725 research outputs found
A spectroscopic study of the globular Cluster NGC 4147
Indexación: Web of ScienceWe present the abundance analysis for a sample of 18 red giant branch stars in the metal-poor globular cluster NGC 4147 based on medium- and high-resolution spectra. This is the first extensive spectroscopic study of this cluster. We derive abundances of C, N, O, Na, Mg, Al, Si, Ca, Ti, Cr, Fe, Ni, Y, Ba, and Eu. We find a metallicity of [Fe/H] = -1.84 +/- 0.02 and an alpha-enhancement of +0.38 +/- 0.05 (errors on the mean), typical of halo globular clusters in this metallicity regime. A significant spread is observed in the abundances of light elements C, N, O, Na, and Al. In particular, we found an Na-O anticorrelation and Na-Al correlation. The cluster contains only similar to 15 per cent of stars that belong to the first generation (Na-poor and O-rich). This implies that it suffered a severe mass-loss during its lifetime. Its [Ca/Fe] and [Ti/Fe] mean values agree better with the Galactic halo trend than with the trend of extragalactic environments at the cluster metallicity. This possibly suggests that NGC 4147 is a genuine Galactic object at odd with what claimed by some author that proposed the cluster to be member of the Sagittarius dwarf galaxy. An antirelation between the light s-process element Y and Na may also be present.https://academic.oup.com/mnras/article-lookup/doi/10.1093/mnras/stw114
The cosmological Lithium problem outside the Galaxy: the Sagittarius globular cluster M54
The cosmological Li problem is the observed discrepancy between Li abundance,
A(Li), measured in Galactic dwarf, old and metal-poor stars (traditionally
assumed to be equal to the initial value A(Li)_0), and that predicted by
standard Big Bang Nucleosynthesis calculations (A(Li)_{BBN}). Here we attack
the Li problem by considering an alternative diagnostic, namely the surface Li
abundance of red giant branch stars that in a colour magnitude diagram populate
the region between the completion of the first dredge-up and the red giant
branch bump. We obtained high-resolution spectra with the FLAMES facility at
the Very Large Telescope for a sample of red giants in the globular cluster
M54, belonging to the Sagittarius dwarf galaxy. We obtain A(Li)=+0.93+-0.11
dex, translating -- after taking into account the dilution due to the dredge
up-- to initial abundances (A(Li)_0) in the range 2.35--2.29 dex, depending on
whether or not atomic diffusion is considered. This is the first measurement of
Li in the Sagittarius galaxy and the more distant estimate of A(Li)_0 in old
stars obtained so far. The A(Li)_0 estimated in M54 is lower by ~0.35 dex than
A(Li)_{BBN}, hence incompatible at a level of ~3sigma. Our result shows that
this discrepancy is a universal problem concerning both the Milky Way and
extra-galactic systems. Either modifications of BBN calculations, or a
combination of atomic diffusion plus a suitably tuned additional mixing during
the main sequence, need to be invoked to solve the discrepancy.Comment: Accepted by MNRAS, 10 pages, 5 figures, 1 tabl
A double stellar generation in the Globular Cluster NGC6656 (M 22). Two stellar groups with different iron and s-process element abundance
AIMS. In this paper we present the chemical abundance analysis from high
resolution UVES spectra of seventeen bright giant stars of the Globular Cluster
M~22. RESULTS. We obtained an average iron abundance of [Fe/H]=-1.76\pm0.02
(internal errors only) and an \alpha enhancement of 0.36\pm0.04 (internal
errors only). Na and O, and Al and O follow the well known anti-correlation
found in many other GCs. We identified two groups of stars with significantly
different abundances of the s-process elements Y, Zr and Ba. The relative
numbers of the two group members are very similar to the ratio of the stars in
the two SGBs of M22 recently found by Piotto (2009). Y and Ba abundances do not
correlate with Na, O and Al. The s-element rich stars are also richer in iron
and have higher Ca abundances. The results from high resolution spectra have
been further confirmed by lower resolution GIRAFFE spectra of fourteen
additional M22 stars. GIRAFFE spectra show also that the Eu -- a pure r-process
element -- abundance is not related to the iron content. We discuss the
chemical abundance pattern of M22 stars in the context of the multiple stellar
populations in GC scenario.Comment: 17 Pages, 21 figures, Accepted for publication in A&
Detailed abundances in stars belonging to ultra-faint dwarf spheroidal galaxies
We report preliminary results concerning the detailed chemical composition of
metal poor stars belonging to close ultra-faint dwarf galaxies (hereafter
UfDSphs). The abundances have been determined thanks to spectra obtained with
X-Shooter, a high efficiency spectrograph installed on one of the ESO VLT
units. The sample of ultra-faint dwarf spheroidal stars have abundance ratios
slightly lower to what is measured in field halo star of the same
metallicity.We did not find extreme abundances in our Hercules stars as the one
found by Koch for his 2 Hercules stars. The synthesis of the neutron capture
elements Ba and Sr seems to originate from the same nucleosynthetic process in
operation during the early stages of the galactic evolution.Comment: 3 pages, 1 figure; OMEG11 conference (Tokyo, Nov 2011
Clues on the Galactic evolution of sulphur from star clusters
(Abridged) The abundances of alpha-elements are a powerful diagnostic of the
star formation history and chemical evolution of a galaxy. Sulphur, being
moderately volatile, can be reliably measured in the interstellar medium (ISM)
of damped Ly-alpha galaxies and extragalactic HII regions. Measurements in
stars of different metallicity in our Galaxy can then be readily compared to
the abundances in external galaxies. Such a comparison is not possible for Si
or Ca that suffer depletion onto dust in the ISM. Furthermore, studying sulphur
is interesting because it probes nucleosynthetic conditions that are very
different from those of O or Mg. The measurements in star clusters are a
reliable tracers of the Galactic evolution of sulphur. We find
NLTE=6.11+/-0.04 for M 4, NLTE=7.17+/-0.02 for NGC 2477, and
NLTE=7.13+/-0.06 for NGC 5822. For the only star studied in Trumpler 5 we
find A(S)NLTE=6.43+/-0.03 and A(S)LTE=6.94+/-0.05. Our measurements show that,
by and large, the S abundances in Galactic clusters trace reliably those in
field stars. The only possible exception is Trumpler 5, for which the NLTE
sulphur abundance implies an [S/Fe] ratio lower by roughly 0.4 dex than
observed in field stars of comparable metallicity, even though its LTE sulphur
abundance is in line with abundances of field stars. Moreover the LTE sulphur
abundance is consistent only with the abundance of another alpha-element, Mg,
in the same star, while the low NLTE value is consistent with Si and Ca. The S
abundances in our sample of stars in clusters imply that the clusters are
chemically homogeneous for S within 0.05 dex.Comment: A&A in pres
New HST WFC3/UVIS observations augment the stellar-population complexity of omega Centauri
We used archival multi-band Hubble Space Telescope observations obtained with
the Wide-Field Camera 3 in the UV-optical channel to present new important
observational findings on the color-magnitude diagram (CMD) of the Galactic
globular cluster omega Centauri. The ultraviolet WFC3 data have been coupled
with available WFC/ACS optical-band data. The new CMDs, obtained from the
combination of colors coming from eight different bands, disclose an even more
complex stellar population than previously identified. This paper discusses the
detailed morphology of the CMDs.Comment: 17 pages, 14 figures (11 in low res), 3 tables. Accepted for
publication in AJ on June 19, 201
The C+N+O abundance of Omega Centauri giant stars: implications on the chemical enrichment scenario and the relative ages of different stellar populations
We present a chemical-composition analysis of 77 red-giant stars in Omega
Centauri. We have measured abundances for carbon and nitrogen, and combined our
results with abundances of O, Na, La, and Fe that we determined in our previous
work. Our aim is to better understand the peculiar chemical-enrichment history
of this cluster, by studying how the total C+N+O content varies among the
different-metallicity stellar groups, and among stars at different places along
the Na-O anticorrelation. We find the (anti)correlations among the light
elements that would be expected on theoretical ground for matter that has been
nuclearly processed via high-temperature proton captures. The overall
[(C+N+O)/Fe] increases by 0.5 dex from [Fe/H] -2.0 to [Fe/H] -0.9. Our results
provide insight into the chemical-enrichment history of the cluster, and the
measured CNO variations provide important corrections for estimating the
relative ages of the different stellar populations.Comment: 26 pages, 9 figure - Accepted for publication in Ap
- …