18 research outputs found
High resolution spectroscopic study of red clump stars in the Galaxy: iron group elements
The main atmospheric parameters and abundances of the iron group elements
(vanadium, chromium, iron, cobalt and nickel) are determined for 62 red giant
"clump" stars revealed in the Galactic field by the Hipparcos orbiting
observatory. The stars form a homogeneous sample with the mean value of
temperature T=4750 +- 160K, of surface gravity log g = 2.41 +- 0.26 and the
mean value of metallicity [Fe/H] = -0.04 +- 0.15 dex. A Gaussian fit to the
[Fe/H] distribution produces the mean [Fe/H] = -0.01 dex and dispersion of
[Fe/H] = 0.08 dex. The near-solar metallicity and small dispersion of [Fe/H] of
clump stars of the Galaxy obtained in this work confirm the theoretical model
of the Hipparcos clump by Girardi & Salaris (2001). This suggests that nearby
clump stars are (in the mean) relatively young objects, reflecting mainly the
near-solar metallicities developed in the local disk during the last few Gyrs
of its history. We find iron group element to iron abundance ratios in clump
giants to be close to solar.Comment: 9 pages, 7 figures, accepted for publication in MNRA
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
The Gaia-ESO Survey: CNO abundances in the open clusters Trumpler 20, NGC 4815, and NGC 6705
Aim of this work is to determine C, N, and O abundances in stars of Galactic
open clusters of the Gaia-ESO survey and to compare the observed abundances
with those predicted by current stellar and Galactic evolution models. In this
pilot paper, we investigate the first three intermediate-age open clusters.
High-resolution spectra, observed with the FLAMES-UVES spectrograph on the ESO
VLT telescope, were analysed using a differential model atmosphere method.
Abundances of carbon were derived using the C2 band heads at 5135 and 5635.5
{\AA}. The wavelength interval 6470- 6490 {\AA}, with CN features, was analysed
to determine nitrogen abundances. Oxygen abundances were determined from the [O
i] line at 6300 {\AA}. The mean values of the elemental abundances in Trumpler
20 as determined from 42 stars are: [Fe/H] = 0.10 +- 0.08 (s.d.), [C/H] = -0.10
+- 0.07, [N/H] = 0.50 +- 0.07, and consequently C/N = 0.98 +- 0.12. We measure
from five giants in NGC 4815: [Fe/H] = -0.01 +- 0.04, [C/H] = -0.17 +- 0.08,
[N/H] = 0.53 +- 0.07, [O/H] = 0.12 +- 0.09, and C/N = 0.79 +- 0.08. We obtain
from 27 giants in NGC 6705: [Fe/H] = 0.0 +- 0.05, [C/H] = -0.08 +- 0.06, [N/H]
= 0.61 +- 0.07, [O/H] = 0.13 +- 0.05, and C/N = 0.83 +- 0.19. The C/N ratios of
stars in the investigated open clusters were compared with the ratios predicted
by stellar evolutionary models. For the corresponding stellar turn-off masses
from 1.9 to 3.3 solar masses, the observed C/N ratio values are very close to
the predictions of standard first dredge-up models as well as to models of
thermohaline extra-mixing. The average [O/H] abundance ratios of NGC 4815 and
NGC 6705 are compared with the predictions of two Galactic chemical evolution
models. The data are consistent with the evolution at the solar radius within
the errors.Comment: 13 pages, 11 figures, accepted for publication in Astronomy &
Astrophysics, 201
The Gaia-ESO Survey : The analysis of high-resolution UVES spectra of FGK-type stars
Date of Acceptance: 01/09/2014Context. The ongoing Gaia-ESO Public Spectroscopic Survey is using FLAMES at the VLT to obtain high-quality medium-resolution Giraffe spectra for about 105 stars and high-resolution UVES spectra for about 5000 stars. With UVES, the Survey has already observed 1447 FGK-type stars. Aims. These UVES spectra are analyzed in parallel by several state-of-the-art methodologies. Our aim is to present how these analyses were implemented, to discuss their results, and to describe how a final recommended parameter scale is defined. We also discuss the precision (method-to-method dispersion) and accuracy (biases with respect to the reference values) of the final parameters. These results are part of the Gaia-ESO second internal release and will be part of its first public release of advanced data products. Methods. The final parameter scale is tied to the scale defined by the Gaia benchmark stars, a set of stars with fundamental atmospheric parameters. In addition, a set of open and globular clusters is used to evaluate the physical soundness of the results. Each of the implemented methodologies is judged against the benchmark stars to define weights in three different regions of the parameter space. The final recommended results are the weighted medians of those from the individual methods. Results. The recommended results successfully reproduce the atmospheric parameters of the benchmark stars and the expected Teff-log g relation of the calibrating clusters. Atmospheric parameters and abundances have been determined for 1301 FGK-type stars observed with UVES. The median of the method-to-method dispersion of the atmospheric parameters is 55 K for Teff, 0.13 dex for log g and 0.07 dex for [Fe/H]. Systematic biases are estimated to be between 50-100 K for Teff, 0.10-0.25 dex for log g and 0.05-0.10 dex for [Fe/H]. Abundances for 24 elements were derived: C, N, O, Na, Mg, Al, Si, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, Zr, Mo, Ba, Nd, and Eu. The typical method-to-method dispersion of the abundances varies between 0.10 and 0.20 dex. Conclusions. The Gaia-ESO sample of high-resolution spectra of FGK-type stars will be among the largest of its kind analyzed in a homogeneous way. The extensive list of elemental abundances derived in these stars will enable significant advances in the areas of stellar evolution and Milky Way formation and evolution.Peer reviewe
Young and Intermediate-age Distance Indicators
Distance measurements beyond geometrical and semi-geometrical methods, rely
mainly on standard candles. As the name suggests, these objects have known
luminosities by virtue of their intrinsic proprieties and play a major role in
our understanding of modern cosmology. The main caveats associated with
standard candles are their absolute calibration, contamination of the sample
from other sources and systematic uncertainties. The absolute calibration
mainly depends on their chemical composition and age. To understand the impact
of these effects on the distance scale, it is essential to develop methods
based on different sample of standard candles. Here we review the fundamental
properties of young and intermediate-age distance indicators such as Cepheids,
Mira variables and Red Clump stars and the recent developments in their
application as distance indicators.Comment: Review article, 63 pages (28 figures), Accepted for publication in
Space Science Reviews (Chapter 3 of a special collection resulting from the
May 2016 ISSI-BJ workshop on Astronomical Distance Determination in the Space
Age
Chemical composition of evolved stars in the open cluster NGCÂ 7789
High-resolution spectra of six giants and three core-helium-burning
âclumpâ stars in the open cluster NGCÂ 7789 have been obtained with the SOFIN
spectrograph on the Nordic Optical Telescope to investigate abundances of up to
20 chemical elements. Abundances of carbon were studied using the
Swan (0, 1) band head at 5635.5Â Ă
. The wavelength interval
7980â8130Â Ă
with strong CN features was analysed in order to determine
nitrogen abundances and isotope ratios.
The oxygen abundances were determined from the [OÂ I] line at 6300Â Ă
.
The overall metallicity of evolved stars in the cluster was found to be close
to solar (). Compared with the Sun and other dwarf
stars of the Galactic disk, mean abundances in the investigated giant stars
suggest that carbon is depleted by about 0.2Â dex, and nitrogen and oxygen are
close to solar. In the clump stars investigated, carbon is depleted
by about 0.2Â dex, the mean abundance of nitrogen is enhanced by 0.26Â dex and
oxygen is lower by 0.14Â dex.
This has the effect of lowering the mean C/N ratios to the value of
in the giant stars
and to the value of in the clump stars.
The mean ratios are lowered
to about the same value of in the giants and clump stars investigated.
Concerning other chemical elements an overabundance
of sodium is noticeable and of silicon and calcium one is suspected.
Abundances of iron-group and heavier chemical elements in all nine stars were
found to be close to solar
The Gaia-ESO Survey: The analysis of high-resolution UVES spectra of FGK-type stars
The article of record as published may be found at http://dx.doi.org/10.1051/0004-6361/201423937Context. The ongoing Gaia-ESO Public Spectroscopic Survey is using FLAMES at the VLT to obtain high-quality medium-resolution Giraffe spectra for about 10(5) stars and high-resolution UVES spectra for about 5000 stars. With UVES, the Survey has already observed 1447 FGK-type stars.
Aims. These UVES spectra are analyzed in parallel by several state-of-the-art methodologies. Our aim is to present how these analyses were implemented, to discuss their results, and to describe how a final recommended parameter scale is defined. We also discuss the precision (method-to-method dispersion) and accuracy (biases with respect to the reference values) of the final parameters. These results are part of the Gaia-ESO second internal release and will be part of its first public release of advanced data products.
Methods. The final parameter scale is tied to the scale defined by the Gaia benchmark stars, a set of stars with fundamental atmospheric parameters. In addition, a set of open and globular clusters is used to evaluate the physical soundness of the results. Each of the implemented methodologies is judged against the benchmark stars to define weights in three different regions of the parameter space. The final recommended results are the weighted medians of those from the individual methods.
Results. The recommended results successfully reproduce the atmospheric parameters of the benchmark stars and the expected T(eff)-log g relation of the calibrating clusters. Atmospheric parameters and abundances have been determined for 1301 FGK-type stars observed with UVES. The median of the method-to-method dispersion of the atmospheric parameters is 55K for T(eff), 0.13 dex for log g and 0.07 dex for [Fe/H]. Systematic biases are estimated to be between 50â100 K for T(eff), 0.10â0.25 dex for log g and 0.05â0.10 dex for [Fe/H]. Abundances for 24 elements were derived: C, N, O, Na, Mg, Al, Si, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, Zr, Mo, Ba, Nd, and Eu. The typical method-to-method dispersion of the abundances varies between 0.10 and 0.20 dex.
Conclusions. The Gaia-ESO sample of high-resolution spectra of FGK-type stars will be among the largest of its kind analyzed in a homogeneous way. The extensive list of elemental abundances derived in these stars will enable significant advances in the areas of stellar evolution and Milky Way formation and evolution.Support by the Swedish National Space Board (SNSB) through several grants