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We present accurate element abundance patterns based on the non-local thermodynamic equilibrium (non-LTE, NLTE) line formation for 14 chemical elements from He to Nd for a sample of nine A9 to B3 type stars with well determined atmospheric parameters and low rotational velocities. We constructed new model atom of Zr II-III and updated model atoms for Sr II and Ba II by implementing the photoionization cross sections from calculations with the Dirac B-spline R-matrix method. The NLTE abundances of He to Fe in the stars HD~17081, HD~32115, HD~160762, and HD~209459 are found to be consistent with the solar abundances, and HD~73666 being a Blue Struggler does not reveal deviations from chemical composition of the Praesepe cluster. Three of these stars with an effective temperature of lower than 10500~K have supersolar abundances of Sr, Zr, Ba, and Nd, and our results suggest the presence of a positive correlation between stellar effective temperature and abundance. For each star, enhancement of Ba is higher than that for any other heavy element. We propose that the solar Ba abundance is not representative of the galactic Ba abundance at modern epoch. The status of HD~145788 was not clarified: this star has solar abundances of C to Si and enhancements of Sr to Ba similar to that for superficially normal stars of similar temperature, while overabundant Ca, Ti, and Fe. The NLTE abundances of Vega support its status of a mild lambda~Bootis star., 16 pages, 8 figures, accepted for publication in MNRAS

We have constructed a new K I model atom using currently available atomic data. We have performed calculations for K I by abandoning the assumption of local thermodynamic equilibrium (non-LTE) for the Sun and three moderately metal-poor dwarf stars. To take into account the inelastic processes in collisions with hydrogen atoms, for the first time we have used the rate constants calculated by including the fine structure of K I levels and analyzed the influence of their application on the non-LTE results compared to the use of the rate constants calculated for combined levels. In agreement with the non-LTE studies available in the literature, K I is subject to overrecombination, which leads to a strengthening of spectral lines and negative abundance corrections. The non-LTE effects are shown to weaken when using new collisional data. We reached the same conclusion when comparing the non-LTE corrections calculated for the Ca II 8662 A line in model atmospheres with $$\textrm{[Fe/H]}={-}4.5$$ using the Ca II $$+$$ H I collision rates derived with and without allowance for the fine structure of Ca II levels. However, the effect is very small for the other two triplet lines, Ca II 8498 and 8542 A. The solar non-LTE abundance $$\log\varepsilon_{\textrm{K}}=5.09\pm 0.08$$ derived from five lines is consistent with the meteoritic one within 0.01 dex. Despite the fact that in the atmospheres of the program stars the departures from LTE are larger than those in the Sun, the differential abundance [K/H] is almost independent of which collisional data set is used: the one calculated with or without allowance for the fine structure of K I levels.

Aims. The aim of this work is to determine fundamental parameters of three Ap stars, GO And (HD 4778), κ Psc (HD 220825), and 84 UMa (HD 120198), using spectroscopic techniques. By analysing these stars, we complete the sample of Ap stars for which fundamental parameters have additionally been derived by means of interferometry. This enables a cross-comparison of results derived by direct and indirect methods. Methods. Our study is based on the analysis of high-resolution spectra with a high signal-to-noise ratio that were obtained with ESPaDOnS spectrograph. We used an iterative method of fundamental parameter determinations that includes self-consistent modelling of the stellar atmosphere, taking individual abundances of chemical elements into account and subsequently fitting a theoretical spectral energy distribution to the observed distribution. The quality of the spectroscopic determinations was evaluated through a comparison with the interferometric results. Results. For all investigated stars, we determined fundamental parameters and derived chemical abundances that are typical for Ap stars. The abundances are mainly characterised by a gradual increase of heavy element atmospheric abundances from an order of magnitude for iron peak elements up to very significant excesses of 3-4 dex of the rare-earth elements relative to the solar values. The only exception is Ba, whose abundance is close to the solar abundance. There is also a significant He deficiency in the atmospheres of HD 120198 and HD 220825, whereas the He abundance in HD 4778 is close to the solar abundance. We do not find significant Fe and Cr stratification. Using these abundances, we constructed self-consistent atmospheric models for each star. The effect of the surface chemical inhomogeneity on the derived fundamental parameters did not exceed ±100 K in effective temperature, which lies within the range of errors in similar self-consistent analyses of Ap stars. Finally, we compared spectroscopically derived effective temperatures, radii, and luminosity for 13 out of 14 Ap stars in a benchmark sample with the interferometric results. While radii and luminosity agree within the quoted errors of both determinations, the spectroscopic effective temperatures are higher than the interferometric temperatures for stars with Teff > 9000 K. The observed hydrogen line profiles favour the spectroscopically derived temperatures. © ESO 2021., This research has made use of the data from Gaia DR2 catalogues through the VizieR catalogue access tool. The use of the VALD database is acknowledged. This work is based on observations obtained with ESpAdOnS spectropolarimeter. We use the archived data from EsPaDOnS spectrograph with programme ID: ID 09BQ78 (HD 4778), ID 16AC27 (HD 120198), and ID 18BC22 (HD 220825). A.R. acknowledges the financial support from the grant RFBR, project number 19-32-90147. D.S. acknowledges the financial support from the State Agency for Research of the Spanish MCIU through the “Center of Excellence Severo Ochoa” award to the Instituto de Astrofísica de Andalucía (SEV-2017-0709).

We performed the non-local thermodynamic equilibrium (non-LTE) calculations for Ti I-II with the updated model atom that includes quantum-mechanical rate coefficients for inelastic collisions with hydrogen atoms. We have calculated for the first time the rate coefficients for bound-bound transitions in inelastic collisions of titanium atoms and ions with hydrogen atoms and for the charge-exchange processes: Ti I + H Ti II + H- and Ti II + H Ti III + H-. The influence of these data on non-LTE abundance determinations has been tested for the Sun and four metal-poor stars. For Ti I and Ti II, the application of the derived rate coefficients has led to an increase in the departures from LTE and an increase in the titanium abundance compared to that, obtained with approximate formulas for the rate coefficients. In metal-poor stars, we have failed to achieve consistent non-LTE abundances from lines of two ionization stages. The known in the literature discrepancy in the non-LTE abundances from Ti I and Ti II lines in metal-poor stars cannot be solved by improvement of the rates of inelastic processes in collisions with hydrogen atoms in non-LTE calculations with classical model atmospheres., Comment: Published in Astronomy Letters

For three stars in the ultra-faint dwarf (UFD) galaxy Bootes I we have determined the atmospheric parameters, performed a new reduction of high-resolution spectra from the Subaru archive, and derived the abundances of eight chemical elements without using the LTE assumption. As a result, among the galaxies of its class Bootes I now has the largest sample of stars (11) with a homogeneous set of atmospheric parameters and chemical abundances, and this makes it the most promising for studying the chemical evolution of UFD galaxies. We show that in the range -3, Comment: 10 pages, 9 figures, 7 tables

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.

Elemental abundances of the most metal-poor stars reflect the conditions in the early Galaxy and the properties of the first stars. We present a spectroscopic follow-up of two ultra metal-poor stars ([Fe/H], Comment: 17 pages, 11 figures, accepted for publication in MNRAS

We present the results of our modeling of the O I line formation under non-LTE conditions in the atmospheres of FG stars. The statistical equilibrium of O I has been calculated using Barklem’s quantum-mechanical rates of inelastic collisions with hydrogen atoms. We have determined the non-LTE oxygen abundance from atomic O I lines for the Sun and 46 FG stars in a wide metallicity range, −2.6 < [Fe/H] < 0.2. The application of accurate atomic data has led to an increase in the departures from LTE and a decrease in the oxygen abundance compared to the use of Drawin’s theoretical approximation. The change in the non-LTE abundance from the infrared O I 7771-5 A triplet lines is 0.11 dex for solar atmospheric parameters and diminishes in absolute value with decreasing metallicity. We have revised the [O/Fe]–[Fe/H] relationship derived by us previously. The change in [O/Fe] is small in the [Fe/H] range from −1.5 to 0.2. For stars with [Fe/H] < −1 the [O/Fe] ratio has increased so that [O/Fe] = 0.60 at [Fe/H] = −0.8 and rises to [O/Fe] = 0.75 at [Fe/H] = −2.6.

The atmospheric parameters and abundances of Mg, Si, Ca, and Ti have been determined for 20 stars using the Gaia DR2 parallaxes, high-resolution spectra, and the non-local thermodynamic equilibrium (non-LTE) line formation modeling. A sample of stars with homogeneous data on the abundances of alpha-process elements has thus been increased to 94. It is shown that applying a non-LTE approach and classical 1D atmospheric models with spectroscopically determined surface gravities based on Fe~I and Fe~II lines yields reliable results. Analysis of the full sample confirms the conclusions of earlier studies indicating enhancements of Mg, Si, Ca, and Ti relative to Fe for halo and thick-disk stars, and larger enhancements for the thick-disk stars compared to the thin-disk stars of similar metallicities. The following new results are obtained. The ratios [Mg/Fe], [Si/Fe], [Ca/Fe], and [Ti/Fe] in the thick disk remain constant and similar to each other at the level 0.3 when [Fe/H] < -0.4, and fall off when the metallicity becomes higher, suggesting the onset of the production of iron in Type Ia supernovae. Halo stars have the same [alpha/Fe] values independent of their distance (within ~ 8 kpc of the Sun), providing evidence for a universal evolution of the abundances of alpha-process elements in different parts of the Galaxy. The enhancements relative to iron for halo stars are, on average, similar, at the level 0.3 dex, for Mg, Si, Ca, and Ti. These data are important for constraining the nucleosynthesis models. The star-to-star scatter of [alpha/Fe] increases for [Fe/H] < -2.6, while the scatter of the ratios between the different alpha-process elements remains small, possibly indicating incomplete mixing of nucleosynthesis products at the epoch of the formation of these stars., 20 pages, 4 tables, 4 figures

For a sample of dwarf stars close to the Sun with well-known atmospheric parameters and an iron abundance in the range −2.6 −0.7, and the titanium overabundance relative to iron is 0.3. The results confirm that not only oxygen but also titanium are synthesized in the α-process. Our data can be used to test the Galactic chemical evolution models.

Non-local thermodynamic equilibrium (non-LTE) line formation for Ca I-Ca II, Ti I-Ti II, and Fe I-Fe II is considered in model atmospheres of giant stars with an effective temperature of 4000 K $\le$ Teff $\le$ 5000 K and a metal abundance of -4 $\le$ [Fe/H] $\le$ 0. The departures from LTE are analyzed depending on atmospheric parameters. We present the non-LTE abundance corrections for 28 lines of Ca I, 42 lines of Ti I, 54 lines of Ti II, and 262 lines of Fe I and a three-dimensional interpolation code to obtain the non-LTE correction online (http://spectrum.inasan.ru/nLTE/) for an individual spectral line and given atmospheric parameters., 14 pages, 7 figures, published in Astronomy Letters, 42, 606-615 (2016)

We construct a comprehensive model atom for TiI-II using more than 3600 measured and predicted energy levels of TiI and 1800 energy levels of TiII, and quantum mechanical photoionisation cross-sections. Non-local thermodynamical equilibrium (NLTE) line formation for the two ions of titanium 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 systematically depleted total absorption in the TiI lines and to positive abundance corrections. The magnitude of NLTE abundance corrections is smaller compared to the available literature data for the cool (FGK type) atmospheres. NLTE leads to strengthened TiII lines and to negative NLTE abundance corrections. For the first time, we performed the NLTE calculations for TiI-II in the 6500 K 4.1, and neglecting inelastic collisions with HI for the VMP giant HD 122563. For the very metal-poor turn-off stars ([Fe/H] < -2 and log g < 4.1), the abundance difference TiI-TiII was obtained to be positive already in LTE and it increases in NLTE. The accurate collisional data for TiI and TiII are desired to find a clue to this problem.

A long sought after goal using chemical abundance patterns derived from metal-poor stars is to understand the Galactic chemical evolution (GCE) and to pin down the nature of the first stars (Pop III). Here, we use a sample of 14 metal-poor stars observed with the high-resolution spectrograph PEPSI at the LBT to derive abundances of 32 elements (34 including limits). We present well-sampled abundance patterns for all stars obtained using local thermodynamic equilibrium (LTE) radiative transfer codes and 1D hydrostatic model atmospheres. It is currently well known that the assumptions of 1D and LTE may hide several issues, thereby introducing biases in our interpretation as to the nature of the first stars and the GCE. Hence, we use non-LTE (NLTE) and correct the abundances using 3D model atmospheres to present a physically more reliable pattern. In order to infer the nature of the first stars, we compare unevolved, cool stars, enriched by a single event (`mono-enriched'), with a set of yield predictions to pin down the mass and energy of the Pop III progenitor. To date, only few bona fide second generation mono-enriched stars are known. A simple x^2-fit may bias our inferred mass and energy just as much as the simple 1D LTE abundance pattern, and we thus carried out our study with an improved fitting technique considering dilution and mixing. Our sample presents Carbon Enhanced Metal-Poor (CEMP) stars, some of which are promising true second generation (mono-enriched) stars. The unevolved, dwarf BD+09_2190 shows a mono-enriched signature which, combined with kinematical data, indicates that it moves in the outer halo and likely has been accreted onto the Milky Way early on. The Pop III progenitor was likely of 25.5M and 0.6 10^51erg (foe)/19.2M and 1.5foe in LTE/NLTE. Finally, we explore the predominant donor and formation site of the rapid and slow neutron-capture elements. Abridged, Comment: 22 pages, 16 figures, online material. Accepted for publication in A&A

Context. Mixing-length theory is used to treat stellar convection. As a simulation in one-dimensional stellar atmospheres models, the mixing-length parameter α is calibrated from the Sun and then applied to other stars. However, there is no strong evidence to suggest that α should be the same for stars of different evolutionary stages. Aims. We evaluate the impact of the α value on the metallicity of different types of stars and investigate the correlation between the metallicity discrepancy (Δ[Fe∕H]) and stellar parameters (Teff, log g). Methods. We selected ten well-studied field stars and one open cluster of nine members for which high-resolution and high signal-to-noise spectra are available. The model atmospheres were calculated with the code MAFAGS-OS. We derived iron abundances from Fe I and Fe II lines both under local thermodynamic equilibrium and non-LTE conditions using a spectrum synthesis method. After deriving [Fe/H] for each line, we calculated Δ[Fe∕H] with two different α values, fixed solar-calibrated α, and α obtained for each star individually. Finally, we investigated the correlation between Δ[Fe∕H] caused by revised α with stellar parameters. Results. For FGK dwarf stars, the Δ[Fe∕H] caused by the α correction is less than 0.02 dex, while for turn-off and giant stars, the Δ[Fe∕H] values are no more than 0.03 dex, which are lower than typical uncertainties in metallicity. For main-sequence stars, Δ[Fe∕H] versus Teff and Δ[Fe∕H] versus log g are well fit by linear relations.

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 A) spectrum range, in contrast with LTE, where the discrepancy between the two groups of lines ranges from -0.5 dex to 0.6 dex for different stars. Our NLTE method reproduces the Ca II 8912-27, 9890 A 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 K < Teff < 13000 K, 3.2 < log g < 5.0, -0.5 < [Fe/H] < 0.5, Vt = 2.0 km/s. Our NLTE results can be applied for calcium NLTE abundance determination from Gaia spectra, given that accurate continuum normalisation 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., accepted for publication in MNRAS, source file contains on-line tables

We performed extensive tests of the accuracy of atmospheric parameter determination for FGK stars based on the spectrum fitting procedure Spectroscopy Made Easy (SME). Our stellar sample consists of 13 objects, including the Sun, in the temperature range 5000--6600~K and metallicity range -1.4 -- +0.4. The analysed stars have the advantage of having parameters derived by interferometry. For each star we use spectra obtained with different spectrographs and different signal-to-noise ratios (S/N). For the fitting we adopted three different sets of constraints and test how the derived parameters depend upon the spectral regions (masks) used in SME. We developed and implemented in SME a new method for estimating uncertainties in the resulting parameters based on fitting residuals, partial derivatives, and data uncertainties. For stars in the 5700--6600 K range the best agreement with the effective temperatures derived by interferometry is achieved when spectrum fitting includes the H$\alpha$ and H$\beta$ lines, while for cooler stars the choice of the mask does not affect the results. The derived atmospheric parameters do not strongly depend on spectral resolution and S/N of the observations, while the uncertainties in temperature and surface gravity increase with increasing effective temperature, with minima at 50~K in Teff and 0.1~dex in log g, for spectra with S/N=150--200. A NLTE analysis of the TiI/TiII and FeI/FeII ionisation equilibria and abundances determined from the atomic CI (NLTE) and molecular CH species supports the parameters we derived with SME by fitting the observed spectra including the hydrogen lines., Comment: 16 pages, 8 figures, 4 tables, accepted in MNRAS

We present Non-Local Thermodynamic Equilibrium (Non-LTE) abundance corrections for Mg, Ca, and Fe in 12 ultra metal-poor (UMP) stars ([Fe/H] < −4.00). We show that they increase in absolute value toward the lower metallicity up to 0.45 dex for Mg, 0.30 dex for Ca, and 1.00 dex for Fe. This represents a first step toward a full Non-LTE analysis of chemical species in the UMP stars that will enable us to put useful constraints on the properties of the “First” stars.

We present the NLTE abundances of 10 chemical species in 65 very metal-poor stars in eight dSphs and the Milky Way halo. The classical dSphs Sculptor, Ursa Minor, Sextans, and Fornax reveal a similar plateau at [α/Fe] = 0.3 for each of Mg, Ca, and Ti, similarly to the MW halo. We provide the evidence for a decline in α/Fe in the Boötes I UFD, that is probably due to the ejecta of SNeIa. The dichotomy in the [Sr/Ba] versus [Ba/H] diagram is observed in the classical dSphs, similarly to the MW halo, calling for two different nucleosynthesis channels for Sr. The Boötes I and UMa II UFDs reveal very similar ratios of [Sr/Mg] = −1.3 and [Ba/Mg] = –1. The stars in the Coma Berenices and Leo IV UFDs are even poorer in Sr and Ba. The subsolar Sr/Ba ratios of Boötes I and UMa II indicate a common r-process origin of their n-capture elements.

Context. Iron plays a crucial role in studies of late-type stars. In their atmospheres, neutral iron is the minority species, and lines of Fe I are subject to the departures from local thermodynamic equilibrium (LTE). In contrast, one believes that LTE is a realistic approximation for Fe II lines. The main source of the uncertainties in the non-LTE (NLTE) calculations for cool atmospheres is a treatment of inelastic collisions with hydrogen atoms. Aims. Our aim is to investigate the effect of Fe I + H I and Fe II + H I collisions and their different treatments on the Fe I/Fe II ionisation equilibrium and iron abundance determinations for three Galactic halo benchmark stars (HD 84937, HD 122563, and HD 140283) and a sample of 38 very metal-poor giants in the dwarf galaxies with well known distances. Methods. We performed the NLTE calculations for Fe I–Fe II by applying quantum-mechanical rate coefficients for collisions with H I from recent papers. Results. We find that collisions with H I serve as efficient thermalisation processes for Fe II, to an extent that the NLTE abundance corrections for Fe II lines do not exceed 0.02 dex, in absolute value, for [Fe/H] ≳−3, and reach +0.06 dex at [Fe/H] ~−4. For a given star, different treatments of Fe I + H I collisions lead to similar average NLTE abundances from the Fe I lines, although discrepancies in the NLTE abundance corrections exist for individual lines. By using quantum-mechanical collisional data and the Gaia-based surface gravity, we obtain consistent abundances from the two ionisation stages, Fe I and Fe II, for red giant HD 122563. For turn-off star HD 84937, and subgiant HD 140283, we analyse the iron lines in the visible and the ultra-violet (UV, 1968–2990 Å) ranges. For either Fe I or Fe II, abundances from the visible and UV lines are found to be consistent in each star. The NLTE abundances from the two ionisation stages agree within 0.10 dex and 0.13 dex for two different treatments of Fe I + H I collisions. The Fe I/Fe II ionisation equilibrium is achieved for each star of our stellar sample in the dwarf galaxies, with the exception of stars at [Fe/H] ≲−3.7.

Solar oxygen abundance is a key parameter for the studies of solar physics. Oxygen abundances of cool stars with different metallicities are important for understanding the galactic chemical evolution. We present non-LTE calculations for O I with the classical plane-parallel (1D) model atmospheres for a set of stellar parameters corresponding to stars of spectral types from A to K. Non-LTE leads to strengthening the O I lines, and the difference between the non-LTE and LTE abundances (non-LTE correction) is negative. The departures from LTE grow toward higher effective temperature and lower surface gravity. In the entire temperature range and log g = 4, the non-LTE correction does not exceed 0.05 dex in absolute value for lines of O I in the visible spectral range. The non-LTE corrections are significantly larger for the infrared O I 7771-5, 8446 Å lines and reach an order of magnitude for A-type stars. To differentiate the effects of inelastic collisions with electrons and neutral hydrogen atoms on the statistical equilibrium (SE) of O I, we derived the oxygen abundance for the five well studied A-type stars. For each star, non-LTE largely removes the difference between the infrared and visible lines found in LTE. In the case of cool stars (Sun and Procyon), inelastic collisions with H I affect the SE of O I, and agreement between the abundances from different lines is achieved when using the Drawin's formalism for collisional rates calculations. The solar mean oxygen abundance from the six lines is ϵ = 8.74 ± 0.05, when using the MAFAGS-OS solar model atmosphere and ϵ = 8.78 ± 0.03, when applying the 3D corrections taken from the literature. The non-LTE abundances of oxygen are derived for the sample of cool dwarfs with various metallicities on high-resolution spectra observed in the Lick observatory.

A spectrum of a hot spot, produced by radiation of accretion shock at T Tauri star's surface, has been calculated taking into account non-LTE effects for HeI, HeII, CaI and CaII, using LTE-calculations of spot's atmospheric structure, calculated by Dodin & Lamzin (2012). Assuming that pre-shock gas number density N_0 and its velocity V_0 are the same across the accretion column, we calculated spectra of a system "star + round spot" for a set of N_0, V_0 values and parameters, which characterized the star and the spot. It has been shown that theoretical spectra with an appropriate choice of the parameters reproduce well observed veiling of photospheric absorption lines in optical band as well as profiles and intensities of so-called narrow components of HeII and CaI emission lines in spectra of 9 stars. We found that the accreted gas density N_0>10^{12} cm^{-3} for all considered stars except DK Tau. Observed spectra of 8 stars were succesfully fitted, asuming solar abundance of calcium, but it appeared possible to fit TW Hya spectrum only under assumption that calcium abundance in accreted gas was three times less than solar. We derive spot's parameters by comparison of theoretical and observed spectra, normalised to continuum level, so our results are independent on unknown value of interstellar extinction. We have found that the predicted flux in CaII lines is less than observed one, but this discrepancy can be resolved if not only high-density but also lower density gas falls onto the star. Theoretical equivalent widths as well as relative intensities of He\,I subordinate lines disagree significantly with observations, presumbly due to a number of reasons: necessity to take into account non-LTE thermal structure of upper layers of a hot spot, poorly known collisional atomic data for HeI upper levels and inhomogeneity of the hot spot.

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, Bo��tes 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] > -3.5 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 hardly achieve consistent 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 these work provide the base for a detailed abundance analysis presented in a companion paper., 25 pages, 7 tables, 7 figures, A&A, accepted

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 A 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 A 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.

For the first time, we present an extensive study of stars with individual non-local thermodynamic equilibrium (NLTE) abundances for 17 chemical elements from Li to Eu in a sample of stars uniformly distributed over the $-2.62 \le$ [Fe/H] $\le +0.24$ 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 and improve earlier results as follows. (i) The element-to-iron ratios for Mg, Si, Ca, and Ti form a MP plateau at a similar height of 0.3~dex, and the knee occurs at common [Fe/H] $\simeq -0.8$. 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] $\simeq -0.5$. (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] $\simeq -2.5$, 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 raises credit to the interpretation of the data in the context of the chemical evolution of the Galaxy., 43 pages, 17 figures

Based on high-resolution spectral observations for a sample of very metal-poor stars, we investigate how well stellar chemical abundances can be derived with available theoretical methods and computational tools., Comment: Two pages, to appear in Rediscovering our Galaxy, Proceedings IAU Symposium No. 334, 2017, C. Chiappini, I. Minchev, E. Starkenburg & M. Valentini, eds

The understanding of the chemical evolution of the Galaxy relies on the stellar chemical composition. Accurate atmospheric parameters is a prerequisite of determination of accurate chemical abundances. For late type stars with known distance, surface gravity (log g) can be calculated from well-known relation between stellar mass, Teff, and absolute bolometric magnitude. This method weakly depends on model atmospheres, and provides reliable log g. However, accurate distances are available for limited number of stars. Another way to determine log g for cool stars is based on ionisation equilibrium, i.e. consistent abundances from lines of neutral and ionised species. In this study we determine atmospheric parameters moving step-by-step from well-studied nearby dwarfs to ultra-metal poor (UMP) giants. In each sample, we select stars with the most reliable Teff based on photometry and the distance-based log g, and compare with spectroscopic gravity calculated taking into account deviations from local thermodinamic equilibrium (LTE). After that, we apply spectroscopic method of log g determination to other stars of the sample with unknown distances.

The abundances of 22 heavy elements from Sr to Pb have been determined for the halo star HD 29907 (Teff = 5500 K log g = 4.64) with [Fe/H] = 1.55 using high quality VLT/UVES spectra (ESO Chile). The star has a moderate enhancement of r process elements (Eu Tm) with [r/Fe] = 0.63. In the range from Ba to Yb the derived abundance pattern agrees well with those for strongly r process enhanced stars (r II stars with [Eu/Fe] > 1 and [Ba/Eu] < 0) such as CS 22892 052 and CS 31082 001 as well as with the scaled solar r process curve and the r process model HEW. Thus Ba Yb in HD 29907 originate in the r process. Just as other moderately r process enhanced stars studied in the literature HD 29907 exhibits higher Sr Y and Zr abundances than those for r II stars. These results confirm the assumption by other authors about the existence of an additional Sr Zr synthesis mechanism in the early Galaxy before the onset of nucleosynthesis in asymptotic giant branch (AGB) stars. The same mechanism can be responsible for the enhancement of Mo Ag in the star being investigated compared to r II stars. There are no grounds to suggest the presence of s nuclei of lead in the material of the star being investigated because its measured abundance ratio log e{open}(Pb/Eu) = 1.20 lies within the range for the comparison stars: from log e{open}(Pb/Eu) = 0.17 (CS 31082 001) to < 1.55 (HE 1219 0312). Thus even if there was a contribution of AGB stars to the heavy element enrichment of the interstellar medium at the epoch with [Fe/H] = 1.55 it was small at the level of the abundance error. © 2011 Pleiades Publishing Ltd.

In order to assess the relationship between metallicity and exoplanetary systems, we compare the abundances of AF-type main-sequence stars with debris disk properties assessed using Herschel observations of an unbiased survey of nearby stars. Hot stars are not as commonly observed, given their unique constraints in data reduction, lack of metal lines, and "astrophysical noise" from rotation speed. Here, we address that deficiency using new and archival spectra of 83 AF-type stars. We measure the abundances of a few species in addition to Fe in order to classify the stars with Ap/Am or Lambda Boo signatures. Lambda Boo stars have a chemical signature of solar-abundant volatile species and sub-solar refractory abundances that is hypothesized to be altered by the pollution of volatiles. Overall, we see no correlation between debris disks and metallicity, primarily because the sample size is cut significantly when using only reliable fits to the spectroscopic data. The abundance measured from the Mg ii 4481 blend is a useful diagnostic because it can be reliably measured at large vcenterdotsin(i) and is found to be lower around stars with bright debris disks. We find that Lambda Boo stars have brighter debris disks compared to a bias-free sample of AF stars. The trend with disk brightness and Mg abundances suggests pollution effects can be significant and used as a marker for the stability of planetary systems. We explore trends with other species, such as with the C/O ratios, but are significantly limited by the low number of reliable detections.

We present the non-local thermodynamic equilibrium (NLTE) abundances of up to 10 chemical species in a sample of 59 very metal-poor (VMP, -4 < [Fe/H] < -2) stars in seven dwarf spheroidal galaxies (dSphs) and in the Milky Way (MW) halo. Our results are based on high-resolution spectroscopic datasets and homogeneous and accurate atmospheric parameters determined in PaperI. We show that once the NLTE effects are properly taken into account, all massive galaxies in our sample, that is, the MW halo and the classical dSphs Sculptor, Ursa Minor, Sextans, and Fornax, reveal a similar plateau at [alpha/Fe] ~ 0.3 for each of the alpha-process elements: Mg, Ca, and Ti. We put on a firm ground the evidence for a decline in alpha/Fe with increasing metallicity in the BootesI ultra-faint dwarf galaxy (UFD), that is most probably due to the ejecta of type Ia supernovae. For Na/Fe, Na/Mg, and Al/Mg, the MW halo and all dSphs reveal indistinguishable trends with metallicity, suggesting that the processes of Na and Al synthesis are identical in all systems, independent of their mass. The dichotomy in the [Sr/Ba] versus [Ba/H] diagram is observed in the classical dSphs, similarly to the MW halo, calling for two different nucleosynthesis channels for Sr. We show that Sr in the massive galaxies is well correlated with Mg suggesting a strong link to massive stars and that its production is essentially independent of Ba, for most of the [Ba/H] range. Our three UFDs: BootesI, UMaII, and LeoIV are depleted in Sr and Ba relative to Fe and Mg, with very similar ratios of [Sr/Mg] ~ -1.3 and [Ba/Mg] ~ -1 on the entire range of their Mg abundances. The subsolar Sr/Ba ratios of Bootes I and UMa II indicate a common r-process origin of their neutron-capture elements. Sculptor remains the classical dSph, in which the evidence for inhomogeneous mixing in the early evolution stage, at [Fe/H] < -2, is the strongest., 29 pages, 5 tables, 9 figures, accepted for publication in A&A

We perform the non-local thermodynamic equilibrium (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, that is, by Barklem and by Mitrushchenkov, Guitou, Belyaev, Yakovleva, Spielfiedel, and Feautrier, and investigate the accuracy of calcium abundance determinations using the Sun, Procyon, and five metal-poor (MP) stars with well-determined stellar parameters. We show that both collisional recipes lead to very similar NLTE results. 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 A agrees well with that from the Ca I subordinate lines. NLTE largely removes abundance discrepancies between the high-excitation lines of Ca I and Ca II 8498 A obtained for our four [Fe/H] < -2 stars under the LTE assumption. We investigate the formation of the Ca I resonance line in the [Fe/H] < -2 stars. Consistent NLTE abundances from the Ca I resonance line and the Ca II lines are found for two hyper metal-poor stars HE0107-5240 and HE1327-2326. We provide the NLTE abundance corrections for 28 lines of Ca I in a grid of model atmospheres suitable for abundance analysis of FGK-type dwarfs and subgiants., 12 pages, 3 tables, 10 figures; accepted for publication in A&A

Based on high-resolution observed spectra, the non-local thermodynamic equilibrium (NLTE) line formation, and precise stellar atmosphere parameters, we present the first complete sample of dwarf spheroidal galaxies (dSphs) with accurate chemical abundances in the very metal-poor (VMP) regime. The obtained stellar elemental ratios are compared with chemical enrichment models, and we show that NLTE is a major step forward for studies of the dSph and the Milky Way (MW) chemical evolution.

HD 144277 was previously discovered by MOST space photometry to be a young and hot delta Scuti star showing regular groups of pulsation frequencies. The first asteroseismic models required lower than solar metallicity to fit the observed frequency range based on a purely photometric analysis. High-resolution, high S/N spectroscopic data obtained with the HARPS spectrograph were used to determine the fundamental parameters and chemical abundances of HD 144277. These values were put into context alongside the results from asteroseismic models. The effective temperature, Teff, of HD 144277 was determined as 8640(+300)(-100) K, log g is 4.14 +/- 0.15 and the projected rotational velocity, vsini, is 62.0 +/- 2.0 km/s. As the vsini value is significantly larger than previously assumed, we refined the first asteroseimic model accordingly. The overall metallicity Z was determined to be 0.011 where the light elements He, C, O, Na, and S show solar chemical composition, but the heavier elements are significantly underabundant. In addition, the radius of HD 144277 was determined to be 1.55 +/- 0.65 Rsun from spectral energy distribution fitting, based on photometric data taken from the literature. From the spectroscopic observations, we could confirm our previous assumption from asteroseismic models that HD 144277 has less than solar metallicity. The fundamental parameters derived from asteroseismology, Teff, log g, L/Lsun and R\Rsun, agree within one sigma to the values found from spectroscopic analysis. As the vsini value is significantly higher than assumed in the first analysis, near-degeneracies and rotational mode coupling were taken into account in the new models. These suggest that HD 144277 has an equatorial rotational velocity of about 80 km/s and is seen equator-on. The observed frequencies are identified as prograde modes., 7 pages, 8 figures, Astronomy & Astrophysics in press

We present the results of special tests of the automatic procedure for atmospheric parameters determination based on spectroscopy.

We present a homogeneous set of accurate atmospheric parameters for a sample of eleven very metal-poor (-3.32 -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 timescales 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., 18 pages, 13 figures, and 8 tables. Accepted by MNRAS

A new time series of high-resolution Stokes I and V spectra of the magnetic B2V star HD 184927 has been obtained in the context of the Magnetism in Massive Stars (MiMeS) Large Program with the ESPaDOnS spectropolarimeter at the Canada-France-Hawaii Telescope and dimaPol liquid crystal spectropolarimeter at 1.8-m telescope of Dominion Astrophysical Observatory. We model the optical and UV spectrum obtained from the IUE archive to infer the stellar physical parameters. Using magnetic field measurements we derive an improved rotational period of 9.53102+-0.0007d. We infer the longitudinal magnetic field from lines of H, He and various metals, revealing large differences between the apparent field strength variations determined from different elements. Magnetic Doppler Imaging using He and O lines yields strongly nonuniform surface distributions of these elements. We demonstrate that the diversity of longitudinal field variations can be understood as due to the combination of element-specific surface abundance distributions in combination with a surface magnetic field that is comprised of dipolar and quadrupolar components. We have reanalyzed IUE high resolution spectra, confirming strong modulation of wind-sensitive Civ and Siv resonance lines. However, we are unable to detect any modulation of the H$\alpha$ profile attributable to a stellar magnetosphere. We conclude that HD 184927 hosts a centrifugal magnetosphere, albeit one that is undetectable at optical wavelengths. The magnetic braking timescale of HD 184927 is computed to be $\tau_J = 0.96$ or $5.8$ Myr. These values are consistent with the slow rotation and estimated age of the star., Comment: 23 pages, 21 figures. Accepted for publication in MNRAS

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) ultra-faint 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., accepted for publication in MNRAS