Bibcode
Moehler, S.; Dreizler, S.; Lanz, T.; Bono, G.; Sweigart, A. V.; Calamida, A.; Monelli, M.; Nonino, M.
Referencia bibliográfica
Hot Subdwarf Stars and Related Objects ASP Conference Series, Vol. 392, Proceedings of the conference held 23-27 July, 2007, at Otto-Friedrich-Universität, Bamberg, Germany. Edited by Ulrich Heber, C. Simon Jeffery, and Ralf Napiwotzki., p.47
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2008
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Descripción
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 30,000 K to 50,000 K 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
ωCen 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.