ANALYSIS OF HELIUM-RICH SUBDWARF-O STARS .1. NLTE MODELS, METHODS, AND FITS FOR 21 PALOMAR GREEN SURVEY SDOS

Atmospheric parameters for 21 helium-rich hot subdwarf O stars from th e Palomar Green survey are found from fits of NLTE models to optical s pectra. About 250 new NLTE models in the parameter range T(eff) from 3 5,000 to 65,000 K, log (g) from 4.0 to 6.5, and epsilon(He) from 50% H e to 99% He have been calculated. A fit for each object is presented. Estimated distances and luminosities are calculated, assuming a mass o f 0.5 M.. Large distances above the Galactic plane are found, and this implies that the majority of the sdO stars belong to a different stel lar population than the sdB, planetary nebulae and white dwarf stars. Possibilities may include the halo and a '' thick disk.'' Kinematical data from the literature are also discussed in view of the found dista nces. Five of the stars have been observed, modeled, and analyzed by D reizler et al., and significant differences exist between their result s and ours for these stars. The reason for these differences is not ye t known, but the problem does not alter the conclusions about populati on membership that we present.