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.
A comprehensive model atom was constructed for C i using the most up-to-date atomic data. We evaluated non-local thermodynamical equilibrium (NLTE) line formation for neutral carbon in classical ...one-dimensional (1D) models representing atmospheres of late-type stars, where carbon abundance varies from the solar value down to C/H = −3. NLTE leads to stronger C i lines compared with their local thermodynamical equilibrium (LTE) strength and negative NLTE abundance corrections, ΔNLTE. The deviations from LTE are large for the strong lines in the infrared (IR), with ΔNLTE = −0.10 to −0.45 dex, depending on stellar parameters, and minor for the weak lines in the visible spectral range, with |ΔNLTE| ≤ 0.03 dex. The NLTE abundance corrections were found to be dependent on the carbon abundance in the model. As the first application of the treated model atom, carbon NLTE abundances were determined for the Sun and eight late-type stars with well-determined stellar parameters that cover the −2.56 ≤ Fe/H ≤ −1.02 metallicity range. Consistent abundances from the visible and IR lines were found for the Sun and the most metal-rich star of our sample, when applying a scaling factor of S
H = 0.3 to the Drawinian rates of C+H collisions. Carbon abundances were also derived from the molecular CH lines and agree with those from the atomic C i lines for each star. We present the NLTE abundance corrections for lines of C i in the grid of model atmospheres applicable to carbon-enhanced (CEMP) stars.
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.
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 have performed statistical equilibrium calculations for Ca I–Ca II, Ti I–Ti II, and Fe I–Fe II by taking into account the nonequilibrium line formation conditions (the non-LTE approach) in model ...atmospheres of giant stars with effective temperatures 4000 K ≤
T
eff
≤ 5000 K and metal abundances −4 ≤ Fe/H ≤ 0. The dependence of departures from LTE on atmospheric parameters has been analyzed. We present the non-LTE abundance corrections for 28 Ca I lines, 42 Ti I lines, 54 Ti II lines, and 262 Fe I lines and a three-dimensional interpolation code to obtain the non-LTE correction online for an individual line and specified atmospheric parameters.
Abstract
The most metal-poor stars are the oldest objects, they provide a unique opportunity to study the earliest epoch of the Galaxy formation and individual nucleosynthesis events. These stars ...should be investigated with a scrupulous care, taking into account all available photometric, spectroscopic, and astrometric informations. We determined atmospheric parameters for 17 ultra metal-poor (UMP) stars, using an extensive method based on colour-Teff calibrations, isochrones, Gaia DR2 trigonometric parallaxes, and non-local thermodynamic equilibrium (NLTE) analysis of the Ca i/Ca ii ionization equilibrium and the Balmer line wings. We updated the model atom of Ca i- ii by including recent quantum-mechanical rate coefficients for the Ca i + H i and Ca ii + H i inelastic collisions. For any line of Ca i and Ca ii in our sample stars, the changes in collisional data result in a shift of smaller than 0.05 dex in the NLTE abundance. We determined magnesium and calcium NLTE and LTE abundances of our sample stars. For 10 stars, we found close-to-solar Ca/Mg NLTE abundance ratios. In the remaining stars, magnesium and calcium abundances do not follow each other, such that Ca/Mg varies between −3.15 and + 0.36, suggesting a contribution to stellar Mg and Ca abundances from a small number of supernova explosions with different properties. The obtained atmospheric parameters will be used in the forthcoming paper to determine NLTE abundances of chemical elements observed in spectra of the UMP stars.
We have improved the Ba II model atom by taking into account the excitation of transitions through collisions with hydrogen atoms with the rate coefficients from the quantum-mechanical calculations ...of Belyaev and Yakovleva (2018). Using high-resolution spectra and Ba II line modeling when abandoning the assumption of LTE, we have determined the fraction of barium isotopes with an odd mass number (
f
odd
) in four Galactic halo giants with well-known atmospheric parameters. We use a method based on the requirement that the abundances from the resonance (Ba II 4554 Å) and subordinate (Ba II 5853, 6496 Å) lines be equal. A accuracy of 0.04 dex in determining the barium abundance from individual lines has been achieved. In three stars (HD 2796, HD 108317, and HD 122563)
f
odd
≳ 0.4. This suggests that ≳80% of the barium observed in these stars was synthesized in the
r
-process. In HD 128279
f
odd
= 0.27 exceeds the fraction of odd barium isotopes in the Solar system, but only slightly. The dominance of the
r
-process at the formation epoch of the stars from our sample is confirmed by the presence of a europium overabundance relative barium in them, with Eu/Ba > 0.3. We have calculated the non-LTE barium abundance corrections for five Ba II lines and investigated their dependence on atmospheric parameters in the ranges of effective temperatures from 4500 to 6500 K, surface gravities log
g
from 0.5 to 4.5, and metallicities Fe/H from 0 to −3.
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