Aims. The statistical equilibrium of neutral and ionized silicon in the atmospheres of metal-poor stars is discussed. Non-local thermodynamic equilibrium effects are investigated and the silicon abundances in metal-poor stars determined. Methods. We
have used high resolution, high signal to noise ratio spectra from the UVES spectragraph at the ESO VLT telescope. Line formation calculations of Si i and Si ii in the atmospheres of metal-poor stars are presented for atomic models of silicon including 174 terms and 1132 line transitions. Recent improved calculations of Si i and Si ii photoionization cross-sections are taken into account, and the influence of the free-free quasi-molecular absorption in the Ly alpha wing is investigated by comparing theoretical and observed fluxes of metal-poor stars. All abundance results are derived from LTE and NLTE statistical equilibrium calculations and spectrum synthesis methods. Results. It is found that the extreme ultraviolet radiation is very important for metal-poor stars, especially for the high temperature, very metal-poor stars. The radiative bound-free cross-sections also play a very important role for these stars. Conclusions. NLTE effects for Si are found to be important for metal-poor stars, in particular for warm metal-poor stars. It is found that these effects depend on the temperature. For warm metal-poor stars, the NLTE abundance correction reaches ~ 0.2 dex relative to standard LTE calculations. Our results indicate that Si is overabundant for metal-poor stars.
The statistical equilibrium of neutral and ionised silicon in the solar photosphere is investigated. Line formation is discussed and the solar silicon abundance determined. High-resolution solar spectra were used to determine solar $log gfepsilon_{rm
Si}$ values by comparison with Si line synthesis based on LTE and NLTE level populations. The results will be used in a forthcoming paper for differential abundance analyses of metal-poor stars. A detailed analysis of silicon line spectra leads to setting up realistic model atoms, which are exposed to interactions in plane-parallel solar atmospheric models. The resulting departure coefficients are entered into a line-by-line analysis of the visible and near-infrared solar silicon spectrum. The statistical equilibrium of ion{Si}{i} turns out to depend marginally on bound-free interaction processes, both radiative and collisional. Bound-bound interaction processes do not play a significant role either, except for hydrogen collisions, which have to be chosen adequately for fitting the cores of the near-infrared lines. Except for some near-infrared lines, the NLTE influence on the abundances is weak. Taking the deviations from LTE in silicon into account, it is possible to calculate the ionisation equilibrium from neutral and ionised lines. The solar abundance based on the experimental $f$-values of Garz corrected for the Becker et al.s measurement is $7.52 pm 0.05$. Combined with an extended line sample with selected NIST $f$-values, the solar abundance is $7.52 pm 0.06$, with a nearly perfect ionisation equilibrium of $Deltalogepsilon_odot(ion{Si}{ii}/ion{Si}{i}) = -0.01$.
We investigate the formation of neutral and singly ionized scandium lines in the solar photospheres. The research is aimed derive solar $log gfepsilon_{odot}$(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 $logepsilon_odot = 3.07$ 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.
Stellar parameters and abundances of Na, Mg, Al, K, Ca, Sr, Ba, and Eu are determined for four very metal-poor stars (-2.66 < [Fe/H] < -2.15) based on non-LTE line formation and analysis of high-resolution (R ~60000 and 90000) high signal-to-noise (S
/N > 200) observed spectra. A model atom for H I is presented. An effective temperature was obtained from the Balmer Halpha and Hbeta line wing fits, the surface gravity from the Hipparcos parallax if available and the non-LTE ionization balance between Ca I and Ca II. Based on the hyperfine structure affecting the Ba II resonance line, the fractional abundance of the odd isotopes of Ba was derived for HD 84937 and HD 122563 from a requirement that Ba abundances from the resonance line and subordinate lines of Ba II must be equal. For each star, non-LTE leads to a consistency of Teff from two Balmer lines and to a higher temperature compared to the LTE case, by up to 60 K. Non-LTE effects are important in spectroscopic determination of surface gravity from Ca I/Ca II. For each star with a known trigonometric gravity, non-LTE abundances from the lines of two ionization stages agree within the error bars, while a difference in the LTE abundances consists of 0.23 dex to 0.40 dex for different stars. Departures from LTE are found to be significant for the investigated atoms, and they strongly depend on stellar parameters. For HD 84937, the Eu/Ba ratio is consistent with the relative solar system r-process abundances, and the fraction of the odd isotopes of Ba, f_odd, equals 0.43+-0.14. The latter can serve as a constraint on r-process models. The lower Eu/Ba ratio and f_odd = 0.22+-0.15 found for HD 122563 suggest that the s-process or the unknown process has contributed significantly to the Ba abundance in this star.
