Author name code: gehren ADS astronomy entries on 2022-09-14 author:"Gehren, Thomas" ------------------------------------------------------------------------ Title: Statistical Equilibrium of Copper in the Solar Atmosphere Authors: Shi, J. R.; Gehren, T.; Zeng, J. L.; Mashonkina, L.; Zhao, G. Bibcode: 2014ApJ...782...80S Altcode: Non-local thermodynamic equilibrium (NLTE) line formation for neutral copper in the one-dimensional solar atmospheres is presented for the atomic model, including 96 terms of Cu I and the ground state of Cu II. The accurate oscillator strengths for all the line transitions in model atom and photoionization cross sections were calculated using the R-matrix method in the Russell-Saunders coupling scheme. The main NLTE mechanism for Cu I is the ultraviolet overionization. We find that NLTE leads to systematically depleted total absorption in the Cu I lines and, accordingly, positive abundance corrections. Inelastic collisions with neutral hydrogen atoms produce minor effects on the statistical equilibrium of Cu I in the solar atmosphere. For the solar Cu I lines, the departures from LTE are found to be small, the mean NLTE abundance correction of ~0.01 dex. It was found that the six low-excitation lines, with excitation energy of the lower level E exc <= 1.64 eV, give a 0.14 dex lower mean solar abundance compared to that from the six E exc > 3.7 eV lines, when applying experimental gf-values of Kock & Richter. Without the two strong resonance transitions, the solar mean NLTE abundance from 10 lines of Cu I is log ɛ⊙(Cu) = 4.19 ± 0.10, which is consistent within the error bars with the meteoritic value 4.25 ± 0.05 of Lodders et al. The discrepancy between E exc = 1.39-1.64 eV and E exc > 3.7 eV lines can be removed when the calculated gf-values are adopted and a mean solar abundance of log ɛ⊙(Cu) = 4.24 ± 0.08 is derived. Title: Statistical equilibrium of silicon in the atmospheres of cool stars Authors: Shi, J. R.; Gehren, T.; Mashonkina, L.; Zhao, G. Bibcode: 2014IAUS..298..437S Altcode: The statistical equilibrium of neutral and ionized silicon in the atmospheres of cool stars is discussed. Non-local thermodynamic equilibrium effects (NLTE) are investigated. It is found that the NLTE effects for Si are important, in particular for warm metal-poor stars. For warm metal-poor stars, the NLTE abundance correction reaches ~0.2 dex relative to standard LTE calculations. Title: Scandium Abundance in Metal-poor Stars Authors: Zhang, H. W.; Gehren, T.; Zhao, G. Bibcode: 2014IAUS..298..453Z Altcode: The Scandium abundances for 85 metal-poor stars are presented. Our result shows that NLTE corrections for Sc II lines are small (-0.04 to +0.06 dex). The abundance trends in stars of different populations are discussed. Title: Statistical equilibrium of silicon in the atmospheres of nearby metal-poor stars Authors: Shi, J. R.; Gehren, T.; Zhao, G. Bibcode: 2011A&A...534A.103S Altcode:
Aims: We discuss the statistical equilibrium of neutral and ionized silicon in the atmospheres of nearby metal-poor stars. We investigated the effects of non-local thermodynamic equilibrium (NLTE) and determined the silicon abundances.
Methods: We used high resolution, high signal-to-noise ratio spectra from the FOCES spectragraph at the DSAZ telescope. Line-formation calculations of Si i and Si ii in the atmospheres of nearby metal-poor stars are presented. All abundance results are derived from local thermodynamic equilibrium (LTE) and NLTE statistical equilibrium calculations and spectrum synthesis methods.
Results: We find that NLTE effects for Si ii optical lines are important for warm stars, and that they depend on effective temperature. The Si abundances of thin and thick disc stars follow distinct trends, as in the case of Mg. We find that [Si/Fe] gradually increases as [Fe/H] decreases in thin disc stars, while it remains around at ~+0.30 dex for halo and thick disc stars, the halo stars showing larger scatter.
Conclusions: The derived dependence between [Si/Fe] and [Fe/H] is inconsistent with the theoretical predictions of published model calculations for the chemical evolution of the Galaxy. The nearly constant [Si/Mg] ratio with some scatter for halo and thick disc stars suggests that the nucleosynthesis of silicon is closely coupled to that of Mg. In addition, our results do not support the suggestion that type Ia supernove produce significant amounts of silicon.

Based on observations collected at the German-Spanish Astronomical Center, Calar Alto, Spain. Title: A non-LTE study of neutral and singly-ionized iron line spectra in 1D models of the Sun and selected late-type stars Authors: Mashonkina, L.; Gehren, T.; Shi, J. -R.; Korn, A. J.; Grupp, F. Bibcode: 2011A&A...528A..87M Altcode: 2011arXiv1101.4570M
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 gf-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 gf-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.

Based on observations collected at the German Spanish Astronomical Center, Calar Alto, Spain and taken from the ESO UVES-POP archive.Table 5 is only available in electronic form at http://www.aanda.org Title: Fe-peak element abundances in disk and halo stars Authors: Bergemann, Maria; Gehren, Thomas Bibcode: 2010IAUS..265..348B Altcode: 2009arXiv0910.3689B At present none of galactic chemical evolution (GCE) models provides a self-consistent description of observed trends for all iron-peak elements with metallicity simultaneously. The question is whether the discrepancy is due to deficiencies of GCE models, such as stellar yields, or due to erroneous spectroscopically-determined abundances of these elements in metal-poor stars. The present work aims at a critical reevaluation of the abundance trends for several odd and even-Z Fe-peak elements, which are important for understanding explosive nucleosynthesis in supernovae. Title: Fe I/Fe II ionization equilibrium in cool stars: NLTE versus LTE Authors: Mashonkina, Lyudmila; Gehren, Thomas; Shi, Jianrong; Korn, Andreas; Grupp, Frank Bibcode: 2010IAUS..265..197M Altcode: 2009arXiv0910.3997M 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 β 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. Title: NLTE analysis of CoI/CoII lines in spectra of cool stars with new laboratory hyperfine splitting constants Authors: Bergemann, Maria; Pickering, Juliet C.; Gehren, Thomas Bibcode: 2010MNRAS.401.1334B Altcode: 2009arXiv0909.2178B; 2009MNRAS.tmp.1703B The analysis of stellar abundances for odd-Z Fe-peak elements requires accurate non-local thermodynamic equilibrium (NLTE) modelling of spectral lines fully taking into account the hyperfine structure (HFS) splitting of lines. Here, we investigate the statistical equilibrium of Co in the atmospheres of cool stars and the influence of NLTE and HFS on the formation of Co lines and abundances. Significant departures from LTE level populations are found for CoI number densities of excited states in CoII also differ from LTE at low metallicity. The NLTE level populations are used to determine the abundance of Co in solar photosphere, logɛ = 4.95 +/- 0.04dex, which is in agreement with that in CI meteorites within the combined uncertainties. The spectral lines of CoI were calculated using the results of recent measurements of hyperfine interaction constants by UV Fourier transform spectrometry. For CoII, the first laboratory measurements of HFS A and B factors were performed. These highly accurate A factor measurements (errors of the order of 3-7 per cent) allow, for the first time, reliable modelling of CoII lines in the solar and stellar spectra and, thus, a test of the CoI/CoII ionization equilibrium in stellar atmospheres. 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 the method of spectrum synthesis. At low [Fe/H], NLTE abundance corrections for CoI lines are as large as +0.6,..., + 0.8dex. Thus, LTE abundances of Co in metal-poor stars are severely underestimated. The stellar NLTE abundances determined from the single UV line of CoII are lower by ~0.5-0.6dex. The discrepancy might be attributed to possible blends that have not been accounted for in the solar CoII line and its erroneous oscillator strength. The increasing [Co/Fe] trend in metal-poor stars, as calculated from the CoI 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 supernova yields.

Based on observations collected at the European Southern Observatory, Chile, 67.D-0086A, and the Calar Alto Observatory, Spain.

E-mail: mbergema@mpa-garching.mpg.de (MB); j.pickering@imperial.ac.uk (JCP); gehren@usm.lmu.de (TG) Title: Statistical equilibrium of silicon in the atmospheres of metal-poor stars Authors: Shi, J. R.; Gehren, T.; Mashonkina, L.; Zhao, G. Bibcode: 2009A&A...503..533S Altcode: 2009arXiv0907.4928S Aims: The statistical equilibrium of neutral and ionized silicon in the atmospheres of metal-poor stars is discussed. Non-local thermodynamic equilibrium effects (NLTE) 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 Lyα 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.

Based on observations obtained in the frame of the ESO programme ID 165.N-0276(A). Title: NLTE abundances of Mn in a sample of metal-poor stars Authors: Bergemann, M.; Gehren, T. Bibcode: 2008A&A...492..823B Altcode: 2008arXiv0811.0681B 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] throughout the range of metallicities analyzed here.
Conclusions: 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.

Based on observations collected at the European Southern Observatory, Chile, 67.D-0086A, and the Calar Alto Observatory, Spain. Title: Solar scandium abundance Authors: Zhang, H. W.; Gehren, T.; Zhao, G. Bibcode: 2008IAUS..252..127Z Altcode: We investigate the formation of neutral and singly ionized scandium lines in the solar photospheres. Extensive statistical equilibrium calculations were carried out for a model atom, which comprises 92 terms for Sc I and 79 for Sc II. Synthetic line profiles calculated from the level populations according to the NLTE departure coefficients were compared with the observed solar spectral atlas. Abundance determinations using the ODF model lead to a solar Sc abundance of between log γ⊙ = 3.07 and 3.13, depending on the choice of f values. Title: Statistical equilibrium of silicon in the solar atmosphere Authors: Shi, J. R.; Gehren, T.; Butler, K.; Mashonkina, L. I.; Zhao, G. Bibcode: 2008A&A...486..303S Altcode: 2008arXiv0805.3564S Aims: The statistical equilibrium of neutral and ionised silicon in the solar photosphere is investigated. Line formation is discussed and the solar silicon abundance determined.
Methods: High-resolution solar spectra were used to determine solar log gf\varepsilon_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.
Results: The statistical equilibrium of 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.
Conclusions: 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 ± 0.05. Combined with an extended line sample with selected NIST f-values, the solar abundance is 7.52 ± 0.06, with a nearly perfect ionisation equilibrium of Δlog\varepsilon_⊙(Si II/Si I) = -0.01.

Table 1 is only available in electronic form at http://www.aanda.org Title: A non-local thermodynamic equilibrum study of scandium in the Sun Authors: Zhang, H. W.; Gehren, T.; Zhao, G. Bibcode: 2008A&A...481..489Z Altcode: Aims: We investigate the formation of neutral and singly ionized scandium lines in the solar photospheres. The research is aimed at derive solar log gf\varepsilon⊙(Sc) values for scandium lines, which will later be used in differential abundance analyses of metal-poor stars.
Methods: Extensive statistical equilibrium calculations were carried out for a model atom, which comprises 92 terms for Sc I and 79 for 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.
Results: The statistical equilibrium of scandium is dominated by a strong underpopulation of 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\varepsilon_⊙ = 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. Title: NLTE study of scandium in the Sun Authors: Zhang, H. W.; Gehren, T.; Zhao, G. Bibcode: 2008arXiv0802.2609Z Altcode: 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. Title: Non-LTE line formation for heavy elements in four very metal-poor stars Authors: Mashonkina, L.; Zhao, G.; Gehren, T.; Aoki, W.; Bergemann, M.; Noguchi, K.; Shi, J. R.; Takada-Hidai, M.; Zhang, H. W. Bibcode: 2008A&A...478..529M Altcode: 2007arXiv0711.4454M 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 Hα and Hβ 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.

Based on observations collected at Subaru Telescope, which is operated by the National Astronomical Observatory of Japan. Title: Formation of Mn I lines in the solar atmosphere Authors: Bergemann, M.; Gehren, T. Bibcode: 2007A&A...473..291B Altcode: 2007arXiv0707.4131B Context: We present a detailed NLTE analysis of 39 Mn I lines in the solar spectrum. The influence of NLTE effects on the line formation and element abundance is investigated.
Aims: Our goal is the derivation of solar log gf\varepsilon values for manganese lines, which will later be used in differential abundance analyses of metal-poor stars.
Methods: The method of spectrum synthesis is employed, which is based on a solar model atmosphere with initially specified element abundances. A manganese abundance of log\varepsilonMn, ⊙= 5.47 dex is used with the theoretical line-blanketed model atmosphere. Statistical equilibrium calculations are carried out for the model atom, which comprises 245 and 213 levels for Mn I and Mn II, respectively. Photoionization cross-sections are assumed hydrogenic.
Results: For line synthesis van der Waals broadening is calculated according to Anstee & O'Mara's formalism. It is shown that hyperfine structure of the Mn lines also has strong broadening effects, and that manganese is prone to NLTE effects in the solar atmosphere. The nature of the NLTE effects and the validity of the LTE approach are discussed in detail. The role of photoionization and collisional interaction is investigated.
Conclusions: Maximum NLTE corrections of +0.1 dex with respect to LTE profiles are found. We propose an absolute solar abundance of 5.36 ± 0.1 dex. The main source of errors in the abundance calculations is uncertain oscillator strengths.

Research supported by the International Max Planck Research School (IMPRS), Munich, Germany. Figure 8 and Table 5 are only available in electronic form at http://www.aanda.org Title: Lithium abundances in metal-poor stars Authors: Shi, J. R.; Gehren, T.; Zhang, H. W.; Zeng, J. L.; Zhao, G. Bibcode: 2007A&A...465..587S Altcode: Aims:Lithium abundances for 19 metal-poor stars are determined using high-resolution spectroscopy. The abundances of stars on the lithium plateau are discussed.
Methods: All abundance results are derived from NLTE statistical equilibrium calculations and spectrum synthesis methods.
Results: In agreement with previous analyses it is found that excitation and de-excitation due to hydrogen collisions are negligible for the lithium line formation process, while charge transfer reactions are an important source of thermalization. However, the resulting NLTE effects on the determination of lithium abundances for metal-poor stars are negligible (<0.06 dex).
Conclusions: .The mean lithium abundance for stars on the lithium plateau determined from NLTE analyses is A(Li) ~ 2.26, while it is 2.21 dex when charge transfer reactions are included. The latter result enhances the discrepancy between the observed lithium abundances and the primordial lithium abundance as inferred by the WMAP analysis of the cosmic microwave background. This discrepancy may be explained by metal diffusion.

Based on observations collected at the Germany-Spanish Astronomical Center, Calar Alto, Spain. Title: Potassium abundances in nearby metal-poor stars Authors: Zhang, H. W.; Gehren, T.; Butler, K.; Shi, J. R.; Zhao, G. Bibcode: 2006A&A...457..645Z Altcode: Aims.The potassium abundances for 58 metal-poor stars are determined using high-resolution spectroscopy. The abundance trends in stars of different population are discussed.
Methods: .All abundance results have been derived from NLTE statistical equilibrium calculations and spectrum synthesis methods.
Results: .The NLTE corrections are significant (-0.20 to -0.55 dex) and they depend on the effective temperatures and surface gravities. The potassium abundances of thin disk, thick disk and halo stars show distinct trends, such as in the case of the α-elements. [K/Fe] gradually increases with a decrease in [Fe/H] for thin disk stars, [K/Fe] of thick disk stars is nearly constant at [K/Fe] ~ +0.30 dex; halo stars also have nearly constant values of [K/Fe] ~ +0.20 dex.
Conclusions: .The derived dependence between [K/Fe] and [Fe/H] is in agreement with the theoretical prediction of published model calculations of the chemical evolution of the Galaxy. The nearly constant [K/Mg] ratio with small scatter suggests that the nucleosynthesis of potassium is closely coupled to the α-elements. Title: NLTE analysis of the solar potassium abundance Authors: Zhang, H. W.; Butler, K.; Gehren, T.; Shi, J. R.; Zhao, G. Bibcode: 2006A&A...453..723Z Altcode: We investigate the formation of neutral potassium lines in the solar photosphere based on extensive statistical equilibrium calculations to determine the solar potassium abundance. The computations are based on a 68-level potassium atomic model. Hydrogen collisions were supplied with an enhancement factor SH = 0.05 to the classical Drawin formula. Synthetic line profiles calculated from the level populations according to the NLTE departure coefficients were compared with the observed solar spectral atlas. The solar potassium abundance based on theoretical model atmospheres is log \varepsilon⊙ (K) = 5.12 ± 0.03, which agrees with the meteoritic value of 5.09 ± 0.05 compiled by Lodders. Title: Na, Mg and Al abundances as a population discriminant for nearby metal-poor stars Authors: Gehren, T.; Shi, J. R.; Zhang, H. W.; Zhao, G.; Korn, A. J. Bibcode: 2006A&A...451.1065G Altcode: 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] or [Mg/H] abundances. The separation of abundance ratios between disk and halo stars is used as a basic criterion for population membership.
Methods.After careful selection of a clean subsample free of suspected or known binaries and peculiar stars, abundances of Mg, Na and Al are based on NLTE kinetic equilibrium calculations applied to spectrum synthesis methods.
Results.The relation between [Na/Mg] and [Fe/H] is a continuous enrichment through all three Galactic populations spanning a range of values between a metal-poor plateau at [ Na/Mg] = -0.7 and solar values. [Al/Mg] displays a step-like difference between stars of the Galactic halo with overline[Al/Mg] ∼ -0.45 and the two disk populations with overline[Al/Mg] ∼ +0.10. [Al/Mg] ratios, together with the [Mg/Fe] 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. Title: VizieR Online Data Catalog: Sodium abundances in nearby disk stars (Shi+, 2004) Authors: Shi, J. R.; Gehren, T.; Zhao, G. Bibcode: 2005yCat..34230683S Altcode: The spectra of our samples were obtained through the years 1995 to 2000 by Klaus Fuhrmann with the fiber-coupled Cassegrain echelle spectrograph FOCES mounted at the 2.2m telescope of the Calar Alto Observatory.

(1 data file). Title: Sodium abundances in nearby disk stars Authors: Shi, J. R.; Gehren, T.; Zhao, G. Bibcode: 2004A&A...423..683S Altcode: 2004astro.ph..5535S We present sodium abundances for a sample of nearby stars. All results have been derived from NLTE statistical equilibrium calculations. The influence of collisional interactions with electrons and hydrogen atoms is evaluated by comparison of the solar spectrum with very precise fits to the Na I line cores. The NLTE effects are more pronounced in metal-poor stars since the statistical equilibrium is dominated by collisions of which at least the electronic component is substantially reduced. The resulting influence on the determination of sodium abundances is in a direction opposite to that found previously for Mg and Al. The NLTE corrections are about -0.1 in thick-disk stars with [Fe/H] ∼-0.6. Our [Na/Fe] abundance ratios are about solar for thick- and thin-disk stars. The increase in [Na/Fe] as a function of [Fe/H] for metal-rich stars found by Edvardsson et al. (\cite{EAG93}) is confirmed. Our results suggest that sodium yields increase with the metallicity, and quite large amounts of sodium may be produced by AGB stars. We find that [Na/Fe] ratios, together with either [Mg/Fe] ratio, kinematic data or stellar evolutionary ages, make possible the individual discrimination between thin- and thick-disk membership.

Based on observations collected at the Germany-Spanish Astronomical Center, Calar Alto, Spain.

Tables \ref{table2} and \ref{table3} are only available in electronic form at http://www.edpsciences.org Title: Abundances of Na, Mg and Al in nearby metal-poor stars Authors: Gehren, T.; Liang, Y. C.; Shi, J. R.; Zhang, H. W.; Zhao, G. Bibcode: 2004A&A...413.1045G Altcode: To determine the population membership of nearby stars we explore abundance results obtained for the light neutron-rich elements 23Na 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 HIPPARCOS 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. Compared with LTE the abundances require systematic corrections, with typical values of +0.05 for [Mg/Fe], -0.1 for [Na/Fe] and +0.2 for [Al/Fe] in thick disk stars where [Fe/H] ∼ -0.6. In more metal-poor halo stars these values reach +0.1, -0.4, and +0.5, respectively, differences that can no longer be ignored.

After careful selection of a clean subsample free from suspected or known binaries and peculiar stars, we find that [Na/Mg] and [Al/Mg], in combination with [Mg/Fe], space velocities and stellar evolutionary ages, make possible an individual discrimination between thick disk and halo stars. At present, this evidence is limited by the small number of stars analyzed. We identify a gap at [Al/Mg] ∼ -0.15 and [Fe/H] ∼ -1.0 that isolates stars of the thick disk from those in the halo. A similar separation occurs at [Na/Mg] ∼ -0.4. 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.

Based on observations collected at the German-Spanish Astronomical Center, Calar Alto (CAHA H01-2.2-002) and at the European Southern Observatory, Chile (ESO 67.D-0086). Title: Kinetic equilibrium of iron in the atmospheres of cool stars. III. The ionization equilibrium of selected reference stars Authors: Korn, A. J.; Shi, J.; Gehren, T. Bibcode: 2003A&A...407..691K Altcode: 2003astro.ph..6337K Non-LTE line formation calculations of Fe I 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 I/Fe Ii ionization equilibria are in agreement with their astrometric counterparts obtained from HIPPARCOS parallaxes.\ High signal-to-noise, high-resolution échelle spectra are analysed to determine individual profile fits and differential abundances of iron lines. Depending on the choice of the hydrogen collision scaling factor SH, we find deviations from LTE in Fe I ranging from 0.00 (SH,= infty ) to 0.46 dex (SH,= 0 for HD 140283) in the logarithmic abundances while Fe Ii 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 SH,= 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 SH leads to average Fe I non-LTE corrections of 0.02 dex and a mean abundance from Fe I lines of log varepsilon (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 HIPPARCOS 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.

Based on observations collected at the German-Spanish Astronomical Centre, Calar Alto, Spain. Title: NLTE Heavy Element Abundances in Cool Dwarfs: an Implication for the Galaxy Chemical Evolution Authors: Mashonkina, L.; Gehren, T. Bibcode: 2003IAUS..210P.E39M Altcode: No abstract at ADS Title: Iron in Non-LTE - Pitfalls and Prospects Authors: Korn, A. J.; Gehren, T. Bibcode: 2003IAUS..210P.B12K Altcode: No abstract at ADS Title: Mg, Ba and Eu abundances in thick disk and halo stars Authors: Mashonkina, L.; Gehren, T.; Travaglio, C.; Borkova, T. Bibcode: 2003A&A...397..275M Altcode: 2002astro.ph.10366M Our sample of cool dwarf stars from previous papers (Mashonkina & Gehren \cite{euba, eubasr}) 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. (\cite{eu99}), 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 Ii resonance line lambda 4554. As expected, the solar ratio 82:18 (Cameron \cite{cam}) adjusts to observations of the Ba Ii 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. \cite{rs99}), 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. Based on observations collected at the European Southern Observatory, Chile, 67.D-0086A, and the German Spanish Astronomical Center, Calar Alto, Spain. Title: Kinetic equilibrium of iron in the atmospheres of cool dwarf stars. II. Weak Fe I lines in the solar spectrum Authors: Gehren, T.; Korn, A. J.; Shi, J. Bibcode: 2001A&A...380..645G Altcode: 2001astro.ph.10605G NLTE line formation calculations of FeI in the solar atmosphere are extended to include weak lines in the visual spectrum of the Sun. Previously established atomic models are used to discriminate between different ways of treating collisional interaction processes. As indicated by the analysis of strong FeI lines, the influence of deviations from LTE in the solar atmosphere on the Fe abundance is small for all lines. To derive a common solar FeI abundance from both strong and weak lines fine-tuning of the microturbulence velocity parameter and the van der Waals damping constants is required. The solar FeI abundances based on all available f-values are dominated by the large scatter already found for the stronger lines. In particular the bulk of the data from the work of May et al. and O'Brian et al. is not adequate for accurate abundance work. Based on f-values measured by the Hannover and Oxford groups alone, the FeI LTE abundances are \log\varepsilonFeI,solar = 7.57 for the empirical and \log\varepsilonFeI,solar = 7.48 \ldots 7.51 for the line-blanketed solar model. The solar Fe ionization equilibrium obtained for different atomic and atmospheric models rules out NLTE atomic models with a low efficiency of hydrogen collisions. At variance with Paper I, it is now in better agreement with laboratory FeII f-values for all types of line-blanketed models. Our final model assumptions consistent with a single unique solar Fe abundance \log\varepsilonFeI,solar ~ 7.48 \ldots 7.51 calculated from NLTE line formation are (a) a line-blanketed solar model atmosphere, (b) an iron model atom with hydrogen collision rates $0.5 < SH < 5 times the standard value to compensate for the large photoionization cross-sections, (c) a microturbulence velocity ξt = 1.0 km s-1, (d) van der Waals damping parameters decreased by Δ\log C6 = -0.10 \ldots -0.15 as compared to Anstee & O'Mara's calculations, depending on SH, (e) FeII f-values as published by Schnabel et al., and (f) FeI f-values published by the Hannover and Oxford groups. Title: Heavy element abundances in cool dwarf stars: An implication for the evolution of the Galaxy Authors: Mashonkina, L.; Gehren, T. Bibcode: 2001A&A...376..232M Altcode: 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 (\cite{Fuhr3, Fuhr5}). 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 \cite{euba}) 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_sun) 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. Based on observations collected at the German Spanish Astronomical Center, Calar Alto, Spain. Title: Kinetic equilibrium of iron in the atmospheres of cool dwarf stars. I. The solar strong line spectrum Authors: Gehren, T.; Butler, K.; Mashonkina, L.; Reetz, J.; Shi, J. Bibcode: 2001A&A...366..981G Altcode: Line formation calculations of Fe I and Fe II in the solar atmosphere are presented for atomic models of iron including all observed terms and line transitions with available f-values. Recent improved calculations of Fe I photoionization cross-sections are taken into account, and the influence of collision processes is investigated by comparing synthesized and observed solar line flux profiles. The background is represented by the opacity of all important non-iron elements with iron lines added. Using a representative sample of sufficiently unblended strong Fe I and Fe II line profiles, it is evident that line formation is affected by (a) velocity fields and (b) deviations from local thermodynamic equilibrium (NLTE). The calculations are extended to a systematic analysis demonstrating that the ionization equilibrium of iron is recovered for solar parameters (T_eff = 5780 K, log g = 4.44) either using the empirical atmospheric model of Holweger & Müller (\cite{HM74}) and assuming LTE for both Fe I and Fe II or a line-blanketed theoretical atmospheric model with NLTE iron line formation. In the latter case the kinetic equilibrium of Fe I shows a substantial underpopulation of Fe I terms which depends sensitively on both the improved photoionization calculations and the choice of hydrogen collision rates while the Fe II ion is well approximated by LTE. Although the source functions of most of the Fe I lines are nearly thermal, their formation is shifted deeper into the photosphere. NLTE wings of strong Fe I lines are therefore shallower than under the LTE assumption, whereas the cores of the strongest lines display the usual chromospheric contributions. Based on both calculated and laboratory f-values the abundances of 37 Fe II lines range between eps {ion {Fe}iI,sun} = 7.50 and 7.56, depending on atomic and atmospheric models, and those of 117 Fe I lines between eps {ion {Fe}iI,sun} = 7.47 and 7.56, both with a relatively large scatter of 0.08 ... 0.12. The collisional coupling of Fe I levels is investigated. Electron collisions seem to play only a minor role. Hydrogen collisions are very important between terms of low excitation, and they efficiently thermalize the line source functions but not necessarily the populations of the lower levels that determine the line optical depth. Thermalization of those low-excitation terms that are responsible for most of the lines analyzed is achieved only if the collisional coupling among highly excited Fe I terms and their Fe II parent terms is increased by large factors compared with standard collision rates. Solar flux profiles are reproduced under the assumption of both LTE or NLTE, with nearly all types of atomic and atmospheric models, because the Fe ionization equilibrium depends on the corresponding sets of f-values. Title: Stellar Abundances of the Galactic Thick Disk Authors: Pettinger, M. M.; Bernkopf, J.; Fuhrmann, K.; Korn, A. J.; Gehren, T. Bibcode: 2001AGM....18.P166P Altcode: 2001AGAb...18Q.218P We present the results from model atmosphere analyses of two G dwarfs of the Galactic thick disk, 72 Her and HD 64606. High resolution, high signal-to-noise échelle spectra were obtained with the FOCES spectrograph on the 2.2m telescope of the Calar Alto observatory, Spain. Due to the well-defined blaze function of FOCES the determination of the continuum within an order and from order to order in the Hα, Hβ (for Teff) and Mg Ib triplett (for log g) region is very precise and leads to very accurately determined spectroscopic stellar parameters. The aim of our analysis is to study the chemical behaviour of the thick disk in particular with respect to the α-, r- and s-process elements. The principal results are as follows: both stars show significant enhancement in all analysed α-elements, in the r-process element Eu as well as in Al and Zn. Mn and the s-process element Ba are underabundant relative to iron while the other iron-peak elements exhibit a slight enhancement. N, Na, Ce and the r-process element Sr also show a weak overabundance. Based on the very accurate HIPPARCOS astrometry the stellar ages were determined to be 13 Gyrs. This allows us to identify both stars as members of the thick disk which is also in accord with their kinematics. The high Eu/Ba ratios are consistent with the ratio expected for stars older than 12 Gyr under the assumption of r-process dominated enrichment in the early phase of Galactic chemical evolution. We argue that the high [Al/Fe] and [Zn/Fe] ratios potentially allow to spectroscopically distinguish between the halo and thick-disk populations. Title: New Gravities for Old Stars Authors: Korn, A. J.; Gehren, T. Bibcode: 2001ASPC..245..337K Altcode: 2001aats.conf..337K No abstract at ADS Title: HIPPARCOS and the Distance Scale to Local Halo Stars Authors: Korn, A. J.; Gehren, T. Bibcode: 2001ASPC..228..494K Altcode: 2001dscm.conf..494K No abstract at ADS Title: Barium and europium abundances in cool dwarf stars and nucleosynthesis of heavy elements Authors: Mashonkina, L.; Gehren, T. Bibcode: 2000A&A...364..249M Altcode: We revise barium abundances in 29 cool stars with metallicities [Fe/H] ranging from -2.20 to 0.07 and europium abundances in 15 stars with [Fe/H] from -1.52 to 0.07. The sample has been extracted from Fuhrmann's lists (\cite{Fuhr3, Fuhr4}) and confined to main-sequence and turnoff stars with only one subgiant added. The results are based on differential NLTE model atmosphere analyses of spectra that have a typical S/N of 200 and a resolution of 40000 or 60000. The statistical equilibrium of Eu Ii is first investigated with a model atom containing 32 levels of Eu Ii plus the ground state of Eu Iii. NLTE effects decrease the equivalent widths of the Eu Ii lines compared with LTE resulting in positive NLTE abundance corrections which are below 0.08 dex for all the stars investigated. The solar barium abundance eps {Ba,sun} = 2.21 and the europium abundance eps {Eu,sun} = 0.53 are found from the Ba Ii and Eu Ii solar flux line profile fitting, and they coincide within error bars with meteoritic abundances of Grevesse et al. (\cite{met96}). Here the usual scale with eps {H} = 12 is used. The isotopic ratio \iso{151}{Eu}: \iso{153}{Eu} = 55: 45 is obtained from solar disk center intensity profile fitting of the Eu Ii lambda 4129 Åline. We report here for the first time that the elemental ratios [Ba/Fe], [Eu/Fe] and [Eu/Ba] show a different behaviour for stars of different Galactic populations. For the halo stars the [Ba/Fe] ratios are approximately solar, europium is overabundant relative to iron and barium with the mean values [Eu/Fe] = 0.62 and [Eu/Ba] = 0.64. For thick disk stars it is found that a) barium is slightly underabundant relative to iron by about 0.1 dex; b) europium is overabundant relative to iron with the [Eu/Fe] ratios between 0.30 and 0.44; and c) europium is overabundant relative to barium with a mean value of [Eu/Ba] = 0.49 +/- 0.03. A step-like change in the [Eu/Ba] and [Ba/Fe] ratios occurs at the thick to thin disk transition; so, nearly solar elemental ratios [Ba/Fe], [Eu/Fe] and [Eu/Ba] are found for the thin disk stars. These data suggest that a) the halo and thick disk stellar population formed quickly during an interval comparable with the evolution time of an AGB progenitor of 3 to 4 M_sun, and the r-process dominated heavy element production at that epoch; b) there was a hiatus in star formation before the early stage of the thin disk developed. The even-to-odd Ba isotope ratios estimated from hyperfine structure (HFS) affecting the Ba Ii resonance line in the halo and thick disk stars favour a significant contribution of \iso{138}{Ba} to barium for a pure r-process, and this is supported by the recent data of Arlandini et al. (\cite{rs99}). Based on observations at the German Spanish Astronomical Center, Calar Alto, Spain Title: Non-LTE analysis of neutral magnesium in cool stars Authors: Zhao, G.; Gehren, T. Bibcode: 2000A&A...362.1077Z Altcode: 2000astro.ph.11289Z Calculations of the statistical equilibrium of magnesium in the solar photosphere have shown that NLTE populations hardly affect Mg line formation in the Sun. However, in metal-poor dwarfs and giants the influence of electron collisions is reduced, and the ultraviolet radiation field, enhanced due to reduced background line opacity, results in more pronounced NLTE effects. In the photosphere of a cool star excitation and ionization due to collisions with neutral hydrogen can outweigh electron collisions. Analyses based on NLTE populations lead to significantly higher Mg abundances than those calculated from LTE. We calculate magnesium abundances in 10 cool dwarfs and subgiants with metallicities from -2.29 to 0.0. The results are based on spectra of high-resolution and high signal-to-noise ratio. Stellar effective temperatures are derived from Balmer line profiles, surface gravities from Hipparcos parallaxes and the wings of the Mg Ib triplet, and metal abundances and microturbulence velocities are obtained from LTE analyses of Fe Ii line profiles. For stars with metallicities between -2.0 < [Fe/H] < -1.0 abundance corrections Delta [Mg/H]NLTE-LTE ~ 0.05-0.11 are found. As expected the corrections increase with decreasing metal abundance, and they increase slightly with decreasing surface gravity. We also calculate the statistical equilibrium of magnesium for series of model atmospheres with different stellar parameters and find that Delta [Mg/H]NLTE-LTE increases with effective temperature between 5200 and 6500 K. For extremely metal-poor stars the abundance corrections approach Delta [Mg/H]NLTE-LTE ~ 0.23 at [Fe/H] ~ -3.0. Based on observations collected at the German-Spanish Astronomical Center, Calar Alto, Spain and Beijing Astronomical Observatory, Xinglong, China Title: Finding the First Stars: The Hamburg/ESO Objective Prism Survey Authors: Christlieb, Norbert; Reimers, Dieter; Wisotzki, Lutz; Reetz, Johannes; Gehren, Thomas; Beers, Timothy C. Bibcode: 2000fist.conf...49C Altcode: 1999astro.ph.11016C The Hamburg/ESO survey (HES, [4]) is an objective-prism survey for bright quasars based on IIIa-J plates taken with the ESO Schmidt telescope and its 4° prism. It covers the total southern extragalactic sky (δ < +2° \vert b\vert ;≳ 30°). All 380 Schmidt plates (effective area ∼ 7000 square degrees) have been taken, and have been digitized and reduced in Hamburg. The spectral range of the HES plates is 3200 Å < λ < 5300 Å, with a seeing-limited spectral resolution of 15 Å at Hγ. This makes it possible to efficiently exploit the stellar content of the survey. Title: Gravities of Metal-Poor Halo Dwarfs and the Age of the Universe Authors: Korn, Andreas J.; Gehren, Thomas Bibcode: 2000fist.conf...75K Altcode: We report on the methodology of an ongoing project to determine accurate stellar parameters (T eff , log g, [Fe/H], [α/Fe]) for a number of metal-poor halo stars located at the turnoff (TO). With the aid of shift-free unified stellar evolutionary models we envision to derive absolute ages for these stars. Their ages will set stringent lower limits to the age of the Galaxy and the Universe. Title: HIPPARCOS and the Spectroscopic Distance to Local Halo Stars Authors: Korn, A. J.; Gehren, T. Bibcode: 2000AGM....16..P58K Altcode: We are currently extending the study of "Nearby stars of the Galactic disk and halo I & II" (Fuhrmann 1998 & 2000) to a sample of metal-poor halo stars out to about 500 pc. Our ultimate aim is to derive spectroscopic ages for these stars. The basis of our analysis are spectra with R between 40,000 and 60,000 and signal-to-noise ratios (after co-addition) above 200 at Hα. These spectra were acquired using FOCES on the 2.2 m telescope at Calar Alto in 1999. Some of the brighter halo stars have significant Hipparcos parallaxes against which we can cross-check our method of gravity determination. For halo stars down to [m/H] ~-2 (to which the strong line method is applicable) the resulting gravities are practically free of systematics when compared to Hipparcos. From 150 pc outward, the spectroscopic uncertainty in distance is smaller than the astrometric one. The question of whether or not metal-poor stars require a temperature label and/or temperature-depth structure different from that of the Sun and solar-metallicity stars may be addressed in the context of non-LTE ionization equilibria of e. g. iron or calcium. Title: NLTE Analysis of magenisum in cool stars Authors: Zhao, G.; Gehren, T. Bibcode: 2000LIACo..35..277Z Altcode: 2000ghgc.conf..277Z No abstract at ADS Title: Photospheric metal abundances of AR Lacertae Authors: Gehren, Thomas; Ottmann, Renate; Reetz, Johannes Bibcode: 1999A&A...344..221G Altcode: High-resolution spectra of the RS CVn system AR Lac taken during total eclipse of the primary component lead to a precise determination of the stellar effective temperature of the secondary, Teff = 5100 +/- 100 K, and a surface gravity log g = 3.65 +/- 0.1. The small-scale motions are well represented by a microturbulence velocity of v_t = 1.6 +/- 0.3 km s(-1) . Examination of a number of spectral windows by means of spectrum synthesis based on solar spectrum reference fits produces a pattern of photospheric metal abundances that is essentially represented by solar or slightly enhanced values. This is at variance with the subsolar metal abundances reported from X-ray observations with the ROSAT and ASCA satellites. The effective temperature determined here restricts the surface fraction of cool matter such as confined in star spots during the present observations to values around 0.3. For a close binary system with an active chromosphere the photospheric spectra are normal. No spectrum variations on a half hour time scale are detected. Based on observations collected at the German-Spanish Astronomical Center, Calar Alto, Spain Title: Barium abundances in cool dwarf stars as a constraint to s- and r-process nucleosynthesis Authors: Mashonkina, L.; Gehren, T.; Bikmaev, I. Bibcode: 1999A&A...343..519M Altcode: We revise barium abundances in 11 cool stars with metallicities ranging from -2.65 to 0.05. The results are based on differential NLTE model atmosphere analyses of spectra that have a typical S/N of 200 and a resolution of 40000 or 60000. To minimize systematic errors of theoretical modeling and to be sure that elemental surface abundances are not contaminated by thermonuclear reaction products from the stellar interior the sample is confined to main-sequence and turnoff stars with only two subgiants added. Stellar fundamental parameters are derived from either (V-K) colours or Balmer line profiles for the effective temperature, from HIPPARCOS parallaxes for the surface gravity and from the LTE analyses of the Fe II line profiles for metal abundance and microturbulence values. The statistical equilibrium of Ba II is investigated with a model atom containing 41 levels of Ba II plus the ground state of Ba III. NLTE effects depend on the metallicity of a star: they increase the equivalent widths compared with LTE for [Fe/H] > -2, and they show the opposite behaviour at lower metallicities. Empirical evidence for the necessity to include H atom collisions in the statistical equilibrium of Ba II is found from comparison of Ba abundances in the metal-poor stars derived from the different spectral lines. The formula of Drawin with a scaling factor of 1/3 gives quite sufficient results. [Ba/Fe] abundance ratios are approximately solar above [Fe/H] ~ - 2.2 where they decrease rapidly by 0.5-0.6 dex. The direct method based on the hyperfine structure (HFS) of the resonance line of the odd isotopes is suggested to estimate the contribution of the s- and r- process to Ba synthesis. Its application requires the knowledge of the total Ba abundance that can be deduced from the subordinate lines free of HFS. Thus, we cannot estimate the ratio of the s- and r- processes for the two most metal-deficient stars of our sample. Our conclusion is that the s-process dominated Ba production, at least, for the metal-poor stars with [Fe/H] > -2.2. Based on observations at the German Spanish Astronomical Center, Calar Alto, Spain Title: An Automated Search for Metal-Poor Halo Stars in the Hamburg/ESO Objective-Prism Survey Authors: Christlieb, Norbert; Wisotzki, Lutz; Reimers, Dieter; Gehren, Thomas; Reetz, Johannes; Beers, Timothy C. Bibcode: 1999ASPC..165..259C Altcode: 1998astro.ph.10183C; 1999gaha.conf..259C An automated search for metal-poor stars is carried out in the course of the Hamburg/ESO objective-prism survey (HES), which covers the full southern sky at Galactic latitudes |b| >~ 30 deg. As the HES reaches ~1 magnitude deeper and covers areas of the sky which have not been touched by the HK Survey of Beers and collaborators, the total survey volume of the HES represents an increase by a factor of 4.5 compared to the HK Survey. Because of the limited availability of telescope time for spectroscopic follow-up observations, we will focus on a search for the most metal-poor, unevolved stars. We present a simulation study of the HES selection function P(B_J, T_eff, [Fe/H], log g) for metal-poor stars, and results from a first follow-up campaign at the ESO-NTT. Title: Analyzing metal-poor halo stars using FOCES Authors: Korn, A. J.; Gehren, T. Bibcode: 1999AGAb...15..102K Altcode: 1999AGM....15..P31F We present first results of an ongoing project to derive fundamental stellar parameters, atmospheric abundances and absolute ages of some of the most metal-poor field stars known. High resolution (R ~ 40 000), high S/N (> 150) spectra obtained using FOCES at the Calar Alto 2.2m telescope serve this purpose very well. While T_eff is reliably deduced from fitting Balmer profiles, log g cannot be determined easily due to the extraordinary metal deficiency of these objects ([m/H]~-3) and the corresponding weakness/absence of log g-sensitive spectroscopic features. Since using the strong line method for deriving log g has been shown to be inapplicable below [m/H]~-2.4, we concentrate our efforts on ionization equilibria, in particular Fe i/ii. However, iron is known to yield results discrepant with Hipparcos parallaxes for these stars thus demanding a full non-LTE approach. The determination of log g will enable us to derive accurate abundances and absolute ages, the latter profitting from unified stellar evolutionary models with a superior treatment of convection and diffusion (Bernkopf 1998). The ages of these stars will set stringent lower limits to the age of the Universe. Title: Testing SN IA progenitor scenarios: SNR 1006 Authors: Wellstein, S.; Langer, N.; Gehren, T.; Burleigh, M.; Heber, U. Bibcode: 1999AGAb...15R..24W Altcode: For Type Ia supernovae several progenitor scenarios have been proposed: 1) Roche lobe overflow from main sequence stars onto white dwarfs, 2) Roche lobe overflow from subgiants onto white dwarfs, 3) Wind accretion from a giant companion 4) Merging white dwarf pairs. So far, none of these scenarios have been clearly demonstrated to occur in nature, and only for scenario 1 we know potential observed counterparts, the supersoft X-ray sources. We have computed detailed evolutionary model grids for scenarios 1 and 2, and relied on literature data for scenario 3, in order to determine the properties of the white dwarf companion star at the time of the supernova explosion. E.g., we found that the companions in scenario 1 stem from 1.7 dots 2.3 M_odot main sequence stars and are underluminous stars of more than 1 M_odot, out of thermal equilibrium, with luminosities in the range 1 dots 10 L_odot, devoid of Li, Be, and B, and partly enriched in carbon. Scenarios 2 and 3 are found to leave low mass O or B subdwarfs. From Scenarios 1 and 2, we expect large space velocities for the companions after the white dwarf explosion (100 dots 1000 km/s). The remnant of the historical Type Ia supernova SN 1006 is close enough to unambiguously identify the former companion star of the white dwarf and, according to its properties, distinguish between scenarios 1, 2 and 3. We propose a detailed observational strategy to find the companion star. The sdO star near the center of SN 1006 has already been investigated in part. A failure of finding any suitable star in SNR 1006 would support scenario 4. In any case, a distinctive test of Type Ia supernova progenitor scenarios appears to be possible with SNR 1006. Title: Sodium in the Sun and in metal-poor stars Authors: Baumueller, D.; Butler, K.; Gehren, T. Bibcode: 1998A&A...338..637B Altcode: Systematic effects in the statistical equilibrium of sodium in cool metal-poor stars are analyzed using full NLTE line formation. To determine the important influence of collision processes and of the atomic model, NLTE effects of neutral sodium are first evaluated in the solar photosphere where the statistical equilibrium of Na I can be followed by examination of a relatively large number of line transitions. In agreement with previous analyses it is found that even very simple atomic models are sufficient to describe the most important interactions. In the solar atmosphere the inner cores of the lines are most affected by deviations from LTE, but the corresponding abundance corrections due to NLTE populations are small. The influence of collisional interactions with electrons and hydrogen atoms is evaluated by comparison of the solar spectrum with very precise fits to the Na I line cores. The profile analysis depends sensitively on the appropriate choice of velocity amplitudes and its variation with depth. The resulting solar sodium abundance is obtained with small scatter, log varepsilon_Na ,sun = 6.30 +/- 0.03. In metal-poor stars NLTE effects are more pronounced since the statistical equilibrium is dominated by collisions in which at least the electronic component is substantially reduced. The resulting influence on the determination of Na abundances is in a direction opposite to that found previously for Al. Stars determined in LTE analyses to have a solar [Na/Fe] ratio reveal a lower [Na/Fe] when NLTE line formation is taken into account. As for aluminium, the extremely metal-poor and the hotter subdwarfs are affected most strongly by Na abundance corrections reaching -0.5 dex for the D lines. The resulting Galactic evolution of the Na/Fe and Na/Mg ratios is not adequately described by current chemical evolution scenarios. Based on observations collected at the European Southern Observatory, La Silla, Chile, and at the German-Spanish Astronomical Center, Calar Alto, Spain Title: Photospheric metal abundances in active stellar atmospheres Authors: Ottmann, R.; Pfeiffer, M. J.; Gehren, T. Bibcode: 1998A&A...338..661O Altcode: With a sophisticated analysis we determine the photospheric metal abundances of active stars to investigate the abundance stratification in active stellar atmospheres. For selected single stars and RS CVn binaries we have taken high-resolution, high-SN and partially phase-resolved spectra of the entire visible range. With a purely spectroscopic, self-consistent method based on line profile synthesis and an equivalent width analysis, we derive the effective temperature Teff, surface gravity log g, microturbulence xi_t and the abundances of Fe, Mg and Si with high internal accuracy (Delta Teff =~ 70 K, Delta log g =~ 0.1 dex, Delta xi_t =~ 0.1 km/s, Delta [Fe/H], Delta [Mg/H], Delta [Si/H] =~ 0.05 dex). Stellar parameters and metal abundances are obtained for the single stars beta Cet, kappa Cet and pi (1) UMa as well as for the RS CVn binaries AY Cet, VY Ari, EI Eri, IM Peg, lambda And and II Peg. The effect of stellar activity features (spots and plages) on the derived parameters is investigated by phase-resolved spectroscopy; we find that one out of four systems shows a significant variation of the Mg and Si abundances with rotational phase. The results help solving the puzzle about the photospheric Fe abundances of RS CVn binaries; specifically, we obtain [Fe/H] >= -0.4 for all systems, indicating that they definitely are not metal-poor. In II Peg and lambda And, the metals are strongly depleted in the corona relative to the photosphere; in beta Cet and pi (1) UMa, Fe and Si are weakly depleted, but Mg is enhanced (with 1sigma significance). The abundance stratifications are discussed in terms of the FIP-effect and the hydrostatic equilibrium stratification. Title: FOCES - a fibre optics Cassegrain Echelle spectrograph Authors: Pfeiffer, M. J.; Frank, C.; Baumueller, D.; Fuhrmann, K.; Gehren, T. Bibcode: 1998A&AS..130..381P Altcode: We have designed and built the echelle spectrograph FOCES fed by 100 mu m optical fibres to be mounted at the Cassegrain focus of either the 2.2 m or the 3.5 m telescope at the Calar Alto Observatory. The spectrograph itself follows a white-pupil design collimated with two off-axis parabolic mirrors. The 15 cm beam leaving the 31.6 lines/mm R2 echelle is refocussed in the vicinity of a small folding mirror which allows efficient removal of scattered light. The cross-dispersion is achieved with a tandem prism mounting, and the beam imaged with an f/3 transmission camera onto a field centered on a 1024(2) thinned Tektronix CCD with 24 mu m pixel diameter. The echelle image covers the visible spectral region from 380 to 750 nm displayed in 70 spectral orders with full spectral coverage. Spectral orders are separated by 20 pixels in the blue and by 10 pixels in the red. The maximum spectral resolution is R = lambda / Delta lambda \ = 40600 with a 2 pixel resolution element; unvignetted resolution as defined by the fibre alone would be obtained at R = 18000. Replacing the CCD by a 2048(2) chip with 15 mu m pixel diameter and taking into account light losses from a reduced entrance slit width a full 2 pixel resolution of R = 65000 is obtained. The above concept has made FOCES an extremely well-defined instrument. A number of successful test installations at the Cassegrain foci of the Wendelstein 80 cm telescope, the Calar Alto 2.2 m and 3.5 m telescopes has produced spectra of high quality for up to 60 min exposures. The limiting magnitude for a 1 hr exposure with an S/N ratio of 100 scales to V = 12 for a 3.5 m telescope which is only slightly less than expected from laboratory tests. In an alternative mode FOCES offers a second fibre carrying the sky background signal to correct extremely faint object spectra. This mode obtains the required higher cross-dispersion from an additional grism resulting in a correspondingly reduced spectral coverage. Based in part on observations collected at the German-Spanish Astronomical Center, Calar Alto, Spain. Title: Non-LTE analysis of neutral magnesium in the solar atmosphere Authors: Zhao, G.; Butler, K.; Gehren, T. Bibcode: 1998A&A...333..219Z Altcode: We investigate the formation of neutral magnesium lines in the solar photosphere with an atomic model containing 83 levels plus the ground state of Mg ii connected via radiative and collisional interactions. Synthetic line flux and intensity profiles are compared with the solar spectrum to study the relevant physical processes and their influence on the level populations and line profiles. For neutral magnesium with the photoionization edges of its three lowest states at lambda lambda 1620, 2514 and 3757 Angstroms the reduction of the ultraviolet radiation field due to metallic line absorption has been taken into account using Kurucz' (\cite{KURUCZ92}) ODF opacities. In the photosphere of a cool star excitation and ionization due to collisions with neutral hydrogen can outweigh electron collisions. Therefore the influence of different types of collisional interactions with electrons and neutral hydrogen atoms is examined. General agreement with solar line profiles in the visible and infrared is found for an atomic model with both electron collisions and strongly reduced but significantly large neutral hydrogen collision rates. Our investigation thus extends previous results to lines of all different excitation energies. The atomic model found from the analysis of the solar spectrum will serve as a reference for the investigation of cool metal-poor stars in which both the reduced electron collision rates and the enhanced UV intensities lead us to expect more pronounced deviations from LTE. Title: Real-time spectroscopy of gravitational microlensing events - probing the evolution of the Galactic bulge. Authors: Lennon, D. J.; Mao, S.; Reetz, J.; Gehren, T.; Yan, L.; Renzini, A. Bibcode: 1997Msngr..90...30L Altcode: 1997astro.ph.11147L Over several observing seasons, and with the help of gravitational microlensing surveys, the authors aim is to perform a systematic spectroscopic investigation of bulge sources. They expect that the results from this campaign will provide a fundamental insight into the formation and evolution of the bulge of the Galaxy. In the rest of this article the authors describe their first steps on this road, and summarise the current status of the project. Title: Aluminium in metal-poor stars. Authors: Baumueller, D.; Gehren, T. Bibcode: 1997A&A...325.1088B Altcode: Previous calculations of the statistical equilibrium of aluminium in the solar photosphere have shown that NLTE populations hardly affect Al line formation in the Sun; however, in metal-poor stars the influence of electron collisions is reduced, and a UV radiation field enhanced due to smaller background line opacity results in more pronounced NLTE effects. Thus analyses based on NLTE populations lead to significantly higher Al abundances than those calculated from LTE. For stars of intermediate metallicity between -1.0<[Fe/H]<-0.5 some overabundance relative to iron is found. For more metal-poor stars the overabundance disappears and approaches the solar ratio, [Al/Fe]=0. Only a weak overabundance in the [Al/Mg] ratio is detected for stars with intermediate metallicity and a small underabundance of -0.2 to -0.3dex for the metal-poor stars. From investigation of both solar and stellar Al spectra the influence of hydrogen collisions could be better estimated. The previously defined atomic model thus had to be slightly modified to fit both metal-rich and metal-poor stars. Compared with LTE analyses the present results completely change the chemical enrichment scenario with [Al/Fe] now following the trend of primary elements for all metal-poor stars. The hump of enhanced Al/Fe values for stars between -1.0<[Fe/H]<-0.5 does not seem to be an artefact. It nearly vanishes for the [Mg/Fe] abundance ratios. It may not necessarily have to be explained in terms of stellar nucleosynthesis because it could result from our reference to LTE abundances for Mg and Fe. Title: The surface gravities of cool dwarf stars revisited. Authors: Fuhrmann, K.; Pfeiffer, M.; Frank, C.; Reetz, J.; Gehren, T. Bibcode: 1997A&A...323..909F Altcode: On the base of high-resolution spectra and standard model atmosphere analyses we propose to employ the pressure-broadened Mg Ib lines to derive the gravity parameter for F and G stars. These lines are advocated to be a much more robust and reliable tracer compared to the ionization equilibrium of, say, Fe I/Fe II, which is susceptible to overionization effects and uncertainties in the temperature structure of the model atmosphere. It is demonstrated that the strong line method circumvents the long-standing discrepancy ({DELTA}logg~0.5dex) of the standard F star Procyon, the surface gravity of which is precisely known due to its nearness and binary nature. We also discuss similar effects on other, predominantly metal-poor stars. In fact, many of the F and hotter G stars deviate in the LTE metal abundances of neutral and ionized species by up to 0.2dex. However, once the surface gravity parameter is fixed, very reliable iron abundances from Fe II can be derived as well. As a consequence a number of stars considered to be standards will require revised stellar parameters in future analyses. This will have some impact on stellar distances, ages, and galactic evolution in particular. Title: The First Spectroscopic Observations of Caustic Crossing in a Binary Microlensing Event Authors: Lennon, Daniel J.; Mao, Shude; Fuhrmann, K.; Gehren, T. Bibcode: 1996ApJ...471L..23L Altcode: We present the first spectroscopic observations of a binary microlensing event when it was undergoing a caustic crossing with a high magnification of A ~ 25. The event 96-BLG-3 was identified in real time by the MACHO collaboration, in the Baade's window field toward the Galactic bulge. Three spectra were taken consecutively, spanning the light-curve peak of the caustic crossing, each integration lasting 30 minutes. The spectrograms covered the wavelength range 3985--6665 A and are almost identical, the third one differing only in having an amplitude ~6% lower than the others. By comparison with reference star spectra and by using spectrum synthesis techniques, we infer that the source star is a G0 IV--V star, with an effective temperature of Teff +/- 150 K, a metallicity in the range [M/H] = +0.3 to +0.6, and a logarithmic surface gravity of log g = 4.25 +/- 0.25. Using theoretical evolutionary tracks, we derive a radius of ~1.4 ^{-0.4}_{+0.6} Rsolar and hence a distance of 6.9 ^{-1.7}_{+3.1} kpc, consistent with the source residing in the Galactic bulge. We also determine its heliocentric radial velocity to be vr +/- 3 km s-1. Caustic-crossing microlensing events such as 96-BLG-3, if they are observed with 8--10 m class telescopes, can resolve the stellar surface of distant sources with a resolution of 1010 cm or better. This permits a detailed study of the center-to-limb variation of the stellar surface and the intrinsic properties of the lensed source. Title: Line formation of neutral aluminium in the Sun. Authors: Baumueller, D.; Gehren, T. Bibcode: 1996A&A...307..961B Altcode: We investigate the formation of neutral aluminium lines in the solar photosphere using an atomic model containing 58 levels plus the ground state of Al II connected via radiative and collisional interaction. Synthetic line flux and intensity profiles are compared with the solar spectrum to study the relevant kinetic processes and their influence on level populations and line profiles. For neutral aluminium with its extremely large ground state photoionization cross-section near 2071 A the reduction of the ultraviolet radiation field due to metallic line absorption has to be taken into account using Kurucz' (1992) ODF opacities. In the photosphere of a cool star excitation and ionization due to collisions with neutral hydrogen can outweight electronic collisions. The influence of different types of collisional interactions with electrons and neutral hydrogen is therefore examined. As expected, the non-LTE effects in most of the solar Al I lines are small, irrespective of the details of the atomic model. The cores and innermost parts of the wings of the resonance lines at 3944 and 3961A are affected by only small deviations from the observed profiles, part of which is due to the uncertainty connected with the proper choice of the continuum flux in the region of the Ca II H+K lines. Since the first excited state, 4s^2^S , is slightly overpopulated with respect to both the 3p^2^P^o^ ground state and to 4p^2^P^o^ the strongest evidence for non-thermal excitation is found in the infrared lines at 1.3 and particularly at 12μm. Empirical evidence for the necessity to include neutral particle collisions in the kinetic equilibrium of aluminium arises from comparison of these lines with observations. General agreement with solar line profiles in the infrared and in the visible is found for an atomic model with both electronic collisions and a strongly reduced amount of neutral particle collisions. The solar model will serve as a reference for the investigation of cool metal-poor stars in which both the reduced electronic collision rates and the enhanced UV intensities lead us to expect more pronounced deviations from LTE. Title: Spectroscopic analyses of metal-poor stars. III. Magnesium abundances. Authors: Fuhrmann, K.; Axer, M.; Gehren, T. Bibcode: 1995A&A...301..492F Altcode: We present magnesium abundances and Mg/Fe ratios for 56 metal-poor dwarfs and subgiants based on differential LTE model atmosphere analyses of spectra that have a typical S/N of 50-100 and a resolution of about 20000-30000. The distribution of Mg/Fe abundance ratios with Fe abundances is characterized by a steep increase of [Mg/Fe] near [Fe/H]=-0.6. At that iron abundance we recognize an abrupt change from a solar value for stars with [Fe/H]>=-0.6 to an upper limit of +0.4dex for the metal-poor stars. Our data suggest that this step in the Mg/Fe abundance ratio at [Fe/H]=-0.6 is the result of the onset of Galactic disk formation, whereas [Mg/Fe]=+0.4 apparently is an upper limit arising from the constraints of nucleosynthesis in massive SN II events during the first stellar generations, the yields of which we observe in the most metal-poor halo stars. Title: Spectroscopic analyses of metal-poor stars. II. The evolutionary stage of subdwarfs. Authors: Axer, M.; Fuhrmann, K.; Gehren, T. Bibcode: 1995A&A...300..751A Altcode: Models of post-main sequence stellar evolution of VandenBerg & Bell (???) have been applied to determine spectroscopic masses and distances for metal-poor stars. Careful consideration of the most important error sources published in more recent papers such as VandenBerg (???) for the first time allow us to draw firm statistical conclusions. It is shown that the evolutionary calculations qualitatively fit to the observed stellar parameters whereas quantitatively they predict too high ages for metal-poor stars. As an important result we confirm that evolutionary sequences need to be calibrated with respect to their metal abundance in order to use their absolute predictions of temperature and luminosity. It turns out that this can be achieved by a simple shift of the evolutionary tracks and isochrones in effective temperature with values {DELTA}log T_eff_<~0.03 which accounts for possible changes of the mixing-length and the O/Fe ratio with metallicity. The stellar luminosities and surface gravities obtained from evolutionary models are much more reliable than their effective temperatures. Therefore we conclude that the accuracy of the corresponding spectroscopic stellar gravities is systematically affected by deviations from LTE, in particular along the subgiant sequence where systematic errors less than {DELTA}log g =~0.3 must be ascribed to the non-LTE ionization equilibrium of Fe II/Fe I. In our spectroscopic analyses the strong dependence between surface gravity and abundances determined from Fe I lines restricts the accuracy of Fe abundances in subgiants to 0.1 dex at best. The most remarkable result of our evolutionary and kinematic investigations of halo stars refers to the large fraction of slightly evolved subgiants among the so-called subdwarfs. Since conventional photometric approaches often assume that the great majority of metal-poor stars are dwarfs this results in distances that are systematically too low for their samples. Consequently, significant differences are found when comparing evolutionary and kinematic parameters obtained from either photometric or spectroscopic data. We demonstrate this by comparing the space velocities of the stars. It appears that stars with particularly high space velocities derived from spectroscopic distances show very often much lower velocities based on their main sequence parallaxes. We find that results refering to main sequence parallaxes are doubtful and can be used only with greatest care. An advantageous side-effect of the application of spectroscopic data to evolutionary calculations is the possibility to identify binary systems that are either standing out from the Toomre diagram with their unusually high space velocities, or from a log g - log T_eff_ diagram with apparently contradictory luminosities. Title: VizieR Online Data Catalog: Metal-poor stars spectroscopy. II (Axer+, 1995) Authors: Axer, M.; Fuhrmann, K.; Gehren, T. Bibcode: 1995yCat..33000751A Altcode: Models of post-main sequence stellar evolution of VandenBerg & Bell have been applied to determine spectroscopic masses and distances for metal-poor stars. Careful consideration of the most important error sources published in more recent papers such as VandenBerg for the first time allow us to draw firm statistical conclusions. It is shown that the evolutionary calculations qualitatively fit to the observed stellar parameters whereas quantitatively they predict too high ages for metal-poor stars. As an important result we confirm that evolutionary sequences need to be calibrated with respect to their metal abundance in order to use their absolute predictions of temperature and luminosity. In our spectroscopic analyses the strong dependence between surface gravity and abundances determined from Fe I lines restricts the accuracy of Fe abundances in subgiants to 0.1 dex at best. The most remarkable result of our evolutionary and kinematic investigations of halo stars refers to the large fraction of slightly evolved subgiants among the so-called subdwarfs. Since conventional photometric approaches often assume that the great majority of metal-poor stars are dwarfs this results in distances that are systematically too low for their samples. (4 data files). Title: Constraints from Element Abundances in the Galaxy Authors: Gehren, T. Bibcode: 1995LNP...463..190G Altcode: 1995gyu..conf..190G; 1996LNP...463..190G No abstract at ADS Title: The galactic distribution of chemical elements as derived from B-stars in open clusters II. NGC6611, S285, and S289. Authors: Kilian-Montenbruck, J.; Gehren, T.; Nissen, P. E. Bibcode: 1994A&A...291..757K Altcode: The chemical composition of the open cluster NGC6611 and the Sharpless regions S285 and S289 have been analysed, and stellar parameters (effective temperature, gravity, microturbulence) and chemical abundances of 4 unevolved B-stars in NGC6611, 2 B-stars in S285, and 1 B-star in S289 are given. Line blanketed LTE model atmospheres and NLTE line formation calculations were used for the abundance determination of C, N, O, Mg, Al, and Si; LTE line formation calculations were used for the abundance determination of Ne, S, and Fe. In agreement with results obtained from B-type stars in the solar neighbourhood and the young open cluster NGC6231 presented in earlier papers, all elements except neon show an average underabundance of 0.1-0.4dex with respect to the solar value. Among the cluster stars abundance differences of 0.2-0.7dex may be observed. First results for the galactic metallicity gradient show an insignificant value for most observed elements, except for neon and sulfur, which is in contrast to results of analyses of HII regions by Shaver et al. (1983). Title: Spectroscopic analyses of metal-poor stars. I. Basic data and stellar parameters. Authors: Axer, M.; Fuhrmann, K.; Gehren, T. Bibcode: 1994A&A...291..895A Altcode: Accurate stellar parameters have been obtained from the analyses of more than thousand spectra of 115 metal-poor dwarfs and subgiants with visual magnitudes brighter than V=~12. The stellar sample was selected mainly by high proper motion, with additional restrictions from B-V colours and U-B excesses. The effective temperatures cover a range from 5000 to 6500K while metal abundances are found between [M/H]=-0.1 and -3.0dex. For the first time a representative number of unevolved cool metal-poor stars has been analysed individually using purely spectroscopic methods. Based on homogeneous ODF blanketed model atmospheres in LTE and working differentially with respect to the Sun we derive a consistent set of stellar parameters, effective temperature, surface gravity, metal abundance and microturbulence velocity. Individual profile synthesis is applied to a number of spectral lines for each star, which has led as a rule to accuracies in T_eff_ of better than 100K, in logg of better than 0.15, and in [Fe/H] of better than 0.1dex. Because of the consistent treatment with only one type of model atmosphere, this sample provides an opportunity to examine the individual parameters statistically and investigate in detail their relation to the formation and evolution of the Galaxy. One aspect of this analysis is a general shift to higher iron abundances for the most metal-poor stars. Along with the recently preferred meteoritic solar iron abundance and effective temperatures from consistent Balmer line profile fits that tend to be 100-200K hotter than found from photometric calibrations, discrepancies of up to 0.5dex in [Fe/H] can be explained in comparison with other abundance analyses. The most important results refer to the evolutionary status of the bona fide subdwarf sample. Irrespective of the different effective temperatures found here, there exists a severe problem when comparing post main sequence evolutionary models of cool stars with our observed parameters. Even more interesting is the fact, demonstrated by the results of a consistent analysis of the iron ionization equilibrium, that roughly half of the subdwarfs are subgiants, some of them having nearly reached the bottom of the giant branch. Title: Hans-Günter Groth (7. Februar 1927 - 20. Oktober 1993). Authors: Gehren, Thomas Bibcode: 1994MitAG..77....9G Altcode: No abstract at ADS Title: Balmer lines in cool dwarf stars II. Effective temperatures and calibration of colour indices Authors: Fuhrmann, K.; Axer, M.; Gehren, T. Bibcode: 1994A&A...285..585F Altcode: Effective temperatures obtained from synthesis of the extended profile wings of the first four Balmer lines are presented for more than 100 dwarfs and subgiants of different metal abundances and surface gravities in the temperature range from 5000 to 6500 K. Line formation is based on homogeneous ODF blanketed model atmospheres in LTE. The resulting temperatures of the more metal-rich stars differ systematically from those determined by reference to synthetic broad- or intermediate-band colours such as B-V , b-y , R-I or V-K . While the Balmer line temperatures give room to only very small individual errors and result in a convincingly small mean error for all four lines, the scatter against temperatures determined from broad-band colours is by far outside the internal errors claimed in recent applications. This may be attributed to either (a) observational errors, (b) dependence on the relative mixture of metal abundances, (c) unknown line blocking in most of the visible and near-infrared spectrum or (d) the inhomogeneity found in the granular patterns of stellar surfaces. Our results suggest that broad-band colours are insufficient individual temperature indicators, reliable only in a statistical sense. Title: FOCES: the fibre-optic-coupled Cassegrain ecelle spectrograph. Authors: Pfeiffer, M. J.; Frank, C.; Fuhrmann, K.; Gehren, T. Bibcode: 1994AGAb...10..107P Altcode: No abstract at ADS Title: Balmer lines in cool dwarf stars. I. Basic influence of atmospheric models. Authors: Fuhrmann, K.; Axer, M.; Gehren, T. Bibcode: 1993A&A...271..451F Altcode: Based on horizontally homogeneous model atmospheres the analysis of the first members of the hydrogen Balmer line series leads to a revision of the convective efficiency near the surfaces of cool dwarf stars. The synthesis of the Balmer lines includes Stark and resonance broadening. It fits the observed profiles of Hα through Hδ in such different stars as the Sun, Procyon, and the extremely metal-poor subdwarfs G41-41 and HD 140283. It turns out that the existence of the two different but equally important line-broadening mechanisms requires a temperature stratification near continuum optical depth unity which cannot be reconciled with a mixing-length 1 ≃ 1... 2 Hp as is usually derived from stellar interior calculations. With such high values of 1 the synthesis of the Balmer lines leads to effective temperatures differing by as much as 400 K. Comparison of Hα and Hβ profiles instead results in a much smaller mixing-length, 1 = (0.5±0.3) Hp, which seems to fit for all FG-type dwarfs even in the metal-poor limit. Thus, for the first time, spectroscopic observations of stars lead directly to a consistent determination of the temperature stratification in the lower photosphere. Title: The Texas/Pennstate/Stanford/Göttingen/Munich Spectroscopic Survey Telescope. Authors: Kudritzki, R. -P.; Gehren, T.; Fricke, K. J.; Beuermann, K.; Nicklas, H. Bibcode: 1993AGAb....9....9K Altcode: No abstract at ADS Title: The galactic abundance gradient: chemical abundances in early B-type stars. Authors: Kilian, J.; Gehren, T.; Nissen, P. E. Bibcode: 1992AGAb....7...17K Altcode: No abstract at ADS Title: Chemical abundances in early B-type stars Authors: Kilian, J.; Becker, S. R.; Gehren, T.; Nissen, P. E. Bibcode: 1992LNP...401...30K Altcode: 1992aets.conf...30K No abstract at ADS Title: The efficiency of convection in cool dwarf stars. Authors: Axer, M.; Fuhrmann, K.; Gehren, T. Bibcode: 1992AGAb....7..128A Altcode: No abstract at ADS Title: Effective temperatures of F and G dwarfs: photometric vs. spectroscopic methods. Authors: Axer, M.; Fuhrmann, K.; Gehren, T. Bibcode: 1992AGAb....7..126A Altcode: No abstract at ADS Title: Chemical abundances in early-type stars. III. NLTE temperature and gravity determination. Authors: Kilian, J.; Becker, S. R.; Gehren, T.; Nissen, P. E. Bibcode: 1991A&A...244..419K Altcode: Effective temperatures Teff and gravities log g of l9 unevolved B stars in the local field and nearby associations are given. Determinations of Teff due to non-LTE analyses of silicon line formation and non-LTE analyses of helium line formation are compared, and a reasonable agreement is obtained. Small differences were observed for four B stars only. The temperature calibration of some important photometric observables is discussed, with particular emphasis on a comparison with synthetic spectral indices. Temperatures obtained from the nonthermodynamic ionization equilibria of helium and silicon were found to be systematically higher by 2000 K than those calculated from synthetic photometry and LTE assumptions. Title: Numerical simulation of photospheric convection in solar-type stars. I - Hydrodynamical test calculations Authors: Reile, C.; Gehren, T. Bibcode: 1991A&A...242..142R Altcode: A computerized model of photospheric convection occurring in granules or similar structures in cool stars has been implemented to investigate the dependence of the granular flow and the structure of cool stellar atmospheres on radiative transfer and various element abundances. The numerical properties of the hydrodynamical method used are examined, emphasizing the treatment of advective terms. The method is applied to a number of idealized but important test problems such as shock waves, the Sedov problem in different dimensions, and standing gravoacoustic waves. It is shown that the combination of monotonic transport and operator-splitting is remarkably stable against error propagation. Title: The ROSAT/XUV data centre: pre-processing, distribution and archiving of ROSAT/XUV pointed phase data for German PIs. Authors: Brunner, H.; Kreysing, H. -C.; Staubert, R.; Herold, H.; Friedrich, S.; Knödler, M.; Gehren, T.; Kunze, D. Bibcode: 1991AGAb....6..116B Altcode: No abstract at ADS Title: Spectroscopic Tests of Late Type Model Atmospheres of Dwarf Stars Authors: Gehren, T.; Reile, C.; Steenbock, W. Bibcode: 1991ASIC..341..387G Altcode: 1991sabc.conf..387G No abstract at ADS Title: Numerical Simulation of Photospheric Convection - Hydrodynamical Test Calculations Authors: Reile, C.; Gehren, T. Bibcode: 1991ASIC..341..303R Altcode: 1991sabc.conf..303R No abstract at ADS Title: Dolidze 25 : a metal-deficient galactic open cluster. Authors: Lennon, D. J.; Dufton, P. L.; Fitzsimmons, A.; Gehren, T.; Nissen, P. E. Bibcode: 1990A&A...240..349L Altcode: High resolution spectra of three OB-type stars in the distant (5-6 kpc) anticenter galactic cluster Dolidze 25 have been analyzed using non-LTE and LTE model atmosphere techniques to derive atmospheric parameters and chemical compositions, respectively. The cluster appears to be significantly deficient in metals, with one star having underabundances for individual elements ranging from -0.55 to -0.93 dex with respect to the spectroscopic standard Tau Scorpii. This in conjunction with previous work on galactic clusters (Fitzsimmons et al. 1990, Gehren et al. 1985) implies that the variation of abundance with galactocentric distance is not linear and indeed may not be unique. Title: NLTE temperature and gravity determination of early B-type stars. Authors: Kilian, J.; Becker, S. R.; Gehren, T.; Nissen, P. E. Bibcode: 1990AGAb....5...61K Altcode: No abstract at ADS Title: Model atmosphere analyses of halo dwarfs. Authors: Axer, M.; Fuhrmann, K.; Gehren, T.; Reetz, J. Bibcode: 1990AGAb....5...70A Altcode: No abstract at ADS Title: Astronomical Information Obtained from Echelle Spectra Authors: Gehren, T. Bibcode: 1990ESOC...34..103G Altcode: 1990daan.work..103G No abstract at ADS Title: The initial mass function in early stages of galactic evolution Authors: Gehren, T. Bibcode: 1990nuas.symp..137G Altcode: No abstract at ADS Title: The initial mass function in early stages of Galactic evolution. Authors: Gehren, T. Bibcode: 1989nuas.conf..137G Altcode: Contents: 1. Introduction. 2. The initial mass function of globular cluster stars. 3. Spectroscopic parallaxes of halo field stars. 4. Masses of halo subdwarfs. 5. Conclusions. Title: Numerical Simulation of Photospheric Convection Authors: Reile, C.; Gehren, T.; Steenbock, W. Bibcode: 1989AGAb....3...50R Altcode: No abstract at ADS Title: Two-dimensional N-body simulations for the Evolution of Elliptical Galaxies Authors: Zeilinger, W.; Spies, W.; Burkert, A.; Gehren, T.; Hensler, G. Bibcode: 1989AGAb....3..110Z Altcode: No abstract at ADS Title: Metal-poor subdwarfs and early galactic nucleosynthesis. Authors: Hartmann, K.; Gehren, T. Bibcode: 1988A&A...199..269H Altcode: Observations of more than 20 metal-poor subdwarfs are presented, discussing the spectroscopic abundance analyses based on high-reolution spectra and scaled solar model atmospheres. Many of the stars are shown to be more evolved than can be reconciled with reasonable time scales based on standard theory of stellar evolution, suggesting that the post main sequence evolution of cool metal-poor stars is influenced by an enhanced O/Fe ratio compared with a standard solar mixture. Radial velocities are given for 60 metal-poor stars, 25 percent of which are suspected to be velocity variable. The kinematic properties of several specific stars are discussed. The abundance ratios Mg/Fe, Ca/Fe, Ti/Fe, Al/Fe, and Mn/Fe as a function of the Fe abundance itself indicate that these elements probably have not been produced in purely explosive nucleosynthesis. It is suggested that a considerable fraction of the isotopes must have been synthesized during preceding hydrostatic burning phases. Title: Chemical Abundances in Stars. Authors: Gehren, T. Bibcode: 1988RvMA....1...52G Altcode: Contents: 1. Introduction. 2. Methods of abundance determination. 3. Chemical elements in the solar system. 4. Abundances in unevolved disk stars. 5. Peculiar signs of stellar evolution. 6. Stellar abundances and galactic chemical evolution. 7. Summary. Title: Proper Motions of Herbig-Haro Objects. VII. The Region of NGC 2068 Authors: Jones, B. F.; Cohen, Martin; Wehinger, Peter A.; Gehren, Thomas Bibcode: 1987AJ.....94.1260J Altcode: The authors have determined proper motions and have obtained long-slit low-resolution spectroscopy for many of the Herbig-Haro (HH) objects in the region of NGC 2068. The HH 24 complex alone contains two dynamically distinct bipolar "jets". The authors believe there are three exciting stars in this region, two within the bounds of HH 24: one (SSV 63) is responsible for HH 24 knots CEA and the objects HH 19 and HH 20 to the northwest of HH 24, and possibly for HH 27 to the south; a second for HH 24 knots G1 - G3 and probably for HH 23. A third star (probably SSV 59) is responsible for HH 25 and HH 26, which also define a bipolar flow. The proper motions and radial velocities suggest that objects have undergone both acceleration in the vicinity of the exciting stars, and deceleration a parsec away. Title: Echelle background correction. Authors: Gehren, T.; Ponz, D. Bibcode: 1986A&A...168..386G Altcode: An improved method for the scattered light background correction of two-dimensional echelle spectra is described, and the results are compared with more approximate methods, such as the widely used interorder minimum subtraction. The new correction is applied to a CCD echelle spectrum, and the accuracy of the continuum level is demonstrated to be better than 1 percent. Title: Non-Lte Analysis of Massive Stars in the Magellanic Clouds Authors: Gehren, T.; Husfeld, D.; Kudritzki, R. P.; Conti, P. S.; Hummer, D. G. Bibcode: 1986IAUS..116..413G Altcode: The massive stars of the Magellanic Clouds are of considerable current interest with regard to questions of initial mass function, star formation mechanisms, stellar evolution with mass loss and the chemical evolution of galaxies. The effective temperatures, surface gravities and helium abundances of 6 main sequence O-type stars, obtained by fitting non-LTE model atmospheres to high quality spectra, are presented. Title: Die Rotverschiebung der Quasare. Authors: Gehren, T. Bibcode: 1985S&W....24..647G Altcode: Theories on the nature of the quasar redshift are discussed. The arguments for the redshift being due to the relativistic Doppler effect, the gravitational fields of compact masses, and the expansion of space are summarized. The surface brightness theorem is addressed, and the theoretical importance of observations of quasar galaxies is examined. The evolution of these galaxies is discussed. Title: Abundance gradients in the galactic disk from young B-type stars in clusters: first results. Authors: Gehren, T.; Nissen, P. E.; Kudritzki, R. P.; Butler, K. Bibcode: 1985ESOC...21..171G Altcode: 1985pdcn.conf..171G; 1985pdce.work..171G First results from detailed spectroscopic analyses of early B-type main sequence stars in galactic clusters and associations are presented, covering a range in galactocentric distances from about 8 to 18 kpc. Nitrogen and oxygen abundances for a subsample of 11 sharp-lined stars do not reveal a significant galactic abundance gradient, at variance with the results obtained from H II-region observations. Since the subsample spans only a very restricted stellar temperature range, the analyses have followed a strictly differential approach leading to very small error limits. Thus any explanation of the resulting abundance scatter in a single cluster or at a given galactocentric distance in terms of observational or systematic errors can probably be ruled out. Title: Photometry of Quasar Host Galaxies and Cosmological Implications Authors: Gehren, T. Bibcode: 1985LNP...232..227G Altcode: 1985nagp.meet..227G Surface photometry of sky-limited red photographic and CCD observations, corrected for galactic extinction, the K term and seeing image degradation, reveals the decomposition of low-redshift quasar images into a central point source and an extended underlying nebulosity. The investigation of the statistical properties of a well-resolved subsample of these nebulosities shows that (1) the underlying nebulosities are in fact the host galaxies of quasar nuclei, (2) quasar host galaxies show strong evidence for luminosity evolution, (3) quasar host galaxies in many cases appear to be heavily distorted by interaction with faint companion galaxies. Title: Host galaxies of quasars and their association with galaxy clusters. Authors: Gehren, T.; Fried, J.; Wehinger, P. A.; Wyckoff, S. Bibcode: 1984ApJ...278...11G Altcode: Seventeen quasars with redshifts ranging from 0.044 to 0.828 have been observed through an r filter using a CCD detector. Using a point-spread function defined by field stars exposed on the same CCD frame as the quasar, point-by-point subtraction of the quasar nucleus from the image reveals some of the structure in the nebulosity underlying essentially all quasars with redshifts ⪉0.5. Under the assumption that the quasar nuclei and the underlying nebulosities have the same cosmological redshifts, absolute magnitudes and metric diameters of the resolved structures again support the interpretation that quasars are central events in distant galaxies. The morphologies of the resolved galaxies underlying the quasars PHL 909, 3Cr 48, and 0241+622 are suggestive of tidal interactions with nearby companion galaxies. Counts of faint galaxies in the vicinity of the quasar images indicate that quasars are usually situated in groups or clusters of galaxies which are dominated by the luminosity contribution of the quasar host galaxy. Title: Host galaxies of quasars Authors: Gehren, T.; Fried, J.; Wehinger, P. A.; Wyckoff, S. Bibcode: 1983LIACo..24..489G Altcode: 1983qgl..conf..489G By analogy with other active extragalactic phenomena, a quasar is defined to consist of at least two components: a central point source and an extended underlying nebulosity, referred to as the host galaxy of a quasar. Here, some integrated properties of the host galaxies are derived from CCD surface photometry obtained for 17 low-redshift quasars at the Cassegrain focus of a 2.2-m telescope. The luminosities, diameters, and intensity profiles derived from the CCD observations are consistent with the assumption that radio quasars reside in host galaxies at cosmological distances comparable with first-ranked elliptical cluster galaxies. A less luminous type of galaxy may be associated with radio-quiet quasars. Title: Statistical properties of quasar galaxies Authors: Wyckoff, S.; Wehinger, P. A.; Gehren, T.; Fried, J.; Spinrad, H.; Tapia, S. Bibcode: 1983LIACo..24..483W Altcode: 1983qgl..conf..483W The authors present data on a statistically significant sample of quasars covering a wide range in redshifts. The results support the view that all quasars sit in galaxies. The correlations presented indicate that the nebulosities surrounding both radio-loud and radio-quiet quasars have metric diameters and intrinsic optical luminosities corresponding to galaxies at the quasar redshifts. Also the authors present evidence for significant differences in the diameters and absolute magnitudes of radio-loud and radio-quiet quasars in the sense that radio-loud quasars sit in more luminous, larger galaxies. Title: Quasar Galaxies - Two-Dimensional Image Deconvolutions Authors: Wehinger, P. A.; Wyckoff, S.; Gehren, T.; Spinrad, H. Bibcode: 1983IAUS..104...49W Altcode: No abstract at ADS Title: Quasargalaxien Authors: Fried, J. W.; Gehren, T. Bibcode: 1983MitAG..60..450F Altcode: No abstract at ADS Title: Properties of Quasar Nebulosities Authors: Wyckoff, S.; Wehinger, P. A.; Gehren, T.; Spinrad, H. Bibcode: 1982BAAS...14Q.908W Altcode: No abstract at ADS Title: Models of stellar evolution and their use in calibrating distances and element abundances of stars Authors: Gehren, T. Bibcode: 1982A&A...109..187G Altcode: Recently published grids of stellar interior models fail to account for the temperature and luminosity of the Sun with reasonable values of mass, age and chemical composition. It is shown how this discrepancy may affect the determination of element abundances and distances of field stars and clusters. Title: Extremely Metal-Poor Subdwarfs Authors: Gehren, T. Bibcode: 1982Msngr..27...22G Altcode: The formation of our Galaxy and its evolution from an extended spherical halo to a highly flattened spiral disk can be convincingly documented by observing cool dwarf stars that have remained essentially unevolved since they formed billions of years ago. Model predictions of nucleosynthesis in stars, starting from a zerometal primordial composition, combined with a theoretical outline of the galactic collapse, have led to a coarse description of the history of our Galaxy, in which the oldest stars are extremely metal-poor and have highly eccentric galactic orbits with relatively small orbital angular momenta as compared with young disk stars. Title: Characteristics of Nebulosity Associated with Parkes Quasars Authors: Wehinger, P.; Wyckoff, S.; Gehren, T. Bibcode: 1982IAUS...97..375W Altcode: No abstract at ADS Title: Resolution of quasar images. Authors: Wyckoff, S.; Wehinger, P. A.; Gehren, T. Bibcode: 1981ApJ...247..750W Altcode: Thirteen low redshift (z approximately less than 0.6) quasars have been resolved on large-scale, sky-limited photographs, and the images were analyzed by digitally removing the plate background and the point-spread function defined by images of nearby field stars, having magnitudes comparable to those of the quasars. The nebulosities around the quasars were found to have an average metric diameter of approximately 90 + or - 30 kpc and an integrated absolute magnitude of approximately -21.8 + or - 0.8. An expression is presented for the average intensity profile of the underlying nebulosity. Correlations (between the isophotal diameters of the resolved nebulosity and quasar redshifts; and the integrated apparent magnitudes of the underlying nebulosity and isophotal diameters) indicate roughly constant diameters and surface brightnesses for the nebulosities associated with the quasars. The physical and statistical properties of the nebulosities surrounding the quasars support the hypothesis that quasars are the luminous nuclei of distant galaxies. Title: The temperature scale of solar-type stars. Authors: Gehren, T. Bibcode: 1981A&A...100...97G Altcode: Effective temperatures for the atmospheres of solar-type stars have been determined by comparison of observed hydrogen lines with profiles computed from scaled solar model atmospheres. The analysis is based on more than one hundred high-dispersion spectra of 20 main sequence stars with spectral types F6 V to G5 V and calibrated directly with spectra of the sun, moon, and daylight sky. For early G-type stars the resulting spectroscopic temperature scale deviates considerably from color-temperature relations derived from synthesis of theoretical color indices using model atmospheres with statistical line distribution functions. The spectroscopic calibration yields effective temperatures for G-type stars that are about 200-300 K higher than those predicted from comparison of observed and synthesized color indices. Title: Properties of Quasar Galaxies Authors: Wehinger, P. A.; Wyckoff, S.; Gehren, T. Bibcode: 1981BAAS...13..806W Altcode: No abstract at ADS Title: Räumliche Auflösung von Quasaren Authors: Wehinger, P. A.; Gehren, T.; Wyckoff, S.; Boksenberg, A. Bibcode: 1981MitAG..52R.157W Altcode: 1981MitAG..52..157W No abstract at ADS Title: Metallarme Subdwarfs der Spektraltypen F und G Authors: Gehren, T.; Hippelein, H.; Münch, G. Bibcode: 1981MitAG..52...68G Altcode: No abstract at ADS Title: Discovery of nebulosity associated with the quasar 3C 273. Authors: Wyckoff, S.; Gehren, T.; Morton, D. C.; Albrecht, R.; Wehinger, P. A.; Boksenberg, A. Bibcode: 1980ApJ...242L..59W Altcode: The discovery of a diffuse nebulosity physically associated with the quasar 3C 273 (excluding its optical jet) is reported. The nebulosity was detected as a diffuse asymmetrical extension in the isophotes of a photographic image of the quasar obtained at the prime focus of a 3.6-m telescope which is resolved with a radial size scale of approximately 15 arcsec at a surface brightness level of 25 mag/sq arcsec. Spectra of the extended structure exhibit narrow emission lines at the redshifted positions of the forbidden O II line at 3727 A and Ne III line at 3869 A, indicating the presence of low-density hot gas at a projected distance of approximately 4.5 arcsec from the quasar. Comparison of photometric and spectroscopic data indicates the dominant emission to be continuum radiation, which may originate from an underlying galaxy associated with 3C 273. Title: Nebulosity Associated with Quasars Authors: Wyckoff, S.; Wehinger, P. A.; Gehren, T. Bibcode: 1980BAAS...12..808W Altcode: No abstract at ADS Title: IPCS Spectroscopy of the Quasar PKS 0812+020 and Its Associated Nebulosity Authors: Wehinger, P. A.; Gehren, T.; Wyckoff, S.; Boksenberg, A. Bibcode: 1980BAAS...12R.807W Altcode: No abstract at ADS Title: The Radio/Optical Structure of the QSO 0812+020 Authors: Wyckoff, S.; Johnston, K.; Ghigo, F.; Rudnick, L.; Wehinger, P.; Gehren, T.; Boksenberg, A. Bibcode: 1980BAAS...12..497W Altcode: No abstract at ADS Title: Quasars Resolved Authors: Wehinger, P. A.; Gehren, T.; Wyckoff, S. Bibcode: 1980Msngr..20R...1W Altcode: While observers have obtained spectra of more than 1,400 quasars since they were discovered in 1963, fewer than one per cent have been studied by direct imaging techniques at significantly faint surface brightness levels and high angular resolution to detect anything more than a bright point-like source. La Silla's very dark sky, excellent seeing, plus the superb optics of the 3.6-m telescope have been combined with digital analysis of sky-limited photographs to produce two-dimensional intensity contour maps and image profiles of quasars. The contour maps reveal extended structure on ascale of 5-10 are sec for 80 per cent of a sample of 20 low redshift (z = 0.1-0.5) quasars. In a significant number of cases the data also show the presence of galaxies near the QSO's, some of which have measured redshifts nearly equal to the QSO redshifts. This programme is the joint effort of Drs. Peter Wehinger and Thomas Gehren of the Max-Planck Institute for Astronomy in Heidelberg and Professor Susan Wyckoff of Arizona State University in Tempe, Arizona. Title: Deep Imagery of Quasar Fields - Techniques Authors: Wehinger, P. A.; Gehren, T.; Wyckoff, S. Bibcode: 1980tdp..conf..401W Altcode: No abstract at ADS Title: Deep Imaging of Quasar Fields Authors: Wehinger, P. A.; Wyckoff, S.; Gehren, T. Bibcode: 1980IAUS...92..133W Altcode: No abstract at ADS Title: Turbulence in main sequence stars Authors: Gehren, T. Bibcode: 1980LNP...114..103G Altcode: 1980sttu.coll..103G; 1980IAUCo..51..103G Turbulence in main sequence stars is discussed, considering micro- and macroturbulence, the horizontally homogeneous atmosphere approach, and small scale velocity fields. With regard to microturbulence and basic stellar parameters, sources of systematic errors are described, concerning abundance analyses based on absolute oscillator strengths, the problem of absolute f-values, departures from LTE, and the damping constants. Also discussed are microturbulence values from narrow band photometry, curve-of-growth analyses, model atmosphere analyses, and Fourier transform analysis. It is noted that a correlation between microturbulence and convective velocities exists, and the question of the nature of the transport mechanism is addressed. Title: BL LAC Objects: Direct Imaging Authors: Wehinger, P. A.; Wyckoff, S.; Tapia, S.; Gehren, T. Bibcode: 1979BAAS...11..693W Altcode: No abstract at ADS Title: Metal abundances and microturbulence in seven solar-type stars. II. Model atmosphere analysis. Authors: Gehren, T. Bibcode: 1979A&A....75...73G Altcode: High-dispersion spectra of seven near main sequence stars of MK spectral types F6 to G0 were investigated with differential model atmosphere analyses relative to the sun in order to obtain metal abundances and microturbulence velocities. It is shown that three of the stars hitherto assumed to be metal-rich from photometric narrow-band observations (9 Com, 14 Boo, and o Aql) are only marginally overabundant with respect to the sun, provided their microturbulence is properly taken into account. It is found that all the metal-overabundant stars examined are younger than the sun. Apart from a general metal over or under-abundance, the details of the abundance patterns are the same as in the sun. Title: On the chemical composition and age of beta Vir. Authors: Gehren, T. Bibcode: 1978A&A....65..427G Altcode: The F8V star Beta Vir is reanalyzed by means of a differential model atmosphere analysis. Taking into account the revision of Holweger's solar model and improved solar equivalent widths as measured from the Liege atlas, the final atmospheric parameters are given. A solar He/H ratio has been assumed. Evolutionary isochrones suggest a stellar age of approximately 3 billion years, in contrast to earlier investigations. Title: Metal abundances and microturbulence in seven solar-type stars: spectroscopic data Authors: Gehren, T.; Reimers, D.; Berthold, L.; Berthold, J.; Hennig, R. Bibcode: 1978A&AS...31..297G Altcode: Basic spectroscopic data for the analyses of seven solar-type stars are presented, including the computational results for individual spectral lines in the wavelength region from 4400 to 6800 A. Line strengths are compared with previous observations by other authors. Title: Model atmosphere analysis of HR 8799. Authors: Gehren, T. Bibcode: 1977A&A....59..303G Altcode: Eight high-dispersion spectrograms of the metal-deficient star HR 8799, covering the wavelength range from 3700 to 6000 A, are analyzed using a model-atmosphere program that assumes LTE but neglects line blanketing. A 'spectroscopic' model atmosphere is obtained, but a discrepancy is found between the model-atmosphere computations and photometric observations. Unsuccessful attempts are made to fit the star's mass-luminosity ratio to various evolutionary tracks; the results indicate that the observations cannot be accounted for by a single stellar object and that the present evolutionary stage of HR 8799 requires the existence of an efficient mass-transfer process. Therefore, a binary model is proposed in which the primary had a mass of approximately 0.85 solar mass at the start of core hydrogen burning, the secondary (HR 8799) had a mass of about 0.80 solar mass at the same stage, and the primary transferred about 0.45 solar mass to HR 8799 after reaching the giant stage. Title: HR 8799-ein metallarmer Doppelstern? Authors: Gehren, T. Bibcode: 1977MitAG..42..139G Altcode: No abstract at ADS Title: Über die Bahnen der regulären Satelliten im Planetensystem Authors: Unsöld, A.; Gehren, T. Bibcode: 1975NW.....62...95U Altcode: No abstract at ADS Title: Entstehung von Na I-Linien in der Sonnenatmosphäre Authors: Gehren, T. Bibcode: 1975MitAG..36..141G Altcode: No abstract at ADS Title: Kinetic equilibrium and line formation of Na I in the solar atmosphere. Authors: Gehren, T. Bibcode: 1975A&A....38..289G Altcode: Summary. The influence of deviations from local thermodynamic equilibrium (LTE) on the formation of Na I lines under solar conditions is investigated. Based on different prescribed solar model atmospheres and model atoms which include up to 9 energy levels and 14 line transitions, the line transfer problem is solved using the integral equation approach, assuming a stationary, plane-parallel and homogeneous atmosphere, and complete redistribution of line photons. The kinetic ionization equilibrium turns out to be determined through radiative interaction between highly excited levels and the continuum. The kinetic excitation equilibrium between the high-lying levels deviates from LTE already in the photosphere as a result of an outwardly increasing radiation loss in the line transitions. The excitation of the Na D lines follows the LTE predictions out to T = I0 2, where the onset of photon escape from the resonance line radiation field yields a stronger population of the ground state. Under the assumption of anisotropic microturbulence with h >= and d /dr <0 for r < I0 , the computed D line cores closely fit observation. Since the formation of the Na D lines is nearly independent of the solar chromospheric temperature, such a fit is obtained for three different solar models. There appears to be ne need for the high chromospheric electron densities suggested in previous investigations. Attempts to fit observation to the computed cores of higher excited lines have failed. A further investigation of the influence of different model parameters reveals that the kinetic ionization equilibrium and the excitation equilibrium between highly excited levels depend on the number of energy levels included in our model atom, as well as on the collision cross-sections. The equivalent widths of the Na ilines are affected by deviations from LTE only a few per cent. Key words: non-LTE kinetic equilibrium Na I lines solar atmosphere Title: Kinetic equilibrium and line formation of Na I in the solar atmosphere Authors: Gehren, Thomas Bibcode: 1974PhDT.......153G Altcode: No abstract at ADS Title: An Interpolation Method for the Solution of Non-LTE Line Transfer Problems Authors: Gehren, T. Bibcode: 1973A&A....27..291G Altcode: Summary: Iterative procedures solving non-LTE line transfer problems through the integral equation approach require the calculation of monochromatic lambda operators. We outline a simple interpolation method that avoids the repeated evaluation of exponential integrals. Based on polynomial approximation of the source function, gaussian integration weights 0cm and divisions tm have to be calculated only once from the integral representation for a set of r-points. While saving more than 3/4 of computing time, first solutions of multilevel line transfer problems indicate that the accuracy is comparable to other methods. Key words: non-LTE - line transfer - interpolation method (t) E,(t) dt = M mZ=i 0cmf(tm)