1,374 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
A Stromgren view of the multiple populations in globular clusters
We discuss a variety of photometric indices assembled from the uvby Stromgren
system. Our aim is to examine the pros and cons of the various indices to find
the most suitable one(s) to study the properties of multiple populations in
globular clusters (GCs) discovered by spectroscopy. We explore in particular
the capabilities of indices like m_1 and c_y at different metallicities. We
define a new index delta_4=(u-v)-(b-y) to separate first and second stellar
generations in GCs of any metal abundance, since it keeps the sensitivity to
multiple stellar populations over all the metallicity range and at the same
time minimizes the sensitivity to photometric errors. We detecte clear
differences in the red giant branches of the GCs examined, like skewness or
bi/multi-modality in color distribution. We connect the photometric information
with the spectroscopic results on O, Na abundances we obtained in our survey of
GCs. Finally, we compute the effects of different chemical composition on the
Stromgren filters and indices using synthetic spectra.Comment: Accepted for publication on Astronomy and Astrophysics. Figures 1,3,5
degraded in resolutio
Helium in first and second-generation stars in Globular Clusters from spectroscopy of red giants
(abridged) Recent spectroscopic and photometric observations show the
existence of various generations of stars in GCs, differing in the abundances
of products of H-burning at high temperatures (the main final product being
He). It is important to study the connections between stars properties and He
content. We consider here the about 1400 stars on the Red Giant Branch (RGB)
observed with FLAMES@VLT in 19 Galactic GCs, part of out Na-O anticorrelation
projet. Stars with different He are expected to have different temperatures
(i.e. colours), slightly different [Fe/H], and different luminosity levels of
the RGB bump. All these differences are small, but our study has the necessary
precision, good statistics, and homogeneity to detect them. We also computed
suitable sets of stellar models (BaSTI) for various assumptions about the
initial helium content. Differences in observable quantities that can be
attributed to variations in He content are generally detectable between stars
of the Primordial (P, first-generation) and Extreme (E, second-generation)
populations, but not between the Primordial and Intermediate ones (I). The only
exception (differences are significant also between P and I populations) is
NGC2808, where three populations are clearly separated also on the Main
Sequence and the Horizontal Branch. The average enhancement in the He mass
fraction Y between P and E stars is about 0.05-0.11, depending on the
assumptions. The differences in Y, for NGC2808 alone, are about 0.11-0.14
between P and I stars, and about 0.15-0.19 between P and E stars, again
depending on the assumptions. The RGB bump luminosity of first and
second-generation stars has different levels; the implied Y difference is more
difficult to quantify, but is in agreement with the other determinations.Comment: In press on A&
Intrinsic iron spread and a new metallicity scale for Globular Clusters
We have collected spectra of about 2000 red giant branch (RGB) stars in 19
Galactic globular clusters (GC) using FLAMES@VLT (about 100 star with GIRAFFE
and about 10 with UVES, respectively, in each GC). These observations provide
an unprecedented, precise, and homogeneous data-set of Fe abundances in GCs. We
use it to study the cosmic scatter of iron and find that, as far as Fe is
concerned, most GCs can still be considered mono-metallic, since the upper
limit to the scatter in iron is less than 0.05 dex, meaning that the degree of
homogeneity is better than 12%. The scatter in Fe we find seems to have a
dependence on luminosity, possibly due to the well-known inadequacies of
stellar atmospheres for upper-RGB stars and/or to intrinsic variability. It
also seems to be correlated with cluster properties, like the mass, indicating
a larger scatter in more massive GCs which is likely a (small) true intrinsic
scatter. The 19 GCs, covering the metallicity range of the bulk of Galactic
GCs, define an accurate and updated metallicity scale. We provide
transformation equations for a few existing scales. We also provide new values
of [Fe/H], on our scale, for all GCs in the Harris' catalogue.Comment: 14 pages, 13 figures, accepted for publication on Astronomy and
Astrophysic
Magnetic field induced non-Fermi liquid to Fermi liquid crossover at the quantum critical point of YbCuAu
The temperature (T) dependence of the muon and Cu nuclear spin-lattice
relaxation rates in YbCu4.4Au0.6 is reported over nearly four decades.
It is shown that for diverges following the behaviour
predicted by the self-consistent renormalization (SCR) theory developed by
Moriya for a ferromagnetic quantum critical point. On the other hand, the
static uniform susceptibility is observed to diverge as and
, a behaviour which is not accounted for by SCR theory.
The application of a magnetic field is observed to induce a crossover to a
Fermi liquid behaviour and for is found to obey the scaling
law .Comment: 4 pages, 4 figure
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
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
Na-O Anticorrelation and HB. VIII. Proton-capture elements and metallicities in 17 globular clusters from UVES spectra
We present homogeneous abundances for Fe and some of the elements involved in
the proton-capture reactions (O, Na, Mg, Al, and Si) for 202 red giants in 17
Galactic globular clusters (GCs) from the analysis of high resolution UVES
spectra obtained with FLAMES@ESO-VLT2. Our programme clusters span almost the
whole range in metallicity of GCs and were selected to sample the widest range
of global parameters (HB morphology, masses, concentration, etc). Here we focus
on the discussion of the Na-O and Mg-Al anticorrelations and related issues.
Our study finds clear Na and O star-to-star abundance variations in all GCs.
Variations in Al are present in all but a few GCs. Finally, a spread in
abundances of Mg and Si are also present in a few clusters. Mg is slightly less
overabundant and Si slightly more overabundant in the most Al-rich stars. The
correlation between Si and Al abundances is a signature of production of 28Si
leaking from the Mg-Al cycle in a few clusters. The cross sections required for
the proper reactions to take over in the cycle point to temperatures in excess
of about 65 MK for the favoured site of production. We used a dilution model to
infer the total range of Al abundances starting from the Al abundances in the
UVES spectra, and the Na abundance distributions found from analysis of the
much larger set of stars for which GIRAFFE spectra were available. We found
that the maximum amount of additional Al produced by first generation polluters
contributing to the composition of the second generation stars in each cluster
is closely correlated with the same combination of metallicity and cluster
luminosity that reproduced the minimum O abundances found from GIRAFFE spectra.
We then suggest that the high temperatures required for the Mg-Al cycle are
only reached in the most massive and most metal-poor polluters.Comment: 20 pages, 13 figures, fig. 3 degraded. Accepted for publication on
Astronomy and Astrophysic
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