ATMOSPHERIC PARAMETERS OF FIELD SUBDWARF B STARS

High signal-to-noise ratio optical spectrophotometry of a sample of fi eld subluminous B stars drawn largely from the Palomar Green ultraviol et-excess survey is analyzed with a new grid of model atmospheres and synthetic spectra. The stellar effective temperatures, surface graviti es, and photospheric helium abundances are determined simultaneously f rom a detailed analysis of hydrogen and helium absorption fine profile s. The derived temperatures and gravities place the subluminous B star s in the theoretical H-R diagram along and bounded below by theoretica l sequences of the zero-age extended horizontal branch, lending strong support to the hypothesis that these stars are composed of helium-bur ning cores of approximately 0.5 M., overlain by very thin layers of hy drogen (less-than-or-similar-to 0.02 M.). Various scenarios for their past evolutionary history are examined in the context of their probabl e future evolution into white dwarfs of lower than average mass.