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.
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.
Context. Heavy element abundances in extremely metal-poor stars provide strong constraints on the processes of forming these elements in the first stars. Aims. We attempt to determine precise ...abundances of strontium in a homogeneous sample of extremely metal-poor stars. Methods. The abundance of strontium in 54 very or extremely metal-poor stars, was redetermined by abandoning the local thermodynamic equilibrium (LTE) hypothesis, and fitting non-LTE (NLTE) profiles to the observed spectral lines. The corrected Sr abundances and previously obtained NLTE Ba abundances are compared to the predictions of several hypothetical formation processes for the lighter neutron-capture elements. Results. Our NLTE abundances confirm the previously determined huge scatter of the strontium abundance in low metallicity stars. This scatter is also found (and is even larger) at very low metallicities (i.e. early in the chemical evolution). The Sr abundance in the extremely metal-poor (EMP) stars is compatible with the main r-process involved in other processes (or by variations of the r-process), as we briefly discuss.
Aims. Aluminium is a key element to constrain the models of the chemical enrichment and the yields of the first supernovae. But obtaining precise Al abundances in extremely metal-poor (EMP) stars ...requires that the non-LTE effects be carefully taken into account. Methods. The NLTE profiles of the blue resonance aluminium lines have been computed in a sample of 53 extremely metal-poor stars with a modified version of the program MULTI applied to an atomic model of the Al atom with 78 levels of Al I and 13 levels of Al II, and compared to the observations. Results. With these new determinations, all the stars of the sample show a ratio Al/Fe close to the solar value: Al/Fe =-0.06\pm0.10 with a very small scatter. These results are compared to the models of the chemical evolution of the halo using different models of SN II and are compatible with recent computations. The sodium-rich giants are not found to be also aluminium-rich and thus, as expected, the convection in these giants only brings to the surface the products of the Ne-Na cycle.
In this work, we present and discuss the observations of the Mn abundances for 247 FGK dwarfs, located in the Galactic disc with metallicity −1 < Fe/H < +0.3. The observed stars belong to the ...substructures of the Galaxy thick and thin disks, and to the Hercules stream. The observations were conducted using the 1.93 m telescope at Observatoire de Haute-Provence (OHP, France) equipped with the echelle-type spectrographs ELODIE and SOPHIE. The abundances were derived under the LTE approximation, with an average error for the Mn/Fe ratio of 0.10 dex. For most of the stars in the sample, Mn abundances are not available in the literature. We obtain an evolution of Mn/Fe ratio with the metallicity Fe/H consistent with previous data compilations. In particular, within the metallicity range covered by our stellar sample, the Mn/Fe ratio is increasing with the increase of metallicity. This due to the contribution to the Galactic chemical evolution of Mn and Fe from thermonuclear supernovae. We confirm the baseline scenario where most of the Mn in the Galactic disc and in the Sun is made by thermonuclear supernovae. In particular, the effective contribution from core-collapse supernovae to the Mn in the Solar system is about 10–20 per cent. However, present uncertainties affecting the production of Mn and Fe in thermonuclear supernovae are limiting the constraining power of the observed Mn/Fe trend in the Galactic discs on, e.g. the frequency of different thermonuclear supernovae populations. The different production of these two elements in different types of thermonuclear supernovae needs to be disentangled by the dependence of their relative production on the metallicity of the supernova progenitor.
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
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.
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.
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.