Recent theoretical progress in modelling H-deficient hot subdwarfs has
had a greater influence on temperature determinations than normal OB stars
since the continuum opacity, dominated by hydrogen in normal stars, is
severely reduced in helium stars. Therefore the line opacity is much more
effective in blocking and redistributing radiation, so the treatment of
line-blanketing is critical (e.g. Haas et al. 1996).
In the case of He-rich sdO stars,
Dreizler (1993) has used ionization equilibrium techniques with line-blanketed
non-LTE models, revealing 44.5kK for LSIV+10
9,
substantially lower than 65kK obtained from UV continuum methods
(Schönberner & Drilling 1984). Temperatures of sdO stars
using blanketed models (Dreizler et al. 1990)
are systematically
5-10kK lower than unblanketed results (Thejll et al. 1994).
Temperature determinations in He-poor sdB
stars
use Balmer line profile fits to blanketed LTE model atmospheres
(e.g. Saffer et al. 1994) and are in good agreement with results
from suitable
Strömgren photometric calibrations (e.g. Bergeron et al. 1984).
As with sdO stars, temperatures of H-deficient extreme helium
(EHe) stars obtained from line-blanketed analyses are systematically lower
(
2000K) than unblanketed studies (e.g. Jeffery & Heber 1992).