Astronomy & Astrophysics

Free access article

Issue		A&A Volume 443, Number 3, December I 2005
Page(s)		L25 - L28
Section		Letters
DOI		10.1051/0004-6361:200500195

A&A 443, L25-L28 (2005)
DOI: 10.1051/0004-6361:200500195

Letter

Extreme helium stars: non-LTE matters

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

N. Przybilla¹, K. Butler², U. Heber¹ and C. S. Jeffery³

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 $\ion{i}$ 10 830 Å transition in particular, in error by up to a factor ~3.

Key words: line: formation -- stars: atmospheres -- stars: fundamental parameters -- stars: individual: V652 Her, HD 144941

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