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Paper: New Views of the Hot Horizontal Branch in ω Centauri
Volume: 392, Hot Subdwarf Stars and Related Objects
Page: 47
Authors: Moehler, S.; Dreizler, S.; Lanz, T.; Bono, G.; Sweigart, A.V.; Calamida, A.; Monelli, M.; Nonino, M.
Abstract: UV observations of some massive globular clusters uncovered a significant population of very hot stars below the hot end of the horizontal branch, the so-called blue hook stars. This feature might be explained either by the late hot flasher scenario where stars experience the helium flash while on the white dwarf cooling curve or by the helium-rich sub-population recently postulated to exist in some clusters. Previous spectroscopic analyses of blue hook stars in ω Cen and NGC 2808 support the late hot flasher scenario, but the stars were found to contain much less helium than expected and the predicted carbon and nitrogen enrichment could not be verified. New moderately high resolution spectra of stars at the hot end of the blue horizontal branch in ω Cen were analysed for atmospheric parameters (Teff, log g, and log [nHe/nH]) and abundances using LTE and non-LTE model atmospheres. In the temperature range 30,000K to 50,000K we find that 35% of our stars are helium-poor (log [nHe/nH] < −2), 51% have solar helium abundance within a factor of 3 (−1.5 ≤ log [nHe/nH] ≤ −0.5) and 14% are helium-rich (log [nHe/nH] > −0.4). We also find carbon enrichment along with helium enrichment, with a maximum carbon abundance of 3% by mass. At least 14% of the hottest horizontal branch stars in ω Cen show helium abundances well above the highest predictions from the helium enrichment scenario (Y ≈ 0.42, corresponding to log [nHe/nH] ≈ −0.74). In addition, the most helium-rich stars show high carbon abundances as predicted by the late hot flasher scenario. We conclude that the helium-rich horizontal branch stars in ω Cen cannot be explained solely by the helium-enrichment scenario invoked to explain the blue main sequence.
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