Related papers: Improved helium line formation for EHe stars
Quantitative analyses of low-mass hydrogen-deficient (super-)giant stars - so-called extreme helium stars - to date face two major difficulties. First, theory fails to reproduce the observed helium lines in their entirety, wings and line…
The near-IR HeI 10830A transition is a highly sensitive diagnostic for non-LTE effects in the helium atom. So far, non-LTE line-formation computations have failed to quantitatively reproduce observations of this line in the entire range of…
Optical high-resolution spectra of V652 Her and HD 144941, the two extreme helium stars with exceptionally low C/He ratios, have been subjected to a non-LTE abundance analysis using the tools TLUSTY and SYNSPEC. Defining atmospheric…
Two evolutionary scenarios are proposed for the formation of extreme helium stars: a post-AGB star suffering from a late thermal pulse, or the merger of two white dwarfs. An identification of the evolutionary channel for individual objects…
Aims: Hydrogen and helium line spectra are crucial diagnostic features for the quantitative analysis of OB stars. We compute synthetic spectra based on a hybrid non-LTE approach in order to test the ability of these models to reproduce…
Extreme helium stars (EHe stars) are hydrogen-deficient supergiants of spectral type A and B. They are believed to result from mergers in double degenerate systems. In this paper we present a detailed quantitative non-LTE spectral analysis…
The lithium abundances in metal-poor halo stars are of importance for cosmology, galaxy evolution and stellar structure. In an attempt to study possible systematic errors in the derived Li abundances, the line formation of LiI lines has…
Stellar parameters and abundances of Na, Mg, Al, K, Ca, Sr, Ba, and Eu are determined for four very metal-poor stars (-2.66 < [Fe/H] < -2.15) based on non-LTE line formation and analysis of high-resolution (R ~60000 and 90000) high…
We report on preliminary results of a hybrid non-LTE analysis of high-resolution, high-S/N spectra of the helium-rich subdwarf B star Feige49 and the helium-poor sdB HD205805. Non-LTE effects are found to have a notable impact on the…
Since its discovery about 50 years ago, HD144941 has generally been classified as a peculiar member of the extreme helium (EHe) supergiant stars, a very rare class of low-mass hydrogen-deficient stars. We report the detection of a strong…
In this paper we report on the real nature of the star HD 145792, classified as He weak in {\it ``The General Catalogue of Ap and Am stars''}. By means of [email protected] high resolution spectroscopic data, we refined the atmospheric…
A non-LTE (NLTE) abundance analysis was carried out for three extreme helium stars (EHes): BD+10 2179, BD-9 4395, and LS IV+6 002, from their optical spectra with NLTE model atmospheres. NLTE TLUSTY model atmospheres were computed with H,…
Heavy-elements, i.e. those beyond the iron peak, mostly form via two neutron capture processes: the s- and r-process. Metal-poor stars should contain fewer isotopes that form via the s-process, according to currently accepted theory. It has…
Neutral sodium is a minority species in the atmospheres of late-type stars, and line formation in local thermodynamic equilibrium (LTE) is often a poor assumption, in particular for strong lines. We present an extensive grid of non-LTE…
Non-local thermodynamical equilibrium (non-LTE) line formation for neutral and singly-ionized iron is considered through a range of spectral types when the effective temperature varies from 6500 K up to 8500 K, the gravity from log g = 4…
A comprehensive model atom was developed for Si I-II-III using the most up-to-date atomic data available so far. Based on the non-local thermodynamic equilibrium (non-LTE = NLTE) line formation for Si I, Si II, and Si III and…
We provide measurements of the Ba isotopic fractions for five metal-poor stars derived with an LTE analysis using 1D model stellar atmospheres. We use high resolution (R\equiv{\lambda}/\Delta{\lambda}=90000-95000), very high signal-to-noise…
Iron plays a crucial role in studies of late-type stars. In their atmospheres, Fe I is the minority species and its lines are subject to the departures from LTE. In contrast, one believes that LTE is a realistic approximation for Fe II…
Several B-type main-sequence stars show chemical peculiarities. A particularly striking class are the $^3$He stars, which exhibit a remarkable enrichment of $^3$He with respect to $^4$He. This isotopic anomaly has also been found in blue…
We report new statistical equilibrium calculations for Fe I and Fe II in the atmosphere of Late-Type stars. We used atomic models for Fe I and Fe II having respectively 256 and 190 levels, as well as 2117 and 3443 radiative transitions.…