Extreme helium stars: non-LTE matters Helium and hydrogen spectra of the unique objects V652 Her and HD144941

TL;DR

This paper demonstrates that applying advanced non-LTE line-formation models resolves longstanding discrepancies in spectral analyses of extreme helium stars, leading to more accurate atmospheric parameters and hydrogen abundances.

Contribution

The study introduces state-of-the-art non-LTE modeling for helium, significantly improving spectral analysis accuracy of extreme helium stars compared to previous LTE and older non-LTE methods.

Findings

Non-LTE models reconcile helium line discrepancies.

Hydrogen abundances are systematically lower than earlier estimates.

Non-LTE effects are crucial for accurate spectral analysis of these stars.

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 HD144941. 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 HeI 10830A transition in particular, in error by up to a factor ~3.