The atmosphere of sublumious B stars.

Abstract

The spectra of eight high galactic latitude B stars, suspected to be subluminous, are analyzed for effective temperature, gravity, and helium to hydrogen ratio. Model atmospheres include (LTE) UV line blanketing (for T/eff/ less than or equal to 30,000 K) and NLTE effects (for T/eff/ greater than 30,000 K). Among the eight confirmed subdwarfs, six have temperatures around 26,000 K. Two objects are considerably hotter (34,000 K and 41,000 K) and belong to the sdOB subclass. Gravity ranges from log g = 5.0 to 5.5 for the sdB's, the sdOB's having log g = 5.7 and 6.0, respectively. Helium is strongly depleted in the sdB's and sdOB's: from He I 4471 (if present) the number fraction obtained is 0.0016 n(He)/n(H) = 0.0016-0.016. From the g, T(eff)-diagram it is concluded that the subluminous B stars and sdOB stars are (generalized) helium main sequence stars with approximately 0.5 solar mass. The helium deficiency of the envelope is brought about by diffusion.