The origins of hot subdwarf stars.

High signal-to-noise optical spectrophotometry of a sample of field subluminous B stars drawn largely from the Palomar Green ultraviolet excess survey is analyzed with a new grid of model atmospheres and synthetic spectra. The stellar effective temperatures, surface gravities, and photospheric helium abundances are determined simultaneously from a detailed analysis of hydrogen and helium absorption line profiles. The derived temperatures and gravities place the subluminous B stars in the theoretical H-R diagram along and bounded below by theoretical sequences of the zero-age extended horizontal branch, lending strong support to the hypothesis that these stars are composed of helium-burning cores of ∼0.5 $M(⊙) overlain by very thin layers of hydrogen (≲0.02 M(⊙)). Various scenarios for their past evolutionary history are examined in the context of their probable future evolution into white dwarfs of lower than average mass. The derived distances above the Galactic plane support a scale height for the population of z₀ = 285 pc, consistent with the identification of their progenitor stars as members of the old disk population. Radial velocities of sdB and sdO stars are analyzed to infer their kinematic characteristics. The results for the sdB stars are inconclusive, but for the sdO stars the results also are consistent with the population belonging to the older part of the thin disk