153 research outputs found
C, N and O abundances in red clump stars of the Milky Way
The Hipparcos orbiting observatory has revealed a large number of
helium-core-burning "clump" stars in the Galactic field. These low-mass stars
exhibit signatures of extra-mixing processes that require modeling beyond the
first dredge-up of standard models. The 12C/13C ratio is the most robust
diagnostic of deep mixing, because it is insensitive to the adopted stellar
parameters. In this work we present 12C/13C determinations in a sample of 34
Galactic clump stars as well as abundances of nitrogen, carbon and oxygen.
Abundances of carbon were studied using the C2 Swan (0,1) band head at 5635.5
A. The wavelength interval 7980-8130 A with strong CN features was analysed in
order to determine nitrogen abundances and 12C/13C isotope ratios. The oxygen
abundances were determined from the [O I] line at 6300 A. Compared with the Sun
and dwarf stars of the Galactic disk, mean abundances in the investigated clump
stars suggest that carbon is depleted by about 0.2 dex, nitrogen is enhanced by
0.2 dex and oxygen is close to abundances in dwarfs. Comparisons to
evolutionary models show that the stars fall into two groups: the one is of
first ascent giants with carbon isotope ratios altered according to the first
dredge-up prediction, and the other one is of helium-core-burning stars with
carbon isotope ratios altered by extra mixing. The stars investigated fall to
these groups in approximately equal numbers.Comment: 8 pages 6 figures Accepted for publication in MNRA
Stellar substructures in the solar neighbourhood. III. Kinematic group 2 in the Geneva-Copenhagen survey
From correlations between orbital parameters, several new coherent groups of
stars were recently identified in the Galactic disc and suggested to correspond
to remnants of disrupted satellites. To reconstruct their origin at least three
main observational parameters - kinematics, chemical composition and age - must
be known. We determine detailed elemental abundances in stars belonging to the
so-called Group 2 of the Geneva-Copenhagen Survey and compare the chemical
composition with Galactic thin- and thick-disc stars, as well as with the
Arcturus and AF06 streams. The aim is to search for chemical signatures that
might give information about the formation history of this kinematic group of
stars. 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. The average value of [Fe/H] for the 32 stars of Group 2 is -0.42 +-
0.10 dex. The investigated group consists mainly of two 8- and 12-Gyr-old
stellar populations. Abundances of oxygen, alpha-elements, and
r-process-dominated elements are higher than in Galactic thin-disc dwarfs. This
elemental abundance pattern has similar characteristics as that of the Galactic
thick-disc. The similarity in chemical composition of stars in Group 2 with
that in stars of the thick-disc might suggest that their formation histories
are linked. The chemical composition together with the kinematic properties and
ages of stars in the investigated stars provides evidence of their common
origin and possible relation to an ancient merging event. A gas-rich satellite
merger scenario is proposed as the most likely origin. Groups 2 and 3 of the
Geneva-Copenhagen Survey might have originated in the same merging event.Comment: 17 pages, 13 figures, accepted for publication in Astronomy &
Astrophysics, 201
Chemical Composition of the RS CVn-TYPE Star Lambda Andromedae
Photospheric parameters and chemical composition are determined for the
single-lined chromospherically active RS CVn-type star {\lambda} And (HD
222107). From the high resolution spectra obtained on the Nordic Optical
Telescope, abundances of 22 chemical elements and isotopes, including such key
elements as 12C, 13C, N and O, were investigated. The differential line
analysis with the MARCS model atmospheres gives T eff=4830 K, log g=2.8,
[Fe/H]=-0.53, [C/Fe]=0.09, [N/Fe]=0.35, [O/Fe]=0.45, C/N=2.21, 12C/13C = 14.
The value of 12C/13C ratio for a star of the RS CVn-type is determined for the
first time, and its low value gives a hint that extra-mixing processes may
start acting in low-mass chromospherically active stars below the bump of the
luminosity function of red giants
Chemical Composition of the RS CVn-type Star 29 Draconis
Photospheric parameters and chemical composition are determined for the
single-lined chromospherically active RS CVn-type star 29 Draconis (HD 160538).
From the high resolution spectra obtained on the Nordic Optical Telescope,
abundances of 22 chemical elements, including the key elements such as 12C,
13C, N and O, were investigated. The differential line analysis with the MARCS
model atmospheres gives Teff=4720 K, log g=2.5, Fe/H]=-0.20, [C/Fe]=-0.14,
[N/Fe]=0.08, [O/Fe]=-0.04, C/N=2.40, 12C/13C=16. The low value of the 12C/13C
ratio gives a hint that extra mixing processes in low-mass chromospherically
active stars may start earlier than the theory of stellar evolution predicts
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
Stellar substructures in the solar neighbourhood IV. Kinematic Group 1 in the Geneva-Copenhagen survey
We determine detailed elemental abundances in stars belonging to the
so-called Group 1 of the Geneva-Copenhagen survey (GCS) and compare the
chemical composition with the Galactic thin- and thick-disc stars, with the GCS
Group 2 and Group 3 stars, as well as with several kinematic streams of similar
metallicities. The aim is to search for chemical signatures that might give
information about the formation history of this kinematic group of stars.
High-resolution spectra were obtained with the Fibre-fed Echelle Spectrograph
(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. The average value of [Fe/H] for the
37 stars of Group 1 is -0.20 +- 0.14 dex. Investigated Group 1 stars can be
separated into three age subgroups. Along with the main 8- and 12-Gyr-old
populations, a subgroup of stars younger than 5 Gyr can be separated as well.
Abundances of oxygen, alpha-elements, and r-process dominated elements are
higher than in Galactic thin-disc dwarfs. This elemental abundance pattern has
similar characteristics to that of the Galactic thick disc and differs slightly
from those in Hercules, Arcturus, and AF06 stellar streams. The similar
chemical composition of stars in Group 1, as well as in Group 2 and 3, with
that in stars of the thick disc might suggest that their formation histories
are linked. The chemical composition pattern together with the kinematic
properties and ages of stars in the investigated GCS groups provide evidence of
their common origin and possible relation to an ancient merging event. A
gas-rich satellite merger scenario is proposed as the most likely origin.Comment: 17 pages, 13 figures, accepted for publication in Astronomy &
Astrophysics, 201
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
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