The atmosphere of subluminous B stars. II. Analysis of 10 helium poor subdwarfs and the birthrate of sdB stars.

Abstract

Ultraviolet and visual spectra of 10 helium-poor hot subluminous subdwarfs are analyzed, and the birthrate of these stars is determined. Effective temperatures range from 25,000 - 75,000 K in the group, which includes three binaries. sdB's occupy a narrow range in gravities and photospheric helium content in contrast to the sdOB ranges of gravities between 5.0 and 6.3 and helium contents from 0.07 to 6 percent. The helium deficiency occurs by diffusion, suggesting that the sdB mass loss rate is twice as small as the sdO mass loss rate. Data suggest that these stars form a single evolutionary group of extended horizon branch (EHB) stars which behave like helium main sequence stars with masses of approximately 0.5 solar mass. The completed survey of EHB stars reveals a birthrate of 2 x 10 to the 14th stars/cu pc/yr, and that their evolution directly into white dwarfs represents only 2 percent of this evolutionary channel; though they contribute significantly to the lowest mass tail of the white dwarf mass distribution.