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Paper: The Chemical Composition of Subdwarf B Stars: Spectroscopic Diagnostics and Clues to Evolution
Volume: 452, Fifth Meeting on Hot Subdwarf Stars and Related Objects
Page: 41
Authors: Jeffery, C. S.; Pereira, C.; Naslim; N.; Behara, N.
Abstract: The principal signature in the optical spectrum of a normal subdwarf B star is the presence of strong gravity-broadened hydrogen lines, and the weakness or absence of neutral helium lines. The latter represents a very low photospheric abundance of helium, and has been attributed to the gravitational settling of helium beneath the photosphere by diffusion. In a few cases, helium lines dominate the spectrum: these helium-rich subdwarfs raise important questions about the formation of all hot subdwarfs. With access to ultraviolet spectra, and also to high signal-to-noise high-resolution optical spectra, it has become possible to explore the abundances of elements other than hydrogen and helium systematically. Questions arise regarding the choice of model atmosphere to use for such chemically peculiar stars. We report on i) a comparison between model grids with radically different chemical distributions and physical assumptions and the consequences for measurement of effective temperature and surface gravity, ii) a multi-wavelength analysis of normal sdB stars, comparing and combining abundances for several stars from FUSE, HST/STIS and optical spectroscopy, and iii) recent work on helium-rich subdwarfs which explores relationships between normal, moderately helium-rich and extremely helium-rich sdB stars.
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