Letter to the Editor

Extreme helium stars: non-LTE matters

Helium and hydrogen spectra of the unique objects V652 Her and HD 144941

¹ Dr. Remeis-Sternwarte Bamberg, Sternwartstrasse 7, 96049 Bamberg, Germany e-mail: przybilla@sternwarte.uni-erlangen.de
² Universitätssternwarte München, Scheinerstrasse 1, 81679 München, Germany
³ Armagh Observatory, College Hill, Armagh BT61 9DG, Northern Ireland

Received: 22 July 2005
Accepted: 1 October 2005

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 10 830 Å transition in particular, in error by up to a factor ~3.