Abstract
High signal-to-noise optical spectrophotometry of a sample of field subluminous B stars drawn largely from the Palomar Green UV excess survey is analyzed with a new grid of model atmospheres and synthetic spectra. The effective temperatures, surface gravities, and photospheric helium abundances are determined simultaneously from an analysis of hydrogen and helium absorption line profiles. The derived temperatures and gravities place the helium-deficient sdB stars in the H—R diagram along and just above 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 ∼0.5M ⊙ overlain by very thin (\(\underset{\raise0.3em\hbox{$\smash{\scriptscriptstyle\thicksim}$}}{<}\), 0.02M ⊙) layers of hydrogen. Various scenarios for their past evolutionary history are examined in the context of their probable future evolution into low-mass white dwarfs.
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