Summary: Astronomy & Astrophysics manuscript no. Hg223 October 3, 2005
(DOI: will be inserted by hand later)
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
Dr. Remeis-Sternwarte Bamberg, Sternwartstrasse 7, D-96049 Bamberg, Germany
Universit¨atssternwarte M¨unchen, Scheinerstrasse 1, D-81679 M¨unchen, Germany
Armagh Observatory, College Hill, Armagh BT61 9DG, Northern Ireland
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 distri-
butions. 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