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T. Lanz (U Maryland & GSFC), T. M. Brown (STScI), A. V. Sweigart (NASA's GSFC), I. Hubeny (NOAO), W. B. Landsman (Science Systems and Applications)
Subdwarf B stars all show significant abundance anomalies. Most are extremely deficient in helium and selected light elements, but a minority are helium-rich. Deficiencies in helium and heavier elements have been attributed to gravitational settling, but the helium-enriched members of the class present a puzzling exception, because radiative levitation should be too weak to prevent gravitational settling.
New evolutionary calculations suggest that these helium-rich sdB stars are the result of a delayed helium-core flash on the white dwarf cooling curve. The convective zone produced by this flash will penetrate the hydrogen envelope, mixing hydrogen into the hot helium-burning interior, where it is rapidly consumed. The resulting star should show greatly enhanced helium and carbon with respect to the other heavy elements. This phenomenon is analogous to the born again scenario for producing hydrogen-deficient R CrB stars following a very late helium-shell flash. We have recently obtained FUSE spectra of two helium-rich sdB stars, revealing huge C III lines at 977 and 1176 Å. Our preliminary analysis yields a surface composition of 97% He and 3% C, in agreement with the new evolutionary scenario.
This work is supported in part by NASA grant NAG5-12383.
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Bulletin of the American Astronomical Society, 34, #4
© 2002. The American Astronomical Soceity.