Astronomy and Astrophysics – Astronomy
Scientific paper
Dec 2001
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001aas...19913704s&link_type=abstract
American Astronomical Society, 199th AAS Meeting, #137.04; Bulletin of the American Astronomical Society, Vol. 33, p.1512
Astronomy and Astrophysics
Astronomy
Scientific paper
Ultraviolet (UV) observations of the globular clusters ω Cen and NGC 2808 have revealed an unexpected population of hot subluminous stars lying up to 0.7 mag below the extreme horizontal branch (EHB) in the UV, which are not explained by canonical stellar models. In order to explore the evolutionary status of these stars, we have evolved a set of low-mass stars from the main sequence through the helium flash to the horizontal branch (HB) for a wide range in the mass loss along the red-giant branch (RGB). Stars with the largest mass loss evolve off the RGB to high effective temperatures before igniting helium in their cores. Our results indicate that the subluminous stars can be explained if these stars undergo a late helium flash while descending the white dwarf cooling curve. Under these conditions the convection zone produced by the helium flash will penetrate into the stellar envelope, thereby mixing the envelope hydrogen into the hot helium-burning interior, where it is rapidly consumed. Such ``flash-mixed'' stars will have helium- and carbon-rich envelopes and will lie at higher effective temperatures than the hottest canonical (i.e., unmixed) EHB stars. Using new stellar atmospheres, we show that these changes in the envelope abundances will suppress the UV flux in the spectra of the flash-mixed stars by the amount needed to explain the hot subluminous stars in ω Cen and NGC 2808. To test this evolutionary scenario, we have obtained medium resolution spectra of a sample of the hottest HB stars in ω Cen. We find that these stars are indeed helium-rich compared to classical EHB stars and also considerably hotter than the hottest EHB models without flash mixing.
Brown Ted M.
Dreizler Stefan
Hubeny Ivan
Landsman Wayne B.
Lanz Thierry
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