Barium abundances in Cepheids Andrievsky, S. M; Lépine, J. R. D; Korotin, S. A ...
Monthly notices of the Royal Astronomical Society,
02/2013, Letnik:
428, Številka:
4
Journal Article
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We derived the barium atmospheric abundances for a large sample of Cepheids, comprising 270 stars. The sample covers a large range of Galactocentric distances, from about 4 to 15 kpc, so it is ...appropriate to investigate the existence of radial barium abundance gradients in the Galactic disc. In fact, this is the first time that such a comprehensive analysis of the distribution of barium abundances in the Galactic disc has been carried out. As a result, we conclude that the Ba abundance distribution can be characterized by a zero gradient. This result is compared with derived gradients for other elements, and some reasons are briefly discussed for the independence of the barium abundances from Galactocentric distances.
ABSTRACT
We have made the first attempt to derive the chemical properties of the Galactic disc at its very central part using high-resolution infrared spectroscopic observations of four classical ...Cepheids. These stars are located at Galactocentric distances smaller than 1 kpc. All investigated stars show near-to-solar elemental abundances. By combining these new data with our previous studies, our results suggest that the radial distribution of iron abundance on a logarithmic scale gradually increases from the outskirts of the Galactic disc to Galactocentric distances of about 2–4 kpc. There, it reaches a maximal value of about +0.4 dex, and then declines sharply to about the solar value at the Galactic Centre.
Using an Non-Local Thermodynamic Equilibrium (NLTE) analysis and a homogenous spectral data base, we have determined Ba abundances in a sample of 210 Galactic Cepheids and related objects. We confirm ...the previous result reported in Andrievsky et al. that the barium distribution in the range of Galactocentric distances from 5 to 18 kpc shows very small gradient value. The value determined here - dBa/H/dR −0.01 dex kpc−1 - is smaller than that for other s-process elements, such as La, Pr, Nd and Sm (Luck & Lambert), that typically show gradients of −0.03 dex kpc−1.
We have investigated the diffuse interstellar band (DIB) at 661.3 nm seen in the spectra of Cepheid stars. After removal of the blending lines of ionized yttrium and neutral iron, we determined the ...equivalent widths (EWs) of the DIB and used these values to investigate the E(B − V)–DIB EW relation. The relation found from Cepheids matches that found in B stars. This relation can help to find the reddening for newly discovered Cepheids without extensive photometric data, and thus determine their distances. The relation between E(B − V) and the DIB EW does not yield precise reddening values. It is not a substitute for better photometric or spectroscopic methods. At best, it is indicative, but it provides some information that may not be otherwise available. Defining R* = E(B − V)/DIB EW, which we consider to be the analogue of R = E(B − V)/Av, we investigated the Galactic longitudinal dependence of R* assuming that the DIB EW value is representative of the interstellar absorption Av. We found that there is an apparent increase of the R* value that corresponds to the abnormal absorption seen towards Cygnus constellation. Finally, we constructed a 2D map of the DIB EW distribution in the Galactic disc basing on our rather limited sample of 253 spectra of 176 Cepheids.
We have derived the abundances of 36 chemical elements in one Cepheid star, ASAS 181024-2049.6, located R
G = 2.53 kpc from the Galactic Centre. This star falls within a region of the inner thin disc ...poorly sampled in Cepheids. Our spectral analysis shows that iron, magnesium, silicon, calcium and titanium local thermodynamic equilibrium abundances in that star support the presence of a plateau-like abundance distribution in the thin disc within 5 kpc of the Galactic Centre, as previously suggested by Martin et al. If confirmed, the flattening of the abundance gradient within that region could be the result of a decrease in the star formation rate due to dynamic effects, possibly from the central Galactic bar.
Context. Calcium is a key element for constraining the models of chemical enrichment of the Galaxy. Aims. Extremely metal-poor stars contain the fossil records of the chemical composition of the ...early Galaxy and it is important to compare Ca abundance with abundances of other light elements, that are supposed to be synthesized in the same stellar evolution phases. Methods. The NLTE profiles of the calcium lines were computed in a sample of 53 extremely metal-poor stars with a modified version of the program MULTI, which allows a very good description of the radiation field. Results. With our new model atom we are able to reconcile the abundance of Ca deduced from the Ca I and Ca II lines in Procyon. This abundance is found to be solar. We find that $\overline{Ca/Fe}$ Ca/Fe = 0.50±0.09 in the early Galaxy, a value slightly higher than the previous LTE estimations. The scatter of the ratios X/Ca is generally smaller than the scatter of the ratio X/Mg where X is a “light metal” (O, Na, Mg, Al, S, and K) with the exception of Al. These scatters cannot be explained by error of measurements, except for oxygen. Surprisingly, the scatter of X/Fe is always equal to, or even smaller than, the scatter around the mean value of X/Ca. We note that at low metallicity, the wavelength of the Ca I resonance line is shifted relative to the (weaker) subordinate lines, a signature of the effect of convection. The Ca abundance deduced from the Ca I resonance line (422.7 nm) is found to be systematically smaller at very low metallicity than the abundance deduced from the subordinate lines. Our computations of the effects of convection (3D effects) are not able to explain this difference. A fully consistent 3D NLTE model atmosphere and line formation scheme would be necessary to fully capture the physics of the stellar atmosphere.
The Gaia RVS benchmark stars Caffau, E.; Bonifacio, P.; Korotin, S. A. ...
Astronomy and astrophysics (Berlin),
07/2021, Letnik:
651
Journal Article
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Context.
The Radial Velocity Spectrometer (RVS) on board the
Gaia
satellite is not provided with a wavelength calibration lamp. It uses its observations of stars with known radial velocity to derive ...the dispersion relation. To derive an accurate radial velocity calibration, a precise knowledge of the line spread function (LSF) of the RVS is necessary. Good-quality ground-based observations in the wavelength range of the RVS are highly desired to determine the LSF.
Aims.
Several radial velocity standard stars are available to the
Gaia
community. The highest possible number of calibrators will surely allow us to improve the accuracy of the radial velocity. Because the LSF may vary across the focal plane of the RVS, a large number of high-quality spectra for the LSF calibration may allow us to better sample the properties of the focal plane.
Methods.
We selected a sample of stars to be observed with UVES at the Very Large Telescope, in a setting including the wavelength range of RVS, that are bright enough to allow obtaining high-quality spectra in a short time. We also selected stars that lack chemical investigation in order to increase the sample of bright, close by stars with a complete chemical inventory.
Results.
We here present the chemical analysis of the first sample of 80 evolved stars. The quality of the spectra is very good, therefore we were able to derive abundances for 20 elements. The metallicity range spanned by the sample is about 1 dex, from slightly metal-poor to solar metallicity. We derived the Rb abundance for all stars and investigated departures from local thermodynamical equilibrium (NLTE) in the formation of its lines.
Conclusions.
The sample of spectra is of good quality, which is useful for a
Gaia
radial velocity calibration. The Rb NLTE effects in this stellar parameters range are small but sometimes non-negligible, especially for spectra of this good quality.
In Galactic halo stars, sulphur has been shown to behave like other alpha-elements, but until now, no comprehensive studies have been done on this element in stars of other galaxies. Here, we use ...high-resolution ESO VLT/FLAMES/GIRAFFE spectra to determine sulphur abundances for 85 stars in the Sculptor dwarf spheroidal galaxy, covering the metallicity range -2.5 < or = Fe/H < or = -0.8. The abundances are derived from the S I triplet at 9213, 9228, and 9238 Angstrom. These lines have been shown to be sensitive to departure from local thermodynamic equilibrium, i.e. NLTE effects. Therefore, we present new NLTE corrections for a grid of stellar parameters covering those of the target stars. The NLTE-corrected sulphur abundances in Sculptor show the same behaviour as other alpha-elements in that galaxy (such as Mg, Si, and Ca). At lower metallicities (Fe/H <, ~ -2) the abundances are consistent with a plateau at S/Fe approximate + 0.16, similar to what is observed in the Galactic halo, S/Fe approximate + 0.2. With increasing Fe/H, the S/Fe ratio declines, reaching negative values at Fe/H > ~ -1.5. The sample also shows an increase in S/Mg with Fe/H, most probably because of enrichment from Type Ia supernovae.
Aims. We obtained spectra for two giants of Crater (Crater J113613-105227 and Crater J113615-105244) using X-Shooter at the VLT, with the purpose of determining their radial velocities and ...metallicities. Methods. Radial velocities were determined by cross-correlating the spectra with that of a standard star. The spectra were analysed with the MyGIsFOS code using a grid of synthetic spectra computed from one-dimensional, local thermodynamic equilibrium (LTE) model atmospheres. Effective temperature and surface gravity were derived from photometry measured from images obtained by the Dark Energy Survey. Results. The radial velocities are 144.3 ± 4.0 km s-1 for Crater J113613-105227 and and 134.1 ± 4.0km s-1 for Crater J113615-105244. The metallicities are Fe/H = −1.73 and Fe/H = −1.67, respectively. In addition to the iron abundance, we were able to determine abundances for nine elements: Na, Mg, Ca, Ti, V, Cr, Mn, Ni, and Ba. For Na and Ba we took into account deviations from LTE because the corrections are significant. The abundance ratios are similar in the two stars and resemble those of Galactic stars of the same metallicity. In the deep photometric images we detected several stars that lie to the blue of the turn-off. Conclusions. The radial velocities imply that both stars are members of the Crater stellar system. The difference in velocity between the two taken at face value implies a velocity dispersion >3.7 km s-1 at a 95% confidence level. Our spectroscopic metallicities agree excellently well with those determined by previous investigations using photometry. Our deep photometry and the spectroscopic metallicity imply an age of 7 Gyr for the main population of the system. The stars to the blue of the turn-off can be interpreted as a younger population that is of the same metallicity and an age of 2.2 Gyr. Finally, spatial and kinematical parameters support the idea that this system is associated with the galaxies Leo IV and Leo V. All the observations favour the interpretation of Crater as a dwarf galaxy.
Aims.
In a previous publication, we reported the first detection of emission in the infrared (IR) triplet at 10 830 Å in the spectra of the classical Cepheid X Cyg. Emission is detectable at phases ...from approximately 0.25 to 0.90. We present and discuss further information on the chromosphere activity of this star, namely International Ultraviolet Explorer (IUE) archive data concerning the emission in the h and k Mg
II
doublet, and near-ultraviolet (NUV) spectra in the vicinity of the H and K Ca
II
doublet. We also present rough estimates of the chromosphere conditions and semi-empirical modelling of the observed emissions in He
I
triplet.
Methods.
A study of the emissions in UV lines of Ca
II
and Mg
II
, and IR lines of He
I
suggests that the emissions observed at certain phases can be explained within the framework of the Gillet's phenomenological model of shock waves in pulsating atmospheres, which was developed from a study of the H
α
behaviour in X Cyg spectra. We used the non-local thermodynamic equilibrium (NLTE) approximation and a simple model of this star's chromosphere for the analysis of the chromosphere indicator profiles.
Results.
We show that under certain assumptions about the properties of the chromosphere, it is possible to describe the presence or absence of emission in the IR lines of He
I
and Ca
II
, and UV lines of Ca
II
and Mg
II
.
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