Astronomy and Astrophysics, volume 443, L25-28 (2005/12-1)
Extreme helium stars: non-LTE matters. Helium and hydrogen spectra of the unique objects V652 Her and HD 144941.
PRZYBILLA N., BUTLER K., HEBER U. and JEFFERY C.S.
Abstract (from CDS):
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 HeI10830Å transition in particular, in error by up to a factor ∼3.
line: formation - stars: atmospheres - stars: fundamental parameters - stars: individual: V652 Her, HD 144941