451 research outputs found
Testing common classical LTE and NLTE model atmosphere and line-formation codes for quantitative spectroscopy of early-type stars
It is generally accepted that the atmospheres of cool/lukewarm stars of
spectral types A and later are described well by LTE model atmospheres, while
the O-type stars require a detailed treatment of NLTE effects. Here model
atmosphere structures, spectral energy distributions and synthetic spectra
computed with ATLAS9/SYNTHE and TLUSTY/SYNSPEC, and results from a hybrid
method combining LTE atmospheres and NLTE line-formation with DETAIL/SURFACE
are compared. Their ability to reproduce observations for effective
temperatures between 15000 and 35000 K are verified. Strengths and weaknesses
of the different approaches are identified. Recommendations are made as to how
to improve the models in order to derive unbiased stellar parameters and
chemical abundances in future applications, with special emphasis on Gaia
science.Comment: 12 pages, 8 figures; accepted for publication in Journal of Physics:
Conference Series, GREAT-ESF Workshop: Stellar Atmospheres in the Gaia Er
Contact-allergy time
The most commonly used techniques for the in vivo evaluation of the cellular
immune response include intracutaneous testing with microbial recall antigens
or sensitization with neoantigens. The reliability of these tests for the individual
patient usually is low due to the lack of standardization and quantification.
Moreover only the efferent branch of the immune response can be judged.
The dinitrochlorobenzene-contact allergy time (DNCB-CAT) is a quantitative
approach for the assessment of the cellular immune response. 2% DNCBointment
is applied on the upper arm in a 1 cm2 area. On the following days
patch-testing with 0.05% DNCB-ointment is done on the homolateral forearm
in alternating localizations till an allergic contact dermatitis reaction appears.
As assessed in patients with malignant melanoma (MM, n=\\5) and with
lymphoproliferative disorders (LD, η = 25), the DNCB-CAT correlates with
the age of the patients and can be expressed by a formula given by the age
(years) χ factor (MM = 0.16; LD = 0.17) + constant figure (MM = 5.5;
LD = 4.3). There was no significant difference between the two groups or subgroups
investigated.
By DNCB-CAT quantitative analysis of the cellular immune response in
vivo is possible. It is an appropriate model for further investigations of the
cellular immunity under different clinical, histological, prognostic, and therapeutic
aspects
Statistical equilibrium equations for trace elements in stellar atmospheres
The conditions of thermodynamic equilibrium, local thermodynamic equilibrium,
and statistical equilibrium are discussed in detail. The equations of
statistical equilibrium and the supplementary equations are shown together with
the expressions for radiative and collisional rates with the emphasize on the
solution for trace elements.Comment: presented at the workshop held in Nice, France, 30.7.-4.8.2007, to
appear in Non-LTE Line Formation for Trace Elements in Stellar Atmospheres,
R. Monier et al. eds., EAS Publ.Se
C II abundances in early-type stars: solution to a notorious non-LTE problem
We address a long-standing discrepancy between non-LTE analyses of the
prominent C II 4267 and 6578/82 A multiplets in early-type stars. A
comprehensive non-LTE model atom of C II is constructed based on critically
selected atomic data. This model atom is used for an abundance study of six
apparently slow-rotating main-sequence and giant early B-type stars.
High-resolution and high-S/N spectra allow us to derive highly consistent
abundances not only from the classical features but also from up to 18 further
C II lines in the visual - including two so far unreported emission features
equally well reproduced in non-LTE. These results require the stellar
atmospheric parameters to be determined with care. A homogeneous (slightly)
sub-solar present-day carbon abundance from young stars in the solar vicinity
(in associations and in the field) of log C/H +12= 8.29+/-0.03 is indicated.Comment: 8 pages, 5 figure
A non-LTE study of neutral and singly-ionized calcium in late-type stars
Non-local thermodynamical equilibrium (NLTE) line formation for neutral and singly-ionized calcium is considered through a range of spectral types when the Ca abundance varies from the solar value down to [Ca/H] = -5. Departures from LTE significantly affect the profiles of Ca I lines over the whole range of stellar parameters considered. However, at [Ca/H] >= -2, NLTE abundance correction of individual lines may be small in absolute value due to the different influence of NLTE effects on line wings and the line core. At lower Ca abundances, NLTE leads to systematically depleted total absorption in the line and positive abundance corrections, exceeding +0.5 dex for Ca I 4226 at [Ca/H] = -4.9. In contrast, NLTE effects strengthen the Ca II lines and lead to negative abundance corrections. NLTE corrections are small, <= 0.02 dex, for the Ca II resonance lines. For the IR lines of multiplet 3d - 4p, they grow in absolute value with decreasing Ca abundance exceeding 0.4 dex in metal-poor stars with [Fe/H] <= -3. Ca abundances are determined for the Sun, Procyon, and seven metal-poor stars, using high S/N and high-resolution spectra at visual and near-IR wavelengths. Lines of Ca I and Ca II give consistent abundances for all objects (except Procyon) when collisions with hydrogen atoms are taken into account. The derived absolute solar Ca abundance (from Ca I and Ca II lines) is \eps{Ca,\odot} = 6.38+-0.01. For Procyon, the mean Ca abundance from Ca I lines is markedly subsolar, [Ca/H] = -0.14+-0.03. The W(Ca I 4226)/W(Ca II 8498) equivalent width ratio is predicted to be sensitive to surface gravity for extremely metal-poor stars, while this is not the case for the ratio involving the Ca II resonance line(s)
A Direct Stellar Metallicity Determination in the Disk of the Maser Galaxy NGC4258
We present the first direct determination of a stellar metallicity in the
spiral galaxy NGC4258 (D=7.6 Mpc) based on the quantitative analysis of a
low-resolution (~5 AE) Keck LRIS spectrum of a blue supergiant star located in
its disk. A determination of stellar metallicity in this galaxy is important
for the absolute calibration of the Cepheid Period-Luminosity relation as an
anchor for the extragalactic distance scale and for a better characterization
of its dependence as a function of abundance. We find a value 0.2 dex lower
than solar metallicity at a galactocentric distance of 8.7 kpc, in agreement
with recent HII region studies using the weak forbidden auroral oxygen line at
4363 AE. We determine the effective stellar temperature, gravity, luminosity
and line-of-sight extinction of the blue supergiant being studied. We show that
it fits well on the flux-weighted gravity--luminosity relation (FGLR),
strengthening the potential of this method as a new extragalactic distance
indicator.Comment: Accepted for publication by ApJ Letters, 5 pages, 5 figure
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