NLTE study of scandium in the Sun

We investigate the formation of neutral and singly ionized scandium lines in the solar photospheres. The research is aimed derive solar $\log gf\epsilon_{\odot}$(Sc) values for scandium lines, which will later be used in differential abundance analyses of metal-poor stars. Extensive statistical equilibrium calculations were carried out for a model atom, which comprises 92 terms for \ion{Sc}{i} and 79 for \ion{Sc}{ii}. Photoionization cross-sections are assumed to be hydrogenic. Synthetic line profiles calculated from the level populations according to the NLTE departure coefficients were compared with the observed solar spectral atlas. Hyperfine structure (HFS) broadening is taken into account. The statistical equilibrium of scandium is dominated by a strong underpopulation of \ion{Sc}{i} caused by missing strong lines. It is nearly unaffected by the variation in interaction parameters and only marginally sensitive to the choice of the solar atmospheric model. Abundance determinations using the ODF model lead to a solar Sc abundance of between $\log\epsilon_\odot = 3.07$ and 3.13, depending on the choice of $f$ values. The long known difference between photospheric and meteoritic scandium abundances is confirmed for the experimental $f$-values.Comment: 10 pages, 6 figures, A&A accepte

Fe I/Fe II ionization equilibrium in cool stars: NLTE versus LTE

Non-local thermodynamic equilibrium (NLTE) line formation for neutral and singly-ionized iron is considered through a range of stellar parameters characteristic of cool stars. A comprehensive model atom for Fe I and Fe II is presented. Our NLTE calculations support the earlier conclusions that the statistical equilibrium (SE) of Fe I shows an underpopulation of Fe I terms. However, the inclusion of the predicted high-excitation levels of Fe I in our model atom leads to a substantial decrease in the departures from LTE. As a test and first application of the Fe I/II model atom, iron abundances are determined for the Sun and four selected stars with well determined stellar parameters and high-quality observed spectra. Within the error bars, lines of Fe I and Fe II give consistent abundances for the Sun and two metal-poor stars when inelastic collisions with hydrogen atoms are taken into account in the SE calculations. For the close-to-solar metallicity stars Procyon and $\beta$ Vir, the difference (Fe II - Fe I) is about 0.1 dex independent of the line formation model, either NLTE or LTE. We evaluate the influence of departures from LTE on Fe abundance and surface gravity determination for cool stars.Comment: 4 pages, to appear in the proceedings of IAU Symposium 265, Chemical Abundances in the Universe: Connecting First Stars to Planets, K. Cunha, M. Spite & B. Barbuy, ed

Finding the First Stars: The Hamburg/ESO Objective Prism Survey

We report on a search for extremely metal-poor ([Fe/H]<-3.0) turnoff stars in the Hamburg/ESO objective prism survey (HES). Metal-poor stars are selected by automatic spectral classification. Extensive simulations show that the selection efficiency for turnoff stars of [Fe/H]25% at B<16.5. Since the HES is more than 1 mag deeper than the HK survey of Beers et al. (1992), the HES offers the possibility to efficiently increase the total number of metal-poor stars by at least a factor of 4.Comment: To appear in: Proceedings of ESO/MPA conference "The First Stars". 2 pages, 1 figur

A non-LTE study of neutral and singly-ionized iron line spectra in 1D models of the Sun and selected late-type stars

A comprehensive model atom for Fe with more than 3000 energy levels is presented. As a test and first application of this model atom, Fe abundances are determined for the Sun and five stars with well determined stellar parameters and high-quality observed spectra. Non-LTE leads to systematically depleted total absorption in the Fe I lines and to positive abundance corrections in agreement with the previous studies, however, the magnitude of non-LTE effect is smaller compared to the earlier results. Non-LTE corrections do not exceed 0.1 dex for the solar metallicity and mildly metal-deficient stars, and they vary within 0.21 dex and 0.35 dex in the very metal-poor stars HD 84937 and HD 122563, respectively, depending on the assumed efficiency of collisions with hydrogen atoms. Based on the analysis of the Fe I/Fe II ionization equilibrium in these two stars, we recommend to apply the Drawin formalism in non-LTE studies of Fe with a scaling factor of 0.1. For the Fe II lines, non-LTE corrections do not exceed 0.01 dex in absolute value. The solar non-LTE abundance obtained from 54 Fe I lines is 7.56+-0.09 and the abundance from 18 Fe II lines varies between 7.41+-0.11 and 7.56+-0.05 depending on the source of the gf-values. Thus, gf-values available for the iron lines are not accurate enough to pursue high-accuracy absolute abundance determinations. Lines of Fe I give, on average, a 0.1 dex lower abundance compared to those of Fe II lines for HD 61421 and HD 102870, even when applying a differential analysis relative to the Sun. A disparity between Fe I and Fe II points to problems of stellar atmosphere modelling or/and effective temperature determination.Comment: 19 pages, 8 figures, online material, accepted by A&

Chromium: NLTE abundances in metal-poor stars and nucleosynthesis in the Galaxy

Aims. We investigate statistical equilibrium of Cr in the atmospheres of late-type stars to show whether the systematic abundance discrepancy between Cr I and Cr II lines, as often encountered in the literature, is due to deviations from LTE. Furthermore, we attempt to interpret the NLTE trend of [Cr/Fe] with [Fe/H] using chemical evolution models for the solar neighborhood. Methods. NLTE calculations are performed for the model of Cr atom, comprising 340 levels and 6806 transitions in total. We make use of the quantum-mechanical photoionization cross-sections of Nahar (2009) and investigate sensitivity of the model to uncertain cross-sections for H I collisions. NLTE line formation is performed for the MAFAGS-ODF model atmospheres of the Sun and 10 metal-poor stars with -3.2 < [Fe/H] < -0.5, and abundances of Cr are derived by comparison of the synthetic and observed flux spectra. Results. We achieve good ionization equilibrium of Cr for the models with different stellar parameters, if inelastic collisions with H I atoms are neglected. The solar NLTE abundance based on Cr I lines is 5.74 dex with {\sigma} = 0.05 dex; it is \sim 0.1 higher than the LTE abundance. For the metal-poor stars, the NLTE abundance corrections to Cr I lines range from +0.3 to +0.5 dex. The resulting [Cr/Fe] ratio is roughly solar for the range of metallicities analyzed here, which is consistent with current views on production of these iron peak elements in supernovae. Conclusions. The tendency of Cr to become deficient with respect to Fe in metal-poor stars is an artifact due to neglect of NLTE effects in the line formation of Cr I, and it has no relation to peculiar physical conditions in the Galactic ISM or deficiencies of nucleosynthesis theory.Comment: 14 pages, 13 figures, to be published in A&

NLTE analysis of Co I/Co II lines in spectra of cool stars with new laboratory hyperfine splitting constants

We investigate the statistical equilibrium of Co in the atmospheres of cool stars, and the influence of NLTE and HFS (hyperfine splitting) on the formation of Co lines and abundances. Significant departures from LTE level populations are found for Co I, also number densities of excited states in Co II differ from LTE at low metallicity. The NLTE abundance of Co in solar photosphere is 4.95 +/- 0.04 dex, which is in agreement with that in C I meteorites within the combined uncertainties. The spectral lines of Co I were calculated using the results of recent measurements of hyperfine interaction constants by UV Fourier transform spectrometry. For Co II, the first laboratory measurements of hyperfine structure splitting A and B factors were performed. A differential abundance analysis of Co is carried out for 18 stars in the metallicity range -3.12 < [Fe/H] < 0. The abundances are derived by method of spectrum synthesis. At low [Fe/H], NLTE abundance corrections for Co I lines are as large as +0.6 >... +0.8 dex. Thus, LTE abundances of Co in metal-poor stars are severely underestimated. The stellar NLTE abundances determined from the single UV line of Co II are lower by ~0.5-0.6 dex. The discrepancy might be attributed to possible blends that have not been accounted for in the solar Co II line and its erroneous oscillator strength. The increasing [Co/Fe] trend in metal-poor stars, as calculated from the Co I lines under NLTE, can be explained if Co is overproduced relative to Fe in massive stars. The models of galactic chemical evolution are wholly inadequate to describe this trend suggesting that the problem is in SN yields.Comment: submitted to MNRAS, 15 page

NLTE abundances of Mn in a sample of metal-poor stars

Following our solar work, we perform NLTE calculations of the Mn abundance for fourteen stars with [Fe/H] for fourteen stars with [Fe/H from 0 to -2.5, mainly to show how NLTE affects Mn abundances in cool stars of different metallicities.The spectrum synthesis and Mn abundances are based on statistical equilibrium calculations using various estimates for the influence of hydrogen collisions.The NLTE abundances of Mn in all studied stars are systematically higher than the LTE abundances. At low metallicities, the NLTE abundance corrections may run up to 0.5 - 0.7 dex. Instead of a strong depletion of Mn relative to Fe in metal-poor stars as found by the other authors, we only find slightly subsolar values of [Mn/Fe] throughout the range of metallicities analyzed here. The [Mn/Fe] trend in metal-poor stars is inconsistent with the predictions of galactic chemical evolution models, where Mn is less produced than Fe.Comment: A&A, accepted 11 page