3,949 research outputs found
The Chemical Compositions of the SRd Variable Stars-- II. WY Andromedae, VW Eridani, and UW Librae
Chemical compositions are derived from high-resolution spectra for three
stars classed as SRd variables in the General Catalogue of Variable Stars.
These stars are shown to be metal-poor supergiants: WY And with [Fe/H] = -1.0,
VW Eri with [Fe/H] = -1.8, and UW Lib with [Fe/H] = -1.2. Their compositions
are identical to within the measurement errors with the compositions of
subdwarfs, subgiants, and less evolved giants of the same FeH. The stars are at
the tip of the first giant branch or in the early stages of evolution along the
asymptotic giant branch (AGB). There is no convincing evidence that these SRd
variables are experiencing thermal pulsing and the third dredge-up on the AGB.
The SRds appear to be the cool limit of the sequence of RV Tauri variables.Comment: 14 pages, 1 figure, 4 table
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
An Abundance Analysis for Five Red Horizontal Branch Stars in the Extremely Metal Rich Globular Cluster NGC 6553
We provide a high dispersion line-by-line abundance analysis of five red HB
stars in the extremely metal rich galactic globular cluster NGC 6553. These red
HB stars are significantly hotter than the very cool stars near the tip of the
giant branch in such a metal rich globular cluster and hence their spectra are
much more amenable to an abundance analysis than would be the case for red
giants.
We find that the mean [Fe/H] for NGC 6553 is -0.16 dex, comparable to the
mean abundance in the galactic bulge found by McWilliam & Rich (1994) and
considerably higher than that obtained from an analysis of two red giants in
this cluster by Barbuy etal (1999). The relative abundance for the best
determined alpha process element (Ca) indicates an excess of alpha process
elements of about a factor of two. The metallicity of NGC 6553 reaches the
average of the Galactic bulge and of the solar neighborhood.Comment: 29 pages, 6 figures, accepted for publication in the Ap
The link between chemical anomalies along the red giant branch and the horizontal branch extension in globular clusters
We find a strong correlation between the extension of the Na-O
anticorrelation observed in red giant branch (RGB) stars and the high
temperature extension of the horizontal branch (HB) blue tails of Galactic
globular clusters (GCs). The longer is the O-depleted tail of the Na-O
anticorrelation observed in the RGB stars, the higher is the maximum
temperature reached by the bluest HB stars in the GC. This result provides a
clear, empirical evidence of a link between the extension of the HB and the
presence of star-to-star abundance variations of proton-capture elements in GC
stars. We discuss the possible interpretation of this correlation.Comment: Comments: 6 pages, 1 figure, uses emulateapj.cls; accepted for
publication in the Astrophysical Journal Letter
Nucleosynthesis in Type II supernovae and the abundances in metal-poor stars
We explore the effects on nucleosynthesis in Type II supernovae of various
parameters (mass cut, neutron excess, explosion energy, progenitor mass) in
order to explain the observed trends of the iron-peak element abundance ratios
([Cr/Fe], [Mn/Fe], [Co/Fe] and [Ni/Fe]) in halo stars as a function of
metallicity for the range [Fe/H] . [Cr/Fe] and [Mn/Fe]
decrease with decreasing [Fe/H], while [Co/Fe] behaves the opposite way and
increases. We show that such a behavior can be explained by a variation of mass
cuts in Type II supernovae as a function of progenitor mass, which provides a
changing mix of nucleosynthesis from an alpha-rich freeze-out of Si-burning and
incomplete Si-burning. This explanation is consistent with the amount of
ejected Ni determined from modeling the early light curves of individual
supernovae. We also suggest that the ratio [H/Fe] of halo stars is mainly
determined by the mass of interstellar hydrogen mixed with the ejecta of a
single supernova which is larger for larger explosion energy and the larger
Str\"omgren radius of the progenitor.Comment: 17 pages, LaTeX, Accepted for publication in the Astrophysical
Journal, more discussion on the Galactic chemical evolutio
Na-O Anticorrelation and HB. IV. Detection of He-rich and He-poor stellar populations in the globular cluster NGC 6218
We used the multifiber spectrograph FLAMES on the ESO Very Large Telescope
UT2 to derive atmospheric parameters, metallicities and abundances of O and Na
for 79 red giant stars in the Galactic globular cluster NGC 6218 (M 12). We
analyzed stars in the magnitude range from about 1 mag below the bump to the
tip of the Red Giant Branch. The average metallicity we derive is
[Fe/H]=-1.31+/-0.004+/-0.028 dex (random and systematic errors, respectively),
with a very small star-to-star scatter (rms=0.033 dex), from moderately
high-resolution Giraffe spectra. This is the first extensive spectroscopic
abundance analysis in this cluster. Our results indicate that NGC 6218 is very
homogeneous as far as heavy elements are concerned. On the other hand, light
elements involved in the well known proton-capture reactions of H-burning at
high temperature, such as O and Na, show large variations, anticorrelated with
each other, at all luminosities along the red giant branch. The conclusion is
that the Na-O anticorrelation must be established in early times at the cluster
formation. We interpret the variation of Na found near the RGB-bump as the
effect of two distinct populations having different bump luminosities, as
predicted for different He content. To our knowledge, NGC 6218 is the first GC
where such a signature has been spectroscopically detected, when combined with
consistent and homogeneous data obtained for NGC 6752 to gain in statistical
significance.Comment: 15 pages, 10 figures; fig.5 degraded in resolution; tables 2,3,5
available at CDS. Accepted for publication on A&
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