3,803 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&
Globular clusters and their contribution to the formation of the Galactic halo
This is a "biased" review because I will show recent evidence on the
contribution of globular clusters (GCs) to the halo of our Galaxy seen through
the lens of the new paradigm of multiple populations in GCs. I will show a few
examples where the chemistry of multiple populations helps to answer hot
questions including whether and how much GCs did contribute to the halo
population, if we have evidence of the GCs-halo link, what are the strengths
and weak points concerning this contribution.Comment: invited review to appear in Proc. IAU Symp. 317, XXIXth IAU General
Assembly, August 2015, Honolulu (HI, USA); eds. A. Bragaglia, M. Arnaboldi,
M. Rejkuba, D. Romano, (7 pages, 4 figures, LaTeX, using iau.cls
Five groups of red giants with distinct chemical composition in the globular cluster NGC 2808
The chemical composition of multiple populations in the massive globular
cluster (GC) NGC~2808 is addressed with the homogeneous abundance re-analysis
of 140 red giant branch (RGB) stars. UVES spectra for 31 stars and GIRAFFE
spectra for the other giants were analysed with the same procedures used for
about 2500 giants in 23 GCs in our FLAMES survey, deriving abundances of Fe, O,
Na, Mg, Si, Ca, Ti, Sc, Cr, Mn, and Ni. Iron, elements from alpha-capture, and
in the Fe-group do not show intrinsic scatter. On our UVES scale the
metallicity of NGC~2808 is [Fe/H]=-1.129+/-0.005+/-0.034$ (+/-statistical
+/-systematic error) with sigma=0.030 (31 stars). Main features related to
proton-capture elements are retrieved, but the improved statistics and the
smaller associated internal errors allow to uncover five distinct groups of
stars along the Na-O anticorrelation. We observe large depletions in Mg,
anticorrelated with enhancements of Na and also Si, suggestive of unusually
high temperatures for proton-captures. About 14% of our sample is formed by
giants with solar or subsolar [Mg/Fe] ratios. Using the [Na/Mg] ratios we
confirm the presence of five populations with different chemical composition,
that we called P1, P2, I1, I2, and E in order of decreasing Mg and increasing
Na abundances. Statistical tests show that the mean ratios in any pair of
groups cannot be extracted from the same parent distribution. The overlap with
the five populations recently detected from UV photometry is good but not
perfect, confirming that more distinct components probably exist in this
complex GC.Comment: 16 pages, 9 tables, 16 figures; accepted for publication on the
Astrophysical Journa
Empirical estimates of the Na-O anti-correlation in 95 Galactic globular clusters
Large star-to-star abundance variations are direct evidence of multiple
stellar populations in Galactic globular clusters (GCs). The main and most
widespread chemical signature is the anti-correlation of the stellar Na and O
abundances. The interquartile range (IQR) of the [O/Na] ratio is well suited to
quantifying the extent of the anti-correlation and to probe its links to global
cluster parameters. However, since it is quite time consuming to obtain precise
abundances from spectroscopy for large samples of stars in GCs, here we show
empirical calibrations of IQR[O/Na] based on the O, Na abundances homogeneously
derived from more than 2000 red giants in 22 GCs in our FLAMES survey. We find
a statistically robust bivariate correlation of IQR as a function of the total
luminosity (a proxy for mass) and cluster concentration c. Calibrated and
observed values lie along the identity line when a term accounting for the
horizontal branch (HB) morphology is added to the calibration, from which we
obtained empirical values for 95 GCs. Spreads in proton-capture elements O and
Na are found for all GCs in the luminosity range from Mv=-3.76 to Mv=-9.98.
This calibration reproduces in a self-consistent picture the link of abundance
variations in light elements with the He enhancements and its effect on the
stellar distribution on the HB. We show that the spreads in light elements seem
already to be dependent on the initial GC masses. The dependence of IQR on
structural parameters stems from the well known correlation between c and Mv,
which is likely to be of primordial origin. Empirical estimates can be used to
extend our investigation of multiple stellar populations to GCs in external
galaxies, up to M31, where even integrated light spectroscopy may currently
provide only a hint of such a phenomenon.Comment: 18 pages, 2 tables, 16 figures; accepted for publication on Astronomy
and Astrophysic
The quite complex "Simple Stellar Populations" of Globular Clusters
There is compelling observational evidence that globular clusters (GCs) are
quite complex objects. A growing body of photometric results indicate that the
evolutionary sequences are not simply isochrones in the observational plane -as
believed until a few years ago- from the main sequence, to the subgiant, giant,
and horizontal branches. The strongest indication of complexity comes however
from the chemistry, from internal dispersion in iron abundance in a few cases,
and in light elements (C, N, O, Na, Mg, Al, etc.) in all GCs. This universality
means that the complexity is intrinsic to the GCs and is most probably related
to their formation mechanisms. The extent of the variations in light elements
abundances is dependent on the GC mass, but mass is not the only modulating
factor; metallicity, age, and possibly orbit can play a role. Finally, one of
the many consequences of this new way of looking at GCs is that their stars may
show different He contents.Comment: 10 pages, 7 composite figures slightly degraded. Invited review, to
appear in the proceedings of IAU Symp. 268 "Light elements in the Universe"
(C. Charbonnel, M. Tosi, F. Primas, C. Chiappini, eds., Cambridge Univ.
Press
Variations in the Na-O anticorrelation in globular clusters: Evidence for a deep mixing episode in red giant branch stars
The Na-O anticorrelation seen in almost all globular clusters ever studied
using high-resolution spectroscopy is now generally explained by the primordial
pollution from the first generation of the intermediate-mass asymptotic giant
branch stars to the proto-stellar clouds of the second generation of stars.
However, the primordial pollution scenario may not tell the whole story for the
observed Na-O anticorrelations in globular clusters. Using the recent data by
Carretta and his collaborators, the different shapes of the Na-O
anticorrelations for red giant branch stars brighter than and fainter than the
red giant branch bump can be clearly seen. If the elemental abundance
measurements by Carretta and his collaborators are not greatly in error, this
variation in the Na-O anticorrelation against luminosity indicates an internal
deep mixing episode during the ascent of the low-mass red giant branch in
globular clusters. Our result implies that the multiple stellar population
division scheme solely based on [O/Fe] and [Na/Fe] ratios of a globular
cluster, which is becoming popular, is not reliable for stars brighter than the
red giant branch bump. Our result also suggests that sodium supplied by the
deep mixing may alleviate the sodium under-production problem within the
primordial asymptotic giant branch pollution scenario.Comment: Accepted for publication in MNRAS. 5 pages, 4 figures and 2 tables
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