric information to our sample of 100 red giants with Na, O
abundances and known classification as first or second-generation stars. We
find that giants stars of the second generation in NGC 3201 show a tendency to
be more centrally concentrated than stars of the first generation, supporting
less robust results from our spectroscopic analysis.Comment: Accepted for publication on Astronomy and Astrophysic
Aluminum abundances of multiple stellar generations in the globular cluster NGC 1851
We study the distribution of aluminum abundances among red giants in the
peculiar globular cluster NGC 1851. Aluminum abundances were derived from the
strong doublet Al I 8772-8773 A measured on intermediate resolution FLAMES
spectra of 50 cluster stars acquired under the Gaia-ESO public survey. We
coupled these abundances with previously derived abundance of O, Na, Mg to
fully characterize the interplay of the NeNa and MgAl cycles of H-burning at
high temperature in the early stellar generation in NGC 1851. The stars in our
sample show well defined correlations between Al,Na and Si; Al is
anticorrelated with O and Mg. The average value of the [Al/Fe] ratio steadily
increases going from the first generation stars to the second generation
populations with intermediate and extremely modified composition. We confirm on
a larger database the results recently obtained by us (Carretta et al. 2011a):
the pattern of abundances of proton-capture elements implies a moderate
production of Al in NGC 1851. We find evidence of a statistically significant
positive correlation between Al and Ba abundances in the more metal-rich
component of red giants in NGC 1851.Comment: Astronomy and Astrophysics, in pres
Detailed abundances of a large sample of giant stars in M 54 and in the Sagittarius nucleus
Homogeneous abundances of light elements, alpha and Fe-group elements from
high-resolution FLAMES spectra are presented for 76 red giant stars in M54, a
massive globular cluster (GC) lying in the nucleus of the Sagittarius dwarf
galaxy. We also derived detailed abundances for 27 red giants belonging to the
Sgr nucleus. Our abundances assess the intrinsic metallicity dispersion (~0.19
dex, rms scatter) of M54, with the bulk of stars peaking at [Fe/H]~-1.6 and a
long tail extending to higher metallicities, similar to omega Cen. The spread
in these probable nuclear star clusters exceeds those of most GCs: these
massive clusters are located in a region intermediate between normal GCs and
dwarf galaxies. M54 shows the Na-O anticorrelation, typical signature of GCs,
which is instead absent in the Sgr nucleus. The light elements (Mg, Al, Si)
participating to the high temperature Mg-Al cycle show that the pattern of
(anti)correlations produced by proton-capture reactions in H-burning is clearly
different between the most metal-rich and most metal-poor components in the two
most massive GCs in the Galaxy, confirming early result based on the Na-O
anticorrelation. As in omega Cen, stars affected by most extreme processing,
i.e. showing the signature of more massive polluters, are those of the
metal-rich component. This can be understood if the burst of star formation
giving birth to the metal-rich component was delayed by as much as 10-30 Myr
with respect to the metal-poor one. The evolution of these massive GCs can be
reconciled in the general scenario for the formation of GCs sketched in
Carretta et al.(2010a) taking into account that omega Cen could have already
incorporated the surrounding nucleus of its progenitor and lost the rest of the
hosting galaxy while the two are still observable as distinct components in M54
and the surrounding field.Comment: 22 pages (3 pages of appendix), 25 figures. Tables 2, 3, 5, 6, and 7
are only available in electronic form at the CDS Accepted for publication on
Astronomy and Astrophysic
Globular Cluster Abundances and What They Can Tell Us About Galaxy Formation
We review the properties of globular clusters which make them useful for
studying the Galactic halo, Galactic chemical evolution, and the early stages
of the formation of the Milky Way. We review the evidence that GCs have a
chemical inventory similar to those of halo field stars. We discuss the
abundance ratios for dSph galaxies and show that it is possible to have formed
at least part the Galactic halo field stellar population by dissolving globular
clusters and/or accreting dSph galaxies but only if this occurred at an early
stage in the formation of the Galaxy. We review the constraints on halo
formation timescales deduced from the low Mg isotopic ratios in metal-poor halo
field dwarfs which indicate that AGB stars did not have time to contribute
significantly, while M71 contains two populations, one without and also one
with a substantial AGB contribution. We review the limited evidence for GCs
with a second population showing additional contributions from SNII, currently
confined to Omega Cen, M54, and M22, all of which may have been the nuclei or
central regions of accreted galaxies. We check our own data for additional such
GCs, and find preliminary indications that NGC 2419, a massive GC far in the
outer Galactic halo, may also belong to this group.Comment: Invited Talk: IAU Symp. 266, Star Clusters - Basic Building Blocks
Throughout Time and Space, proceedings to be published by Cambridge
University Pres
A sequence of nitrogen-rich very red giants in the globular cluster NGC 1851
We present the abundances of N in a sample of 62 stars on the red giant
branch (RGB) in the peculiar globular cluster NGC 1851. The values of [N/Fe]
ratio were obtained by comparing the flux measured in the observed spectra with
that from synthetic spectra for up to about 15 features of CN. This is the
first time that N abundances are obtained for such a large sample of RGB stars
from medium-resolution spectroscopy in this cluster. With these abundances we
provide a chemical tagging of the split red giant branch found from several
studies in NGC 1851. The secondary, reddest sequence on the RGB is populated
almost exclusively by N-rich stars, confirming our previous suggestion based on
Stromgren magnitudes and colours. These giants are also, on average, enriched
in s-process elements such as Ba, and are likely the results of pollution from
low mass stars that experienced episodes of third dredge-up in the asymptotic
giant branch phase.Comment: Version to match the one in press on Astronomy and Astrophysic
A view from inside iron-based superconductors
Muon spin spectroscopy is one of the most powerful tools to investigate the
microscopic properties of superconductors. In this manuscript, an overview on
some of the main achievements obtained by this technique in the iron-based
superconductors (IBS) are presented. It is shown how the muons allow to probe
the whole phase diagram of IBS, from the magnetic to the superconducting phase,
and their sensitivity to unravel the modifications of the magnetic and the
superconducting order parameters, as the phase diagram is spanned either by
charge doping, by an external pressure or by introducing magnetic and
non-magnetic impurities. Moreover, it is highlighted that the muons are unique
probes for the study of the nanoscopic coexistence between magnetism and
superconductivity taking place at the crossover between the two ground-states.Comment: 28 pages, 18 figure
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