102 research outputs found
Lithium abundances in the old open cluster NGC 3960 from VLT/FLAMES observations
Old open clusters are very useful targets to investigate mechanisms
responsible for lithium (Li) depletion during the main sequence. Comparison of
the Li abundances in clusters of different age allows us to understand the
efficiency of the Li destruction process. Our goal is the determination of
membership and Li abundance in a sample of candidate members of the open
cluster NGC 3960 (age ~1 Gyr), with the aim to fill the gap between 0.6 and 2
Gyr in the empirical description of the behavior of the average Li abundance as
a function of the stellar age. We use VLT/FLAMES Giraffe spectra to determine
the radial velocities and thus the membership of a sample of 113
photometrically selected candidate cluster members. From the analysis of the Li
line we derive Li abundances for both cluster members and non-members. 39 stars
have radial velocity consistent with membership, with an expected fraction of
contaminating field stars of about 20%. Li is detected in 29 of the RV members;
we consider these stars as cluster members, while we make the reasonable
assumption that the remaining 10 RV members without Li, are among the
contaminating stars. Li abundances of the stars hotter than about 6000 K are
similar to those of stars in the Hyades, while they are slightly smaller for
cooler stars. This confirms that NGC 3960 is older than the Hyades. The average
Li abundance of stars cooler than about 6000 K indicates that the Li Pop. I
plateau might start already at ~1 Gyr rather than 2 Gyr that is the upper limit
previously derived in the literature. We also find that the fraction of field
stars with high Li abundance (>1.5) is about one third of the whole sample,
which is in agreement with previous estimates. The fraction of contaminating
field stars is consistent with that previously derived by us from photometry.Comment: 12 pages,10 figures, in press in Astronomy and Astrophysic
CNO abundances and carbon isotope ratios in evolved stars of the open clusters NGC 2324, NGC 2477, and NGC 3960
Our main aim is to determine carbon-to-nitrogen and carbon isotope ratios for
evolved giants in the open clusters NGC 2324, NGC 2477, and NGC 3960, which
have turn-off masses of about 2 Msun, and to compare them with predictions of
theoretical models. High-resolution spectra were analysed using a differential
synthetic spectrum method. Abundances of carbon were derived using the C2 Swan
(0,1) band heads at 5135 and 5635.5 A. The wavelength interval 7940-8130 A with
strong CN features was analysed to determine nitrogen abundances and carbon
isotope ratios. The oxygen abundances were determined from the [Oi] line at
6300 A. The mean values of the CNO abundances are [C/Fe]=-0.35+-0.06 (s.d.),
[N/Fe]=0.28+-0.05, and [O/Fe]=-0.02+-0.10 in seven stars of NGC 2324;
[C/Fe]=-0.26+-0.02, [N/Fe]=0.39+-0.04, and [O/Fe]=-0.11+-0.06 in six stars of
NGC 2477; and [C/Fe]=-0.39+-0.04, [N/Fe]=0.32+-0.05, and [O/Fe]=-0.19+-0.06 in
six stars of NGC 3960. The mean C/N ratio is equal to 0.92+-0.12, 0.91+-0.09,
and 0.80+-0.13, respectively. The mean 12C/13C ratio is equal to 21+-1, 20+-1,
and 16+-4, respectively. The 12C/13C and C/N ratios of stars in the
investigated open clusters were compared with the ratios predicted by stellar
evolution models. The mean values of the 12C/13C and C/N ratios in NGC 2324 and
NGC 2477 agree well with the first dredge-up and thermohaline-induced
extra-mixing models, which are similar for intermediate turn-off mass stars.
The 12C/13C ratios in the investigated clump stars of NGC 3960 span from 10 to
20. The mean carbon isotope and C/N ratios in NGC 3960 are close to predictions
of the model in which the thermohaline- and rotation-induced (if rotation
velocity at the zero-age main sequence was 30% of the critical velocity)
extra-mixing act together.Comment: 9 pages, 11 figures. arXiv admin note: text overlap with
arXiv:1603.0952
The Metallicity of the Open Cluster Tombaugh 2
We investigate the nature of the chemical composition of the outer disc open
cluster Tombaugh 2, that a recent study by Frinchaboy et al. (2008) suggested
to possess an intrinsic metal abundance dispersion. We aim to investigate such
claims by high resolution spectra obtained for a number of stars in the
Tombaugh 2 field, together with independent UBVIc photometry. The spectra,
together with input atmospheric parameters and model atmospheres, are used to
determine detailed chemical abundances for a variety of elements in 13 members
having good spectra. We find the mean metallicity to be [Fe/H]=-0.31+-0.02 with
no evidence for an intrinsic abundance dispersion, in contrary to the recent
results of Frinchaboy et al. (2008). We find Ca and Ba to be slightly enhanced
while Ni and Sc are solar. The r-process element Eu was found to be enhanced,
giving an average [Eu/Ba]=+0.17. The Li abundance decreases with Teff on the
upper giant branch and maintains a low level for red clump stars. The mean
metallicity we derive is in good agreement with that expected from the radial
abundance gradient in the disc for a cluster at its Galactocentric distance.
The surprising result found by Frinchaboy et al. (2008), that is the presence
of 2 distinct abundance groups within the cluster, implying either a completely
unique open cluster with an intrinsic metallicity spread, or a very unlikely
superposition of a cold stellar stream and a very distant open cluster, is not
supported by our new result.Comment: 11 pages, 5 eps figures, in press on A&
The extent of mixing in stellar interiors: the open clusters Collinder 261 and Melotte 66
Context: Determining carbon and nitrogen abundances in red giants provides
useful diagnostics to test mixing processes in stellar atmospheres. Aims: Our
main aim is to determine carbon-to-nitrogen and carbon isotope ratios for
evolved giants in the open clusters Collinder 261 and Melotte 66 and to compare
the results with predictions of theoretical models. Methods: High-resolution
spectra were analysed using a differential model atmosphere method. Abundances
of carbon were derived using the C_2 Swan (0,1) band head at 5635.5 A. The
wavelength interval 7940-8130 A, which contains CN features, was analysed to
determine nitrogen abundances and carbon isotope ratios. The oxygen abundances
were determined from the [O_I] line at 6300 A. Results: The mean values of the
elemental abundances in Collinder 261, as determined from seven stars, are:
[C/Fe]=-0.23 +- 0.02 (s.d.), [N/Fe]=0.18 +- 0.09, [O/Fe]=-0.03 +- 0.07. The
mean 12^C/13^C ratio is 11 +- 2, considering four red clump stars and 18 for
one star above the clump. The mean C/N ratios are 1.60 +- 0.30 and 1.74,
respectively. For the five stars in Melotte 66 we obtained: [C/Fe]=-0.21 +-
0.07 (s.d.), [N/Fe]=0.17 +- 0.07, [O/Fe]=0.16 +- 0.04. The 12^C/13^C and C/N
ratios are 8 +- 2 and 1.67 +- 0.21, respectively. Conclusions: The 12^C/13^C
and C/N ratios of stars in the investigated open clusters were compared with
the ratios predicted by stellar evolution models. The mean values of 12^C/13^C
ratios in Collinder 261 and Melotte 66 agree well with models of
thermohaline-induced extra-mixing for the corresponding stellar turn-off masses
of about 1.1 - 1.2 Msun. The mean C/N ratios are not decreased as much as
predicted by the model in which the thermohaline- and rotation-induced
extra-mixing act together.Comment: 6 pages, 8 figure
X-shooter spectroscopy of young stellar objects: I - Mass accretion rates of low-mass T Tauri stars in \sigma Orionis
We present high-quality, medium resolution X-shooter/VLT spectra in the range
300-2500 nm for a sample of 12 very low-mass stars in the \sigma Orionis
cluster. The sample includes stars with masses ranging from 0.08 to 0.3
M. The aim of this first paper is to investigate the reliability of the
many accretion tracers currently used to measure the mass accretion rate in
low-mass, young stars. We use our spectra to measure the accretion luminosity
from the continuum excess emission in the UV and visual; the derived mass
accretion rates range from 10 M yr down to
5 M yr, allowing us to investigate the
behavior of the accretion-driven emission lines in very-low mass accretion rate
regimes. We compute the luminosity of ten accretion-driven emission lines, from
the UV to the near-IR, obtained simultaneously. Most of the secondary tracers
correlate well with the accretion luminosity derived from the continuum excess
emission. We confirm the validity of the correlations between accretion
luminosities and line luminosities given in the literature, with the possible
exception of H\alpha. When looking at individual objects, we find that the
Hydrogen recombination lines, from the UV to the near-IR, give good and
consistent measurements of accretion luminosities, often in better agreement
than the uncertainties introduced by the adopted correlations. The average
accretion luminosity derived from several Hydrogen lines, measured
simultaneously, have a much reduced error. This suggests that some of the
spread in the literature correlations may be due to the use of non-simultaneous
observations of lines and continuum. Three stars in our sample deviate from
this behavior, and we discuss them individually.Comment: 15 pages, 14 figure
Twin stellar clusters with a different DNA
Gaia-ESO, a spectroscopic survey carried out at one of the four VLT telescopes of the ESO, has successfully completed its first two years of observations. More than 5,000 high-resolution stellar spectra are now available to the international scientific community. Among the results published in the first Gaia-ESO papers, we present here the study of the chemical content of three open clusters located in the inner part of our Galaxy.Gaia-ESO è una grande survey spettroscopica pubblica in corso presso uno dei quattro telescopi VLT dell’ESO. La survey ha completato con successo i suoi primi due anni di osservazioni e sono stati resi disponibili alla comunità scientifica internazionale più di 5000 spettri stellari ad alta risoluzione. In questa nota presentiamo i risultati di uno dei primi lavori scientifici pubblicati in cui si discute il contenuto chimico di tre ammassi stellari aperti situati nella parte più interna del disco Galattico
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