Context. Magnesium abundances of cool stars with different metallicities are important for understanding the galactic chemical evolution. Aims. This study tests atomic data used in stellar magnesium ...abundance analyses. Methods. We evaluate nonlocal thermodynamical equilibrium (NLTE) line formation for Mg i, using the most up-to-date theoretical and experimental atomic data available so far, and check the Mg abundances from individual lines in the Sun, four well studied A-type stars, and three reference metal-poor stars. Results. With the adopted gf-values, NLTE abundances derived from the Mg i 4703 Å, 5528 Å, and Mg ib lines are consistent within 0.05 dex for each A-type star. The same four Mg i lines in the solar spectrum give consistent NLTE abundances at log NMg/NH = −4.45, when correcting van der Waals damping constants inferred from the perturbation theory. Inelastic Mg+H collisions as treated by Barklem, Belyaev, Spielfiedel, Guitou, and Feautrier serve as an efficient thermalizing process for the statistical equilibrium of Mg i in the atmospheres of metal-poor stars. The use of Mg+H collision data improves Mg abundance determinations for HD 84937 and HD 122563, though it does not completely remove the differences between different lines.
We construct a model atom for Ti i–ii using more than 3600 measured and predicted energy levels of Ti i and 1800 energy levels of Ti ii, and quantum mechanical photoionization cross-sections. ...Non-local thermodynamical equilibrium (NLTE) line formation for Ti i and Ti ii is treated through a wide range of spectral types from A to K, including metal-poor stars with Fe/H down to −2.6 dex. NLTE leads to weakened Ti i lines and positive abundance corrections. The magnitude of NLTE corrections is smaller compared to the literature data for FGK atmospheres. NLTE leads to strengthened Ti ii lines and negative NLTE abundance corrections. For the first time, we have performed NLTE calculations for Ti i–ii in the 6500 ≤ T
eff ≤ 13 000 K range. For four A-type stars, we derived in LTE an abundance discrepancy of up to 0.22 dex between Ti i and Ti ii, which vanishes in NLTE. For four other A–B stars, with only Ti ii lines observed, NLTE leads to a decrease of line-to-line scatter. An efficiency of inelastic Ti i + H i collisions was estimated from an analysis of Ti i and Ti ii lines in 17 cool stars with −2.6 ≤ Fe/H ≤ 0.0. Consistent NLTE abundances from Ti i and Ti ii were obtained by applying classical Drawinian rates for the stars with log g ≥ 4.1, and neglecting inelastic collisions with H i for the very metal-poor (VMP) giant HD 122563. For the VMP turn-off stars (Fe/H ≤ −2 and log g ≤ 4.1), we obtained the positive abundance difference Ti i–ii already in LTE, which increases in NLTE. Accurate collisional data for Ti i and Ti ii are necessary to help solve this problem.
ABSTRACT
We present a new model atom of Zn i-Zn ii based on the most up-to-date photoionization cross-sections, electron-impact excitation rates, and rate coefficients for the Zn i + H i and Zn ii ... + H− collisions. The latter were calculated using the multichannel quantum asymptotic treatment based on the Born–Oppenheimer approach. Non-LTE analysis was performed for the first time for lines of Zn i and Zn ii in the ultraviolet (UV) spectra of two very metal-poor reference stars: HD 84937 and HD 140283. We found consistent non-LTE abundance from the resonance Zn i 2138 Å line, the subordinate lines, and the lines of Zn ii. In both stars, non-LTE leads to 0.17 dex higher average abundance from Zn i, while, for Zn ii lines, non-LTE corrections are minor and do not exceed 0.06 dex. Using lines of Zn i in the high-resolution spectra, we determined the non-LTE abundances for a sample of 80 stars in the −2.5 ≤ Fe/H ≤ 0.2 metallicity range. The Zn/Fe versus Fe/H diagram reveals a dip, with Zn/Fe ≃ 0.3 in the most metal-poor stars, a close-to-solar value for Fe/H ∼−1.2, and increasing Zn/Fe up to 0.3 in the thick disc stars. The close-to-solar metallicity stars have subsolar Zn/H ≃ −0.1, on average. Non-LTE abundances of zinc were derived for the first time for seven reference F- to B-type stars. We provide a grid of the non-LTE abundance corrections.
We performed the non-local thermodynamic equilibrium (non-LTE, NLTE) calculations for Ca i-ii with the updated model atom that includes new quantum-mechanical rate coefficients for Ca i + H i ...collisions from two recent studies and investigated the accuracy of calcium abundance determinations using the Sun, Procyon, and five metal-poor (MP, −2.6 ≤ Fe/H ≤−1.3) stars with well-determined stellar parameters. Including H i collisions substantially reduces over-ionisation of Ca i in the line formation layers compared with the case of pure electronic collisions and thus the NLTE effects on abundances derived from Ca i lines. We show that both collisional recipes lead to very similar NLTE results. As for Ca ii, the classical Drawinian rates scaled by SH = 0.1 are still applied. When using the subordinate lines of Ca i and the high-excitation lines of Ca ii, NLTE provides the smaller line-to-line scatter compared with the LTE case for each star. For Procyon, NLTE removes a steep trend with line strength among strong Ca i lines seen in LTE and leads to consistent Ca/H abundances from the two ionisation stages. In the MP stars, the NLTE abundance from Ca ii 8498 Å agrees well with the abundance from the Ca i subordinate lines, in contrast to LTE, where the abundance difference grows towards lower metallicity and reaches 0.46 dex in BD −13°3442 (Fe/H = −2.62). NLTE largely removes abundance discrepancies between the high-excitation lines of Ca ii and Ca ii 8498 Å obtained for our four Fe/H < −2 stars under the LTE assumption. We investigated the formation of the Ca i resonance line in the Fe/H < −2 stars. When the calcium abundance varies between Ca/H ≃ −1.8 and −2.3, photon loss in the resonance line itself in the uppermost atmospheric layers drives the strengthening of the line core compared with the LTE case, and this effect prevails over the weakening of the line wings, resulting in negative NLTE abundance correction and underestimation of the abundance derived from Ca i 4226 Å compared with that from the subordinate lines, by 0.08 to 0.32 dex. This problem may be related to the use of classical homogeneous (1D) model atmospheres. The situation is improved when the calcium abundance decreases and the Ca i 4226 Å line formation depths are shifted into deep atmospheric layers that are dominated by over-ionisation of Ca i. However, the departures from LTE are still underestimated for Ca i 4226 Å at Ca/H ≃ −4.4 (HE 0557-4840). Consistent NLTE abundances from the Ca i resonance line and the Ca ii lines are found for HE 0107-5240 and HE 1327-2326 with Ca/H ≤−5. Thus, the Ca i/Ca ii ionisation equilibrium method can successfully be applied to determine surface gravities of Ca/H ≾ −5 stars. We provide the NLTE abundance corrections for 28 lines of Ca i in a grid of model atmospheres with 5000 K ≤ Teff ≤ 6500 K, 2.5 ≤ log g ≤ 4.5, −4 ≤ Fe/H ≤ 0, which is suitable for abundance analysis of FGK-type dwarfs and subgiants.
We present a homogeneous set of accurate atmospheric parameters for a complete sample of very and extremely metal-poor stars in the dwarf spheroidal galaxies (dSphs) Sculptor, Ursa Minor, Sextans, ...Fornax, Boötes I, Ursa Major II, and Leo IV. We also deliver a Milky Way (MW) comparison sample of giant stars covering the − 4 < Fe/H < − 1.7 metallicity range. We show that, in the Fe/H ≿ − 3.7 regime, the non-local thermodynamic equilibrium (NLTE) calculations with non-spectroscopic effective temperature (Teff) and surface gravity (log g) based on the photometric methods and known distance provide consistent abundances of the Fe i and Fe ii lines. This justifies the Fe i/Fe ii ionisation equilibrium method to determine log g for the MW halo giants with unknown distance. The atmospheric parameters of the dSphs and MW stars were checked with independent methods. In the Fe/H > − 3.5 regime, the Ti i/Ti ii ionisation equilibrium is fulfilled in the NLTE calculations. In the log g − Teff plane, all the stars sit on the giant branch of the evolutionary tracks corresponding to Fe/H = − 2 to − 4, in line with their metallicities. For some of the most metal-poor stars of our sample, we achieve relatively inconsistent NLTE abundances from the two ionisation stages for both iron and titanium. We suggest that this is a consequence of the uncertainty in the Teff-colour relation at those metallicities. The results of this work provide the basis for a detailed abundance analysis presented in a companion paper.
Studies of the r-process enhanced stars are important for understanding the nature and origin of the r-process better. We present a detailed abundance analysis of a very metal-poor giant star ...discovered in the HERES project, HE 2252-4225, which exhibits overabundances of the r-process elements with r/Fe = +0.80. No firm conclusion can be drawn about the relationship between the first neutron-capture peak elements, Sr to Ru, in HE 2252-4225 and the solar r-process, owing to the uncertainty in the solar r-process. The investigated star has an anomalously high Th/Eu abundance ratio, so that radioactive dating results in a stellar age of tau = 1.5 + or - 1.5 Gyr that is not expected for a very metal-poor halo star.
ABSTRACT
For the first time, we present an extensive study of stars with individual non-LTE (NLTE) abundances for 17 chemical elements from Li to Eu in a sample of stars uniformly distributed over ...the −2.62 ≤ Fe/H ≤ +0.24 metallicity range that is suitable for the Galactic chemical evolution research. The star sample has been kinematically selected to trace the Galactic thin and thick disks and halo. We find new results and improve earlier ones as follows: (i) the element-to-iron ratios for Mg, Si, Ca, and Ti form a metal-poor (MP) plateau at a similar height of 0.3 dex, and the knee occurs at common Fe/H ≃ −0.8. The knee at the same metallicity is observed for O/Fe, and the MP plateau is formed at O/Fe = 0.61. (ii) The upward trend of C/O with decreasing metallicity exists at Fe/H < −1.2, supporting the earlier finding of Akerman et al. (iii) An underabundance of Na relative to Mg in the Fe/H < −1 stars is nearly constant, with the mean Na/ Mg ≃ −0.5. (iv) The K/Sc, Ca/Sc, and Ti/Sc ratios form well-defined trends, suggesting a common site of the K–Ti production. (v) Sr follows the Fe abundance down to Fe/H ≃ −2.5, while Zr is enhanced in MP stars. (vi) The comparisons of our results with some widely used Galactic evolution models are given. The use of the NLTE element abundances gives increased credit to the interpretation of the data in the context of the chemical evolution of the Galaxy.
A new Sc II model atom has been constructed using up-to-date atomic data. To test it, we have carried out nonlocal thermodynamic equilibrium (non-LTE) calculations for three stars with reliably ...determined atmospheric parameters: the Sun, HD 61421 (Procyon), and HD 84937. Allowance for the departures from LTE leads to a decrease in the root-mean-square abundance error compared to the LTE case and agreement, within the limits of this error, of the abundances deduced from different Sc II lines. The solar non-LTE abundance
exceeds the meteoritic abundance recommended by Lodders (2021) by 0.08 dex. However, agreement within 0.02 dex with the meteoritic abundance has been obtained for Procyon. Based on high-resolution spectra, we have determined the LTE and non-LTE scandium abundances for 56 stars in a wide metallicity range,
. The dependence of Sc/Fe on Fe/H demonstrates a similarity with the behavior of the
-process elements: scandium is enhanced relative to iron (Sc/Fe
0.2) in stars with
, and Sc/Fe decreases with increasing Fe/H for a higher metallicity. The scandium abundance correlates with the titanium abundance. The results obtained are important for solving the problem of the origin of scandium.
ABSTRACT
We present atmospheric parameters and abundances for chemical elements from carbon to barium in metal-poor stars in Segue 1 (seven stars), Coma Berenices (three stars), and Triangulum ii ...(one star) ultrafaint dwarf galaxies (UFDs). The effective temperatures rely on new photometric observations in the visible and infra-red bands, obtained with the 2.5 m telescope of the SAI MSU Caucasian observatory. Abundances of up to fourteen chemical elements were derived under the non-local thermodynamic equilibrium (NLTE) line formation, and LTE abundances were obtained for up to five more elements. For the first time, we present abundance of oxygen in Seg 1 S1 and S4, silicon in ComaBer S2 and Tri ii S40, potassium in Seg 1 S1−S6 and ComaBer S1−S3, and barium in Seg 1 S7. Three stars in Segue 1, two stars in Coma Berenices, and Triangulum ii star have very low Na/Mg of −1.08 to −1.67 dex, which is usually attributed in the literature to an odd–even effect produced by nucleosynthesis in massive metal-free stars. We interpret this chemical property as a footprint of first stars, which is not blurred due to a small number of nucleosynthesis events that contributed to chemical abundance patterns of the sample stars. Our NLTE abundances of Sr and Ba in Coma Berenices, Segue 1, and Triangulum ii report on lower Sr/Ba abundance ratio in the UFDs compared to that in classical dwarf spheroidal galaxies and the Milky Way halo. However, in UFDs, just as in massive galaxies, Sr/Ba is not constant and it can be higher than the pure r-process ratio. We suggest a hypothesis of Sr production in metal-poor binaries at the earliest epoch of galactic evolution.
We constructed a comprehensive model atom for C i–C ii using the most up-to-date atomic data available and evaluated the non-local thermodynamic equilibrium (NLTE) line formation for C i and C ii in ...classical 1D models representing the atmospheres of A- and late B-type stars. Our NLTE calculations predict the emission that appears at effective temperature of 9250 to 10 500 K depending on log g in the C i 8335, 9405 Å singlet lines and at T
eff> 15 000 K (log g = 4) in the C i 9061–9111 Å, 9603–9658 Å triplet lines. A pre-requisite of the emission phenomenon is the overionization-recombination mechanism resulting in a depopulation of the lower levels of C i to a greater extent than the upper levels. Extra depopulation of the lower levels of the transitions corresponding to the near-infrared lines, is caused by photon loss in the UV lines C i 2479, 1930, and 1657 Å. We analysed the lines of C i and C ii in Vega, HD 73666, Sirius, 21 Peg, π Cet, HD 22136, and ι Her taking advantage of their observed high-resolution spectra. The C i emission lines were detected in the four hottest stars, and they were well reproduced in our NLTE calculations. For each star, the mean NLTE abundances from lines of the two ionization stages, C i and C ii, including the C i emission lines, were found to be consistent. We show that the predicted C i emission phenomenon depends strongly on whether accurate or approximate electron-impact excitation rates are applied.
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