25 research outputs found
C, N, O abundances and carbon isotope ratios in evolved stars of the open clusters Collinder 261 and NGC 6253
Context. Investigations of abundances of carbon and nitrogen in the
atmospheres of evolved stars of open clusters may provide comprehensive
information on chemical composition changes caused by stellar evolution. Aims.
Our main aim is to increase the number of open clusters with determined
carbon-to nitrogen and carbon isotope ratios. 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 {\AA} (FEROS
spectra) and the C_2 Swan (1,0) band head at 4737 {\AA} (UVES spectra). The
wavelength interval 7980-8130 {\AA}, with strong CN features was analysed to
determine nitrogen abundances and 12^C/13^C isotope ratios. The oxygen
abundances were determined from the [Oi] line at 6300 {\AA}. Results. The
average value of 12^C/13^C isotope ratios of Cr 261 is equal to 18 \pm 2 in
four giants and to 12 \pm 1 in two clump stars; it is equal to 16 \pm 1 in four
clump stars of the open cluster NGC 6253. The mean C/N ratios in Cr 261 and NGC
6253 are equal to 1.67 \pm 0.06 and 1.37 \pm 0.09, respectively. Conclusions.
The 12^C/13^C and C/N values in Cr 261 and NGC 6253 within limits of
uncertainties agree with the theoretical model of thermohaline-induced mixing
as well as with the cool-bottom processing modelComment: 7 pages, 8 figure
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
Stellar substructures in the solar neighbourhood. II. Abundances of neutron-capture elements in the kinematic Group 3 of the Geneva-Copenhagen survey
The evolution of chemical elements in a galaxy is linked to its star
formation history. Variations in star formation history are imprinted in the
relative abundances of chemical elements produced in different supernova events
and asymptotic giant branch stars. We determine detailed elemental abundances
of s- and r-process elements in stars belonging to Group3 of the
Geneva-Copenhagen survey and compare their chemical composition with Galactic
disc stars. The aim is to look for possible chemical signatures that might give
information about the formation history of this kinematic group of stars, which
is suggested to correspond to remnants of disrupted satellites. High-resolution
spectra were obtained with the FIES spectrograph at the Nordic Optical
Telescope, La Palma, and were analysed with a differential model atmosphere
method. Comparison stars were observed and analysed with the same method.
Abundances of chemical elements produced mainly by the s-process are similar to
those in the Galactic thin-disc dwarfs of the same metallicity, while
abundances of chemical elements produced predominantly by the r-process are
overabundant. The similar elemental abundances are observed in Galactic
thick-disc stars. The chemical composition together with the kinematic
properties and ages of stars in Group3 of the Geneva-Copenhagen survey support
a gas-rich satellite merger scenario as the most likely explanation for the
origin. The similar chemical composition of stars in Group3 and the thick-disc
stars might suggest that their formation histories are linked.Comment: 8 pages, 8 figures, accepted for publication in Astronomy &
Astrophysics, 2013. arXiv admin note: text overlap with arXiv:1203.619
Chemical composition of giant stars in the open cluster IC 4756
Context: Homogeneous investigations of red giant stars in open clusters
contribute to studies of internal evolutionary mixing processes inside stars,
which are reflected in abundances of mixing-sensitive chemical elements like
carbon, nitrogen, and sodium, while alpha- and neutron-capture element
abundances are useful in tracing the Galactic chemical evolution. Aims: The
main aim of this study is a comprehensive chemical analysis of red giant stars
in the open cluster IC 4756, including determinations of 12C/13C and C/N
abundance ratios, and comparisons of the results with theoretical models of
stellar and Galactic chemical evolution. Methods: We used a classical
differential model atmosphere method to analyse high-resolution spectra
obtained with the FEROS spectrograph on the 2.2m MPG/ESO Telescope. The carbon,
nitrogen, and oxygen abundances, 12C/13C ratios, and neutron-capture element
abundances were determined using synthetic spectra, and the main atmospheric
parameters and abundances of other chemical elements were determined from
equivalent widths of spectral lines. Results: We have determined abundances of
23 chemical elements for 13 evolved stars and 12C/13C ratios for six stars of
IC 4756. The mean metallicity of this cluster, as determined from nine definite
member stars, is very close to solar - [Fe/H] = -0.02 +/- 0.01. Abundances of
carbon, nitrogen, and sodium exhibit alterations caused by extra-mixing: the
mean 12C/13C ratio is lowered to 19 +/- 1.4, the C/N ratio is lowered to 0.79
+/- 0.05, and the mean [Na/Fe] value, corrected for deviations from the local
thermodynamical equilibrium encountered, is enhanced by 0.14 +/- 0.05 dex. We
compared our results to those by other authors with theoretical models
The role of radial migration in open cluster and field star populations with Gaia dr3
The survival time of a star cluster depends on its total mass, density, and
thus size, as well as on the environment in which it was born and in which
lies. Its dynamical evolution is influenced by various factors such as
gravitational effects of the Galactic bar, spiral structures, and molecular
clouds. Overall, the factors that determine the longevity of a cluster are
complex and not fully understood. This study aims to investigate if open
clusters and field stars respond differently to the perturbations that cause
radial migration. In particular, we aim at understanding the nature of the
oldest surviving clusters. We compared the time evolution of the kinematic
properties of two Gaia DR3 samples: the first sample is composed of 40
open clusters and the second one of 66,000 MSTO field stars. Both
selected samples are composed of stars selected with the same quality
criterion, belonging to the thin disc, in a similar metallicity range, located
in the same Galactocentric region [7.5-9 kpc] and with ages >1 Gyr. We
performed a statistical analysis comparing the properties of the samples of
field stars and of open clusters. A qualitative comparison of kinematic and
orbital properties reveals that clusters younger than 2-3 Gyr are more
resistant to perturbations than field stars and they move along quasi-circular
orbits. Conversely, clusters older than approximately 3 Gyr have more eccentric
and inclined orbits than isolated stars in the same age range. Such orbits lead
them to reach higher elevations on the Galactic plane, maximising their
probability to survive several Gyr longer. A formal statistical analysis
reveals that there are differences among the time evolution of most of the
kinematic and orbital properties of field stars and open clusters. Our results
suggest that oldest survived clusters are usually more massive and move on
orbits with higher eccentricity.Comment: 13 pages, 20 figures, 2 tables. Article accepted for publication in
A&
Chemical composition of evolved stars in the open cluster NGC 2506
In this study we present abundances of 12^C, 13^C, N, O and up to 26 other
chemical elements in two first ascent giants and two core-helium-burning
'clump' stars of the open cluster NGC 2506. Abundances of carbon were derived
using the C_2 Swan (0,1) band head at 5635.5 A. The wavelength interval
7940-8130 A, with strong CN features, was analysed in order to determine
nitrogen abundances and carbon isotope ratios. The oxygen abundances were
determined from the [O I] line at 6300 A. NGC 2506 was found to have a mean
[Fe/H]=-0.24 +- 0.05 (standard deviation). Compared with the Sun and other
dwarf stars of the Galactic disc, mean abundances in the investigated clump
stars suggest that carbon is depleted by about 0.2 dex, nitrogen is
overabundant by about 0.3 dex and other chemical elements have abundance ratios
close to solar. The C/N and 12^C/13^C ratios are lowered to 1.25 +- 0.27 and 11
+- 3, respectively.Comment: MNRAS accepte