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ADS astronomy entries on 2022-09-14
author:"Gehren, Thomas"
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Title: Statistical Equilibrium of Copper in the Solar Atmosphere
Authors: Shi, J. R.; Gehren, T.; Zeng, J. L.; Mashonkina, L.; Zhao, G.
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
<SUB>exc</SUB> <= 1.64 eV, give a 0.14 dex lower mean solar abundance
compared to that from the six E <SUB>exc</SUB> > 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<SUB>⊙</SUB>(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 <SUB>exc</SUB>
= 1.39-1.64 eV and E <SUB>exc</SUB> > 3.7 eV lines can be removed
when the calculated gf-values are adopted and a mean solar abundance
of log ɛ<SUB>⊙</SUB>(Cu) = 4.24 ± 0.08 is derived.
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Title: Statistical equilibrium of silicon in the atmospheres of
cool stars
Authors: Shi, J. R.; Gehren, T.; Mashonkina, L.; Zhao, G.
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.
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Title: Scandium Abundance in Metal-poor Stars
Authors: Zhang, H. W.; Gehren, T.; Zhao, G.
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.
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Title: Statistical equilibrium of silicon in the atmospheres of
nearby metal-poor stars
Authors: Shi, J. R.; Gehren, T.; Zhao, G.
2011A&A...534A.103S Altcode:
<BR /> 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. <BR /> 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. <BR /> 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. <BR /> 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. <P />Based on
observations collected at the German-Spanish Astronomical Center,
Calar Alto, Spain.
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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.
2011A&A...528A..87M Altcode: 2011arXiv1101.4570M
<BR /> 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. <BR /> 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. <BR /> 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. <P />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 <A href="http://www.aanda.org">http://www.aanda.org</A>
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Title: Fe-peak element abundances in disk and halo stars
Authors: Bergemann, Maria; Gehren, Thomas
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.
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Title: Fe I/Fe II ionization equilibrium in cool stars: NLTE
versus LTE
Authors: Mashonkina, Lyudmila; Gehren, Thomas; Shi, Jianrong; Korn,
Andreas; Grupp, Frank
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.
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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
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. <P />Based on observations collected at the
European Southern Observatory, Chile, 67.D-0086A, and the Calar Alto
Observatory, Spain. <P />E-mail: mbergema@mpa-garching.mpg.de (MB);
j.pickering@imperial.ac.uk (JCP); gehren@usm.lmu.de (TG)
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Title: Statistical equilibrium of silicon in the atmospheres of
metal-poor stars
Authors: Shi, J. R.; Gehren, T.; Mashonkina, L.; Zhao, G.
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. <BR />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. <BR
/>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. <BR />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. <P
/>Based on observations obtained in the frame of the ESO programme
ID 165.N-0276(A).
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Title: NLTE abundances of Mn in a sample of metal-poor stars
Authors: Bergemann, M.; Gehren, T.
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. <BR />Methods: The spectrum synthesis and Mn abundances
are based on statistical equilibrium calculations using various
estimates for the influence of hydrogen collisions. <BR />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. <BR />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. <P />Based
on observations collected at the European Southern Observatory, Chile,
67.D-0086A, and the Calar Alto Observatory, Spain.
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Title: Solar scandium abundance
Authors: Zhang, H. W.; Gehren, T.; Zhao, G.
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<SUB>⊙</SUB> = 3.07 and 3.13, depending on the choice
of f values.
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Title: Statistical equilibrium of silicon in the solar atmosphere
Authors: Shi, J. R.; Gehren, T.; Butler, K.; Mashonkina, L. I.;
Zhao, G.
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. <BR />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. <BR />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. <BR />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. <P />Table 1 is only available in electronic form
at http://www.aanda.org
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Title: A non-local thermodynamic equilibrum study of scandium in
the Sun
Authors: Zhang, H. W.; Gehren, T.; Zhao, G.
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<SUB>⊙</SUB>(Sc) values for scandium
lines, which will later be used in differential abundance analyses of
metal-poor stars. <BR />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. <BR />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.
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Title: NLTE study of scandium in the Sun
Authors: Zhang, H. W.; Gehren, T.; Zhao, G.
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.
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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.
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. <BR
/>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. <BR />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. <P />Based on observations collected at Subaru Telescope,
which is operated by the National Astronomical Observatory of Japan.
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Title: Formation of Mn I lines in the solar atmosphere
Authors: Bergemann, M.; Gehren, T.
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. <BR />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. <BR />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\varepsilon<SUB>Mn,
⊙</SUB>= 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. <BR
/>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. <BR />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. <P
/>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
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Title: Lithium abundances in metal-poor stars
Authors: Shi, J. R.; Gehren, T.; Zhang, H. W.; Zeng, J. L.; Zhao, G.
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. <BR />Methods: All abundance results are derived
from NLTE statistical equilibrium calculations and spectrum synthesis
methods. <BR />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). <BR />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. <P
/>Based on observations collected at the Germany-Spanish Astronomical
Center, Calar Alto, Spain.
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Title: Potassium abundances in nearby metal-poor stars
Authors: Zhang, H. W.; Gehren, T.; Butler, K.; Shi, J. R.; Zhao, G.
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.<BR /> Methods: .All abundance
results have been derived from NLTE statistical equilibrium calculations
and spectrum synthesis methods.<BR /> 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. <BR /> 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.
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Title: NLTE analysis of the solar potassium abundance
Authors: Zhang, H. W.; Butler, K.; Gehren, T.; Shi, J. R.; Zhao, G.
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 S<SUB>H</SUB> = 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.
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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.
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. <BR />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. <BR />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. <BR />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.
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Title: VizieR Online Data Catalog: Sodium abundances in nearby disk
stars (Shi+, 2004)
Authors: Shi, J. R.; Gehren, T.; Zhao, G.
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. <P />(1 data file).
---------------------------------------------------------
Title: Sodium abundances in nearby disk stars
Authors: Shi, J. R.; Gehren, T.; Zhao, G.
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. <P
/>Based on observations collected at the Germany-Spanish Astronomical
Center, Calar Alto, Spain. <P />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.
2004A&A...413.1045G Altcode:
To determine the population membership of nearby stars we explore
abundance results obtained for the light neutron-rich elements
<SUP>23</SUP>Na and <SUP>27</SUP> 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. <P
/>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. <P />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.
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 S<SUB>H</SUB>, we find deviations from
LTE in Fe I ranging from 0.00 (S<SUB>H</SUB>,= infty ) to 0.46 dex
(S<SUB>H</SUB>,= 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 S<SUB>H</SUB>,= 3. This
value is much higher than what is found for simple atoms like Li or Ca,
both from laboratory measurements and inference of stellar analyses.\
The correct choice of collisional damping parameters (“van-der-Waals”
constants) is found to be generally more important for these little
evolved metal-poor stars than considering departures from LTE. For
the Sun the calibrated value for S<SUB>H</SUB> 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. <P />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.
2003IAUS..210P.E39M Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Iron in Non-LTE - Pitfalls and Prospects
Authors: Korn, A. J.; Gehren, T.
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.
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.
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\varepsilon<SUB>FeI,solar</SUB>
= 7.57 for the empirical and \log\varepsilon<SUB>FeI,solar</SUB> =
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\varepsilon<SUB>FeI,solar</SUB> ~ 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 < S<SUB>H</SUB> < 5 times the standard value to compensate
for the large photoionization cross-sections, (c) a microturbulence
velocity ξ<SUB>t</SUB> = 1.0 km s<SUP>-1</SUP>, (d) van der Waals
damping parameters decreased by Δ\log C<SUB>6</SUB> = -0.10 \ldots
-0.15 as compared to Anstee & O'Mara's calculations, depending on
S<SUB>H</SUB>, (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.
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.
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.
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 T<SUB>eff</SUB>) 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.
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.
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.
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.
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]<SUB>NLTE-LTE</SUB>
~ 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]<SUB>NLTE-LTE</SUB> increases with
effective temperature between 5200 and 6500 K. For extremely metal-poor
stars the abundance corrections approach Delta [Mg/H]<SUB>NLTE-LTE</SUB>
~ 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.
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
2000fist.conf...75K Altcode:
We report on the methodology of an ongoing project to determine accurate
stellar parameters (T<SUB> eff </SUB>, 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.
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.
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
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, T<SUB>eff</SUB> = 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.
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.
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.
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.
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.
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.
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
T<SUB>eff</SUB>, surface gravity log g, microturbulence xi_t and
the abundances of Fe, Mg and Si with high internal accuracy (Delta
T<SUB>eff</SUB> =~ 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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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
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.
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.
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.
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 H<SUB>p</SUB>
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) H<SUB>p</SUB>, 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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
1990AGAb....5...61K Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Model atmosphere analyses of halo dwarfs.
Authors: Axer, M.; Fuhrmann, K.; Gehren, T.; Reetz, J.
1990AGAb....5...70A Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Astronomical Information Obtained from Echelle Spectra
Authors: Gehren, T.
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.
1990nuas.symp..137G Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The initial mass function in early stages of Galactic
evolution.
Authors: Gehren, T.
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.
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.
1989AGAb....3..110Z Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Metal-poor subdwarfs and early galactic nucleosynthesis.
Authors: Hartmann, K.; Gehren, T.
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.
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
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.
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.
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.
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.
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.
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.
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.
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.
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.
1983IAUS..104...49W Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Quasargalaxien
Authors: Fried, J. W.; Gehren, T.
1983MitAG..60..450F Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Properties of Quasar Nebulosities
Authors: Wyckoff, S.; Wehinger, P. A.; Gehren, T.; Spinrad, H.
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.
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.
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.
1982IAUS...97..375W Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Resolution of quasar images.
Authors: Wyckoff, S.; Wehinger, P. A.; Gehren, T.
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.
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.
1981BAAS...13..806W Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Räumliche Auflösung von Quasaren
Authors: Wehinger, P. A.; Gehren, T.; Wyckoff, S.; Boksenberg, A.
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.
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.
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.
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.
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.
1980BAAS...12..497W Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Quasars Resolved
Authors: Wehinger, P. A.; Gehren, T.; Wyckoff, S.
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.
1980tdp..conf..401W Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Deep Imaging of Quasar Fields
Authors: Wehinger, P. A.; Wyckoff, S.; Gehren, T.
1980IAUS...92..133W Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Turbulence in main sequence stars
Authors: Gehren, T.
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.
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.
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.
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.
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.
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.
1977MitAG..42..139G Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Über die Bahnen der regulären Satelliten im Planetensystem
Authors: Unsöld, A.; Gehren, T.
1975NW.....62...95U Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Entstehung von Na I-Linien in der Sonnenatmosphäre
Authors: Gehren, T.
1975MitAG..36..141G Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Kinetic equilibrium and line formation of Na I in the solar
atmosphere.
Authors: Gehren, T.
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
1974PhDT.......153G Altcode:
No abstract at ADS
---------------------------------------------------------
Title: An Interpolation Method for the Solution of Non-LTE Line
Transfer Problems
Authors: Gehren, T.
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)
