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The Hot Horizontal-Branch Stars in omega CentauriContext. UV observations of some massive globular clusters have revealed a significant population of stars hotter and fainter than the hot end of the horizontal branch (HB), 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 progeny of the helium-enriched sub-population recently postulated to exist in some clusters. Previous spectroscopic analyses of blue hook stars in co Cen and NGC 2808 support the late hot flasher scenario, but the stars contain much less helium than expected and the predicted C, N enrichment could not be verified. Aims. We compare observed effective temperatures, surface gravities, helium abundances, and carbon line strengths (where detectable) of our targets stars to the predictions of the two scenarios. Methods. Moderately high resolution spectra of hot HB stars in the globular cluster omega-Cen were analysed for radial velocity variations, atmospheric parameters and abundances using LTE and non-LTE model atmospheres. Results. We find no evidence for close binaries among our target stars. All stars below 30 000 K are helium-poor and very similar to HB stars observed in that temperature range in other globular clusters. In the temperature range 30000 K to 50000 K we find that 28% of our stars are helium-poor (log ((sup n)He/(sup n )H)< - 1.6), while 72% have roughly solar or super-solar helium abundance (log ((sup n)He/(sup n )H) >/= -1.5). We also find carbon enrichment strongly correlated with helium enrichment, with a maximum carbon enrichment of 3% by mass. Conclusions. The strong carbon enrichment in tandem with helium enrichment is predicted by the late hot flasher scenario, but not by the helium-enrichment scenario. We conclude that the helium-rich HB stars in omega-Cen cannot be explained solely by the helium-enrichment scenario invoked to explain the blue main sequence.
Document ID
20100024389
Acquisition Source
Goddard Space Flight Center
Document Type
Preprint (Draft being sent to journal)
Authors
Moehler, S.
(European Southern Observatory Garching, Germany)
Dreizler, S.
(Georg-August Univ. Goettingen, Germany)
Lanz, T.
(Maryland Univ. College Park, MD, United States)
Bono, G.
(Tor Vergata Univ. Rome, Italy)
Sweigart, A. V.
(NASA Goddard Space Flight Center Greenbelt, MD, United States)
Calamida, A.
(European Southern Observatory Garching, Germany)
Nonino, M.
(Trieste Astronomical Observatory Trieste, Italy)
Date Acquired
August 24, 2013
Publication Date
May 19, 2010
Subject Category
Astronomy
Distribution Limits
Public
Copyright
Other

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