Extreme helium stars: non-LTE matters

Abstract

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 cores. Second, a general mismatch exists for effective temperatures derived from ionization equilibria and from spectral energy distributions. Here, we demonstrate how the issue can be resolved using state-of-the-art non-LTE line-formation for these chemically peculiar objects. Two unique high-gravity B-type objects are discussed in detail, the pulsating variable V652 Her and the metal-poor star HD 144941. In the first case atmospheric parameters from published LTE analyses are largely recovered, in the other a systematic offset is found. Hydrogen abundances are systematically smaller than previously reported, by up to a factor ∼2. Extreme helium stars turn out to be important testbeds for non-LTE model atoms for helium. Improved non-LTE computations show that analyses assuming LTE or based on older non-LTE model atoms can predict equivalent widths, for the He  10 830 Å transition in particular, in error by up to a factor ∼3.

Non-local Thermodynamic Equilibrium Abundance Analyses of the Extreme Helium Stars V652 Her and HD 144941

“…The Image Reduction and Analysis Facility (IRAF) software package was used to reduce these recorded spectra. HD 144941's spectrum obtained from Anglo-Australian Telescope (AAT), and analysed by Przybilla et al (2005), was made available by Simon Jeffery (private communication) for comparison.…”

Subaru and Swift observations of V652 Herculis: resolving the photospheric pulsation★

“…Other abundances in the model are nC = 0.00005, nN = 0.0014, nO = 0.0001, nNe = 0.00026, nSi = 0.000076, and nFe = 0.00003. Given that the atmosphere is certainly not in hydrostatic equilibrium and some ions are probably not in LTE (Przybilla et al 2005), the model is used primarily for line identification and to approximate the atmospheric properties. Greater consistency in the wings of Stark broadened helium lines, for example, would be desirable.…”

“…Analysis of the spectrum shows a surface that is mostly normal in metal abundances, except for carbon and oxygen, which are weak, and nitrogen, which is sufficiently overabundant to demonstrate that the surface helium is the product of nearly complete CNO burning (Jeffery et al 1999). The 0.5 per cent contamination by hydrogen represents a potential puzzle (Przybilla et al 2005). …”

On the 3He anomaly in hot subdwarf B stars

“…The 3 He model atom was constructed in analogy to that of 4 He by Przybilla (2005), whereby isotopic line shifts as measured by Fred et al (1951) were taken into account. The hybrid LTE/NLTE approach used has only been applied to hot subdwarf B stars before by Przybilla et al (2005);Geier et al (2007) and Latour et al (2016). In the framework of this work, it is indispensable to perform the analysis in NLTE since the most important signature for 3 He enrichment (He 6678 Å) is strongly affected by NLTE effects (see Figure 1).…”