Aims. We evaluate non-local thermodynamical equilibrium (non-LTE) line formation for the two ions of iron and check the ionization equilibrium between Fe i and Fe ii in model atmospheres of the cool reference stars based on the best available complete model atom for neutral and singly-ionized iron.Methods. We present a comprehensive model atom for Fe with more than 3000 measured and predicted energy levels. As a test and first application of the improved model atom, iron abundances are determined for the Sun and five stars with well determined stellar parameters and high-quality observed spectra. The efficiency of inelastic collisions with hydrogen atoms in the statistical equilibrium of iron is empirically estimated from inspection of their different influence on the Fe i and Fe ii lines in the selected stars. Results. 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 such corrections is smaller compared to the earlier results. These 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 over the whole range of stellar parameters that are considered. This study reveals two problems. The first one is that g f -values available for the Fe i and Fe ii lines are not accurate enough to pursue high-accuracy absolute stellar abundance determinations. For the Sun, the mean non-LTE abundance obtained from 54 Fe i lines is 7.56 ± 0.09 and the mean abundance from 18 Fe ii lines varies between 7.41 ± 0.11 and 7.56 ± 0.05 depending on the source of the g f -values. The second problem is that lines of Fe i give, on average, a 0.1 dex lower abundance compared with those of Fe ii lines for HD 61421 and HD 102870, even when applying a differential line-by-line analysis with regard to the Sun. A disparity between neutral atoms and first ions points to problems of stellar atmosphere modelling or/and effective temperature determination.

Aims. Following our solar work, we perform NLTE calculations of the Mn abundance 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. Methods. The spectrum synthesis and Mn abundances are based on statistical equilibrium calculations using various estimates for the influence of hydrogen collisions. Results. 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]

Abstract.To determine the population membership of nearby stars we explore abundance results obtained for the light neutronrich elements 23 Na and 27 Al in a small sample of moderately metal-poor stars. Spectroscopic observations are limited to the solar neighbourhood so that gravities can be determined from H parallaxes, and the results are confronted with those for a separate sample of more metal-poor typical halo stars. Following earlier investigations, the abundances of Na, Mg and Al have been derived from NLTE statistical equilibrium calculations used as input to line profile synthesis. We do not confirm the age gap between thin and thick disk found by Fuhrmann. Instead we find an age boundary between halo and thick disk stars, however, with an absolute value of 14 Gyr that must be considered as preliminary. While the stellar sample is by no means complete, the resulting abundances indicate the necessity to revise current models of chemical evolution and/or stellar nucleosynthesis to allow for an adequate production of neutron-rich species in early stellar generations.

Abstract. Non-LTE line formation calculations of Fe  are performed for a small number of reference stars to investigate and quantify the efficiency of neutral hydrogen collisions. Using the atomic model that was described in previous publications, the final discrimination with respect to hydrogen collisions is based on the condition that the surface gravities as determined by the Fe /Fe  ionization equilibria are in agreement with their astrometric counterparts obtained from H parallaxes.High signal-to-noise, high-resolutionéchelle spectra are analysed to determine individual profile fits and differential abundances of iron lines. Depending on the choice of the hydrogen collision scaling factor S H , we find deviations from LTE in Fe  ranging from 0.00 (S H = ∞) to 0.46 dex (S H = 0 for HD 140283) in the logarithmic abundances while Fe  follows LTE. With the exception of Procyon, for which a mild temperature correction is needed to fulfil the ionization balance, excellent consistency is obtained for the metal-poor reference stars if Balmer profile temperatures are combined with S H = 3. This value is much higher than what is found for simple atoms like Li or Ca, both from laboratory measurements and inference of stellar analyses. The correct choice of collisional damping parameters ("van-der-Waals" constants) is found to be generally more important for these little evolved metal-poor stars than considering departures from LTE. For the Sun the calibrated value for S H leads to average Fe  non-LTE corrections of 0.02 dex and a mean abundance from Fe  lines of log ε(Fe) = 7.49 ± 0.08.We confront the deduced stellar parameters with comparable spectroscopic analyses by other authors which also rely on the iron ionization equilibrium as a gravity indicator. On the basis of the H astrometry our results are shown to be an order of magnitude more precise than published data sets, both in terms of offset and star-to-star scatter.

Aims. Parameters for 55 nearby metal-poor stars are determined using high-resolution spectroscopy. Together with similar data taken from a recent analysis, they are used to show trends of their Galactic evolution with stellar [Fe/H] ratios, asymmetric drift velocities V, and stellar evolutionary ages, make possible the individual discrimination between stars of the thick disk and the halo. At present, this evidence is limited by the small number of stars, and by the theoretical and empirical uncertainties of stellar age determinations, but it achieves a high significance. Conclusions. While the stellar sample is not complete with respect to space volume, the resulting abundances indicate the necessity to revise current models of chemical evolution to allow for an adequate production of Al in early stellar generations.Key words. line: formation -line: profiles -stars: abundances -stars: late-type -Galaxy: evolution IntroductionStellar populations were originally recognized and defined according to their kinematic properties (Oort 1926;Baade 1944;Roman 1955), and it was not until Eggen et al. (1962) that kinematic data were correlated with mean metal abundances. In a first attempt to understand the formation of the Galaxy, their investigation established the meaning of eccentric stellar orbits as well as that of the asymmetric drift velocity. At about the same time, Wallerstein (1961) recognized that there is some evidence for non-solar abundance ratios of the α-elements, particularly in kinematically extreme stars of population II, which then was related to the stellar component of the Galactic halo. Later, Gilmore & Reid (1983) found that the Galactic disk, originally associated with population I stars, is actually composed of two (sub)populations, a thin and a thick disk.At that time, the topic was controversial (see e.g. Bahcall & Soneira 1984;Bahcall et al. 1985), and even today the notion of a third Galactic population is not very popular among some astronomers. Roughly ten years later, Edvardsson et al. (1993) presented a first comprehensive abundance analysis of disk stars resulting in a more or less smooth change of individual element abundance ratios [X/Fe] 1 . While this chemical enrichment wasBased on observations collected at the German-Spanish Astronomical Center, Calar Alto, Spain.1 As usual, [X/Fe] = log (N X /N Fe )−log (N X /N Fe ) represents the normalized logarithmic abundance ratio of elements X and Fe.interpreted as a continuous evolution of stellar generations, the first clear separation of thin and thick-disk stars was presented by Fuhrmann (1998Fuhrmann ( , 2004 who was able to show that the thick disk is a distinct stellar component, both in terms of abundances and age, whereas the kinematic properties are by far less conclusive when looking for an old metal-poor population. Moreover, he gives evidence for the speculation that the thick disk is not at all a minor component of the Milky Way, and that instead the halo component, i.e. the original population II, consists of only ...

Abstract. We present revised strontium, barium and europium abundances for 63 cool stars with metallicities [Fe/H] ranging from −2.20 to 0.25. The stellar sample has been extracted from Fuhrmann's lists (1998, 2001). It is confined to main-sequence and turnoff stars. The results are based on NLTE line formation obtained in differential model atmosphere analyses of spectra that have a typical S/N of 200 and a resolution of 40 000 to 60 000. The element abundance ratios reveal a distinct chemical history of the halo and thick disk compared with that of the thin disk. Europium is overabundant relative to iron and barium in halo and thick disk stars suggesting that during the formation of these galactic populations high-mass stars exploding as SNe II dominated nucleosynthesis on a short time scale of the order of 1 Gyr. We note the importance of [Eu/Mg] determinations for halo stars. Our analysis leads to the preliminary conclusion that Eu/Mg ratios found in halo stars do not support current theoretical models of the r-process based on low-mass SNe; instead they seem to point at a halo formation time much shorter than 1 Gyr. A steep decline of [Eu/Fe] and a slight decline of [Eu/Ba] with increasing metallicity have been first obtained for thick disk stars. This indicates the start of nucleosynthesis in the lower mass stars, in SN I and AGB stars, which enriched the interstellar gas with iron and the most abundant s-process elements. From a decrease of the Eu/Ba ratio by ∼0.10 . . . 0.15 dex the time interval corresponding to the thick disk formation phase can be estimated. The step-like change of element abundance ratios at the thick to thin disk transition found in our previous analysis (Mashonkina & Gehren 2000) is confirmed in this study: [Eu/Ba] and [Eu/Fe] are reduced by ∼0.25 dex and ∼0.15 dex, respectively; [Ba/Fe] increases by ∼0.1 dex. This is indicative of an intermediate phase before the early stage of the thin disk developed, during which only evolved middle and low mass (<8 M ) stars contributed to nucleosynthesis. Our data provide an independent method to calculate the duration of this phase. The main s-process becomes dominant in the production of heavy elements beyond the iron group during the thin disk evolution. We find that in the thin disk stars Ba/Fe ratios increase with time from [Ba/Fe] = −0.06 in stars older than 8 Gyr to [Ba/Fe] = 0.06 in stars that are between 2 and 4 Gyr old.

Aims. Stellar parameters and abundances of Na, Mg, Al, K, Ca, Sr, Ba, and Eu are determined for four very metal-poor (VMP) stars (−2.15 ≥ [Fe/H] ≥ −2.66). For two of them, HD 84937 and HD 122563, the fraction of the odd isotopes of Ba derived for the first time.Methods. Determination of an effective temperature, surface gravity, and element abundances was based on non-local thermodynamic equilibrium (non-LTE) line formation and analysis of high-resolution (R ∼ 60 000 and 90 000) 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 H α and H β line wing fits. The surface gravity was calculated 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 λ 4554, the fractional abundance of the odd isotopes of Ba was derived from a requirement that Ba abundances from the resonance line and subordinate lines of Ba ii must be equal.Results. For each star, non-LTE leads to a consistency of T eff 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 the ionization balance between Ca i and Ca ii. For each star with a known trigonometric surface gravity, non-LTE abundances from the lines of two ionization stages, Ca i and Ca ii, 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 all 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 an observational 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.

Abstract. Our sample of cool dwarf stars from previous papers (Mashonkina & Gehren 2000) is extended in this study including 15 moderately metal-deficient stars. The samples of halo and thick disk stars have overlapping metallicities with [Fe/H] in the region from −0.9 to −1.5, and we compare chemical properties of these two kinematically different stellar populations independent of their metallicity. We present barium, europium and magnesium abundances for the new sample of stars. The results are based on NLTE line formation obtained in differential model atmosphere analyses of high resolution spectra observed mainly using the UVES spectrograph at the VLT of the European Southern Observatory. We confirm the overabundance of Eu relative to Mg in halo stars as reported in our previous papers. Eight halo stars show [Eu/Mg] values between 0.23 and 0.41, whereas stars in the thick and thin disk display a solar europium to magnesium ratio. The [Eu/Ba] values found in the thick disk stars to lie between 0.35 and 0.57 suggest that during thick disk formation evolved low-mass stars started to enrich the interstellar gas by s-nuclei of Ba, and the s-process contribution to barium thus varies from 30% to 50%. Based on these results, and using the chemical evolution calculations by Travaglio et al. (1999), we estimate that the thick disk stellar population formed on a timescale between 1.1 to 1.6 Gyr from the beginning of the protogalactic collapse. In the halo stars the [Eu/Ba] values are found mostly between 0.40 and 0.67, which suggests a duration of the halo formation of about 1.5 Gyr. For the whole sample of stars we present the even-to-odd Ba isotope ratios as determined from hyperfine structure seen in the Ba  resonance line λ 4554. As expected, the solar ratio 82:18 (Cameron 1982) adjusts to observations of the Ba  lines in the thin disk stars. In our halo stars the even-to-odd Ba isotope ratios are close to the pure r-process ratio 54:46 (Arlandini et al. 1999), and in the thick disk stars the isotope ratio is around 65:35 (±10%). Based on these data we deduce for thick disk stars the ratio of the s/r-process contribution to barium as 30:70 (±30%), in agreement with the results obtained from the [Eu/Ba] values.