We have performed non-LTE calculations for O I with a multilevel model atom using currently available atomic data for a set of parameters corresponding to stars of spectral types from A to K. Departures from LTE lead to a strengthening of O I lines, and the difference between the non-LTE and LTE abundances (non-LTE correction) is negative. The non-LTE correction does not exceed 0.05 dex in absolute value for visible O I lines for main-sequence stars in the entire temperature range. For the infrared O I 7771 Å line, the non-LTE correction can reach −1.9 dex. The departures from LTE are enhanced with increasing temperature and decreasing surface gravity. We have derived the oxygen abundance for three A-type mainsequence stars with reliably determined parameters (Vega, Sirius, HD 32115). For each of the stars, allowance for the departures from LTE leads to a decrease in the difference between the abundances from infrared and visible lines, for example, for Vega from 1.17 dex in LTE to 0.14 dex when abandoning LTE. In the case of Procyon and the Sun, inelastic collisions with HI affect the statistical equilibrium of OI, and agreement between the abundances from different lines is achieved when using Drawin’s classical formalism. Based on the O I 6300, 6158, 7771-5, and 8446 Å lines of the solar spectrum, we have derived the mean oxygen abundance log ɛ = 8.74 ± 0.05 using a classical plane-parallel model solar atmosphere and log ɛ _+3D = 8.78 ± 0.03 by applying the 3D corrections taken from the literature.

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.

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 present non-local thermodynamic equilibrium (NLTE) line formation calculations for Ca i and Ca ii in B-F stars. The sign and the magnitude of NLTE abundance corrections depend on line and stellar parameters. We determine calcium abundances for nine stars with reliable stellar parameters. For all stars, where the lines of both species could be measured, the NLTE abundances are found to be consistent within the error bars. We obtain consistent NLTE abundances from Ca ii lines in the visible and near infra-red (IR, 8912−27, 9890 Å) spectrum range, in contrast with LTE, where the discrepancy between the two groups of lines ranges from −0.5 to 0.6 dex for different stars. Our NLTE method reproduces the Ca ii 8912−27, 9890 Å lines observed in emission in the late B-type star HD 160762 with the classical plane-parallel and LTE model atmosphere. NLTE abundance corrections for lines of Ca i and Ca ii were calculated in a grid of model atmospheres with 7000 ≤ T_eff ≤ 13 000 K, 3.2 ≤ log g ≤ 5.0, −0.5 ≤ [Fe/H] ≤0.5, ξ_t = 2.0 km s⁻¹. Our NLTE results can be applied for calcium NLTE abundance determination from Gaia spectra, given that accurate continuum normalization and proper treatment of the hydrogen Paschen lines are provided. The NLTE method can be useful to refine calcium underabundances in Am stars and to provide accurate observational constraints on the models of diffusion.

We present a homogeneous set of accurate atmospheric parameters for a sample of eleven very metal-poor (−3.32 ≤ [Fe/H] ≤−2.61) stars in the Sextans dwarf spheroidal galaxy (dSph) and the non-local thermodynamic equilibrium (NLTE) abundances of, at least, seven chemical elements based on high-resolution UVES/VLT and HDS/Subaru spectra. For each star, its effective temperature and surface gravity were derived from the spectral energy distribution and the known distance, and the Fe abundance was obtained as the average from lines of Fe i and Fe ii. Based on abundances of Mg, Ca, and Ti, we show that all the investigated stars reveal α-enhancements of 0.4 dex to 0.2 dex, and there is a hint of a decline in α/Fe for [Fe/H] >−2.8. The Sextans stars are deficient in Ba. The new result is an extremely tight relation between Ba and Mg, suggesting their common origin in massive stars and Ba synthesis in the r-process events on the time-scales of standard supernovae. The exception is a C-enhanced star S 15-19 which is strongly enhanced in Ba. This star is unlikely to be a CEMP-s star because of low abundances of Sr and Y ([Y/Fe] <−1) that are produced in the s-process as efficiently as Ba and a non-detection of variation in the radial velocity. No distinctions from the Milky Way halo and the Sculptor and Ursa Minor dSphs were found in a history of early enrichment in Na and Ni, namely, the Sextans stars are deficient in Na and have close-to-solar Ni/Fe.