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

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

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

Unravelling the baffling mystery of the ultrahot wind phenomenon in white dwarfs

The presence of ultra-high excitation (UHE) absorption lines (e.g., O VIII) in the optical spectra of several of the hottest white dwarfs poses a decades-long mystery and is something that has never been observed in any other astrophysical object. The occurrence of such features requires a dense environment with temperatures near $10^6$K, by far exceeding the stellar effective temperature. Here we report the discovery of a new hot wind white dwarf, GALEXJ014636.8+323615. Astonishingly, we found for the first time rapid changes of the equivalent widths of the UHE features, which are correlated to the rotational period of the star ($P=0.242035$d). We explain this with the presence of a wind-fed circumstellar magnetosphere in which magnetically confined wind shocks heat up the material to the high temperatures required for the creation of the UHE lines. The photometric and spectroscopic variability of GALEXJ014636.8+323615 can then be understood as consequence of the obliquity of the magnetic axis with respect to the rotation axis of the white dwarf. This is the first time a wind-fed circumstellar magnetosphere around an apparently isolated white dwarf has been discovered and finally offers a plausible explanation of the ultra hot wind phenomenon.Comment: Published in MNRAS Letter

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

Quantitative Spectroscopy of Blue Supergiants in Metal-Poor Dwarf Galaxy NGC 3109

We present a quantitative analysis of the low-resolution (4.5 A) spectra of 12 late-B and early-A blue supergiants (BSGs) in the metal-poor dwarf galaxy NGC 3109. A modified method of analysis is presented which does not require use of the Balmer jump as an independent temperature indicator, as used in previous studies. We determine stellar effective temperatures, gravities, metallicities, reddening, and luminosities, and combine our sample with the early-B type BSGs analyzed by Evans et al. (2007) to derive the distance to NGC 3109 using the Flux-weighted Gravity-Luminosity Relation (FGLR). Using primarily Fe-group elements, we find an average metallicity of [Z] = -0.67 +/- 0.13, and no evidence of a metallicity gradient in the galaxy. Our metallicities are higher than those found by Evans et al. (2007) based on the oxygen abundances of early-B supergiants ([Z] = -0.93 +/- 0.07), suggesting a low alpha/Fe ratio for the galaxy. We adjust the position of NGC 3109 on the BSG-determined galaxy mass-metallicity relation accordingly and compare it to metallicity studies of HII regions in star-forming galaxies. We derive an FGLR distance modulus of 25.55 +/- 0.09 (1.27 Mpc) that compares well with Cepheid and tip of the red giant branch (TRGB) distances. The FGLR itself is consistent with those found in other galaxies, demonstrating the reliability of this method as a measure of extragalactic distances.Comment: 50 pages, 23 figures; Accepted for publication in The Astrophysical Journa

The Araucaria Project: the Local Group Galaxy WLM--Distance and metallicity from quantitative spectroscopy of blue Supergiants

The quantitative analysis of low resolution spectra of A and B supergiants is used to determine a distance modulus of 24.99 +/- 0.10 mag (995 +/- 46 Kpc) to the Local Group galaxy WLM. The analysis yields stellar effective temperatures and gravities, which provide a distance through the Flux weighted Gravity--Luminosity Relationship (FGLR). Our distance is 0.07 mag larger than the most recent results based on Cepheids and the tip of the RGB. This difference is within the 1-sigma overlap of the typical uncertainties quoted in these photometric investigations. In addition, non-LTE spectral synthesis of the rich metal line spectra (mostly iron, chromium and titanium) of the A supergiants is carried out, which allows the determination of stellar metallicities. An average metallicity of -0.87 +/- 0.06 dex with respect to solar metallicity is found.Comment: 46 pages, 14 figures (2 low resolution color figures). Accepted for publication by Ap