We derived elemental abundances in 27 Cepheids, the great majority situated within a zone of Galactocentric distances ranging from 5 to 7 kpc. One star of our sample, SU Sct, has a Galactocentric ...distance of about 3 kpc, and thus falls in a poorly investigated region of the inner thin disc. Our new results, combined with data on abundances in the very central part of our Galaxy taken from the literature, show that iron, magnesium, silicon, sulphur, calcium and titanium local thermodynamic equilibrium abundance radial distributions, as well as non-local thermodynamic equilibrium distribution of oxygen reveal a plateau-like structure or even positive abundance gradient in the region extending from the Galactic Centre to about 5 kpc.
We performed a non-local thermodynamic equilibrium (NLTE) analysis of the infrared oxygen triplet for a large number of Cepheid spectra obtained with the Hobby–Eberly Telescope. These data were ...combined with our previous NLTE results for stars observed with the Max Planck Gesellschaft Telescope with the aim of investigating the oxygen abundance distribution in the Galactic thin disc. We found the slope of the radial (O/H) distribution to be equal −0.058 dex kpc−1. However, we found some evidence that the distribution might become flatter in the outer parts of the disc. This is supported by the results of other authors who have studied open clusters, planetary nebulae and H ii regions. Some mechanisms of flattening are discussed.
Abstract
We have developed a model atom for Cu with which we perform statistical equilibrium computations that allow us to compute the line formation of Cu i lines in stellar atmospheres without ...assuming local thermodynamic equilibrium (LTE). We validate this model atom by reproducing the observed line profiles of the Sun, Procyon and 11 metal-poor stars. Our sample of stars includes both dwarfs and giants. Over a wide range of stellar parameters, we obtain excellent agreement among different Cu i lines. The 11 metal-poor stars have iron abundances in the range − 4.2 ≤ Fe/H ≤ -1.4, the weighted mean of the Cu/Fe ratios is −0.22 dex, with a scatter of −0.15 dex. This is very different from the results from LTE analysis (the difference between NLTE and LTE abundances reaches 1 dex) and in spite of the small size of our sample, it prompts for a revision of the Galactic evolution of Cu.
Recently, several studies have shown that young, open clusters are characterized by a considerable overabundance in their barium content. In particular, D'Orazi et al. reported that in some younger ...clusters Ba/Fe can reach values as high as ∼0.6 dex. The work also identified the presence of an anticorrelation between Ba/Fe and cluster age. For clusters in the age range ∼4.5 Gyr-500 Myr, this is best explained by assuming a higher contribution from low-mass asymptotic giant branch stars to the Galactic chemical enrichment.
The purpose of this work is to investigate the ubiquity of the barium overabundance in young stellar clusters. We analysed high-resolution spectroscopic data, focusing on the s-process elemental abundance for three nearby young associations, i.e. AB Doradus, Carina-Near and Ursa Major. The clusters have been chosen such that their age spread would complement the D'Orazi et al. study.
We find that while the s-process elements Y, Zr, La and Ce exhibit solar ratios in all three associations, Ba is overabundant by ∼0.2 dex. Current theoretical models cannot reproduce this abundance pattern; thus, we investigate whether this unusually large Ba content might be related to chromospheric effects. Although no correlation between Ba/Fe and several activity indicators seems to be present, we conclude that different effects could be at work which may (directly or indirectly) be related to the presence of hot stellar chromospheres.
Aims. LTE abundances of light elements in extremely metal-poor (EMP) stars have been previously derived from high quality spectra. New derivations, free from the NLTE effects, will better constrain ...the models of the Galactic chemical evolution and the yields of the very first supernovae. Methods. The NLTE profiles of the magnesium and potassium lines have been computed in a sample of 53 extremely metal-poor stars with a modified version of the program MULTI and adjusted to the observed lines in order to derive the abundances of these elements. Results. The NLTE corrections for magnesium and potassium are in good agreement with the works found in the literature. The abundances are slightly changed, reaching a better precision: the scatter around the mean of the abundance ratios has decreased. Magnesium may be used with confidence as reference element. Together with previously determined NLTE abundances of sodium and aluminum, the new ratios are displayed, for comparison, along the theoretical trends proposed by some models of the chemical evolution of the Galaxy, using different models of supernovae.
Context. We present a grid of computed non-local thermodynamic equilibrium (NLTE) equivalent widths (EW) and NLTE abundance corrections for four Ba ii lines: 4554, 5853, 6141, and 6496 Å. Aims. The ...grid can be useful in deriving the NLTE barium abundance in stars having parameters in the following ranges: effective temperature from 4000 K to 6500 K, surface gravity log g from 0 to 5, microturbulent velocity 0 km s-1 to 3 km s-1, metallicity Fe/H from –2 to +0.5, and Ba/Fe from –0.4 to +0.6. The NLTE abundance can be either derived by EW interpolation (using the observed Ba ii line EW) or by using the NLTE correction applied to a previously determined LTE abundance. Methods. Ba ii line equivalent widths and the NLTE corrections were calculated using the updated MULTI code and the Ba ii atomic model that was previously applied to determine the NLTE barium abundance in different types of stars. Results. The grid is available on-line through the web, and we find that the grid Ba NLTE corrections are almost as accurate as direct NLTE profile fitting (to within 0.05–0.08 dex). For the weakest Ba ii line (5853 Å) the LTE abundances almost agree with the NLTE abundances, whereas the other three Ba ii lines, 4554, 6141, and 6496 Å, need NLTE corrections even at the highest metallicities tested here. The 4554 Å line is extremely strong and should not be used for abundance analysis above Fe/H = −1. Furthermore, we tested the impact of different model atmospheres and spectrum synthesis codes and found average differences of 0.06 dex and 0.09 dex, respectively, for all four lines. At these metallicities we find an average ΔNLTE of ± 0.1 dex for the three useful Ba lines for subsolar cool dwarfs.
ABSTRACT
For the first time, we describe a detection of the emission in the infrared (IR) He i triplet at 10 830 Å in the classical Cepheid X Cyg. Emissions are clearly seen at phases approximately ...from 0.25 to 0.85. The IR redshifted He i emission is excited by shock wave in the uppermost layers of the star’s envelope, in its falling layers. This is a first detection of the helium IR emission in the classical Cepheids.
ABSTRACT
We present a study of departures from Local Thermodynamic Equilibrium (LTE) in the formation of infrared (IR) lines of Na i, Mg i, Al i, S i, K i, and Sr ii in unevolved stars of spectral ...types F, G, K and metallicities around the solar metallicity. The purpose of this investigation is to identify lines of these species that can be safely treated with the LTE approximation in the IR spectra of these types of stars. We employ a set of 40 stars observed with the GIANO spectrograph at the 3.5 m Telescopio Nazionale Galileo and previously investigated by Caffau et al. We were able to identify many lines that can be treated in LTE for all the above-mentioned species, except for Sr ii. The latter species can only be studied using three lines in the J band, but all three of them display significant departures from LTE. With our small-size, but high-quality sample, we can determine robustly the trends of the abundance ratios with metallicity, confirming the trends apparent from a sample that is larger by several orders of magnitude, but of lower quality in terms of resolution and S/N ratio.
We have checked the consistency between the copper abundance derived in six metal-poor stars using ultraviolet (UV) Cu II lines, which are assumed to form in the local thermodynamic equilibrium ...(LTE), and UV Cu I lines, treated in the non-local thermodynamic equilibrium (NLTE). The stars in our programme cover the atmospheric parameters that are typical for intermediate temperature dwarfs: the effective temperature is in the range of approximately 5800-6100 K, the surface garvity is from 3.6 to 4.5 and the metallicity is from about -1 to -2.6 dex. We have obtained good agreement for the abundances from these two sets of lines, and this demonstrates the reliability of our NLTE copper atomic model. We confirm that no underabundace of this element is seen at low metallicities: the mean Cu/Fe value is about -0.2 dex while, as follows from previous LTE studies, copper behaves as a secondary element and the Cu/Fe ratio in the range of Fe/H from -2 to -3 dex should be about -1 dex. According to our NLTE data, copper behaves as a primary element in the low-metallicity regime. We also conclude that our new NLTE copper abundance in metal-poor stars requires significant reconsideration of this element's yields in explosive nucleosynthesis.
Aims . The purpose of this work is to spectroscopically analyse the classical Cepheid V708 Car. A preliminary check of the spectrum of V708 Car showed that this is a lithium-rich supergiant. We also ...found that V708 Car has an unusual chemical composition in that the abundances of various elements correlate with their condensation temperatures. We tried to find an explanation of this feature, which is unusual for classical Cepheids. Methods . For the spectroscopic analysis, we used methods based on the assumption of local and non-local thermodynamic equilibrium. Results . We determined the fundamental parameters of our program star V708 Car. This long-period Cepheid has a mass of about 12 M ⊙ . We derived the abundances of 27 chemical elements in this star. They are clearly correlated with their condensation temperature: the higher the condensation temperature, the lower the abundance (there are exceptions for sodium and barium, however). We explain this peculiar chemical composition of the V708 Car atmosphere by the gas–dust separation in the envelope of this star. A similar mechanism leads to the observed peculiarities of the chemical composition of λ Boo, W Vir, and asymptotic giant branch stars.
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