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Title:
The hottest horizontal-branch stars in ω Centauri. Late hot flasher vs. helium enrichment
Authors:
Moehler, S.; Dreizler, S.; Lanz, T.; Bono, G.; Sweigart, A. V.; Calamida, A.; Monelli, M.; Nonino, M.
Affiliation:
AA(European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching, Germany ), AB(Georg-August-Universität, Institut für Astrophysik, Friedrich-Hund-Platz 1, 37077 Göttingen, Germany ), AC(Department of Astronomy, University of Maryland, College Park, MD 20742-2421, USA ), AD(INAF - Rome Astronomical Observatory, via Frascati 33, 00040 Monte Porzio Catone, Italy ), AE(NASA Goddard Space Flight Center, Code 667, Greenbelt, MD 20771, USA ), AF(INAF - Rome Astronomical Observatory, via Frascati 33, 00040 Monte Porzio Catone, Italy ), AG(IAC - Instituto de Astrofisica de Canarias, Calle via Lactea, 38200 La Laguna, Tenerife, Spain ), AH(INAF - Trieste Astronomical Observatory, via G.B. Tiepolo 11, 40131 Trieste, Italy )
Publication:
Astronomy and Astrophysics, Volume 475, Issue 2, November IV 2007, pp.L5-L8 (A&A Homepage)
Publication Date:
11/2007
Origin:
EDP Sciences
Keywords:
stars: horizontal-branch, stars: evolution, techniques: spectroscopic, Galaxy: globular clusters: individual: ω Centauri
DOI:
10.1051/0004-6361:20078509
Bibliographic Code:
2007A&A...475L...5M

Abstract

Context: UV observations of some massive globular clusters have revealed a significant population of very hot stars below the hot end of the horizontal branch (HB), the so-called blue hook stars. This feature might be explained either as a result of 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 ω 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, and abundances of He, C, and N of blue hook and canonical extreme horizontal branch (EHB) star candidates to the predictions of the two scenarios.
Methods: Moderately high resolution spectra of stars at the hot end of the blue HB in the globular cluster ω Cen were analysed for atmospheric parameters and abundances using LTE and non-LTE model atmospheres.
Results: In the temperature range 30 000 K to 50 000 K we find that 35% of our stars are helium-poor (log{{n_He}/{n_H}} < -2), 51% have solar helium abundance within a factor of 3 (-1.5 ⪉ log{{n_He}/{n_H}} ⪉ -0.5) and 14% are helium-rich (log{{n_He}/{n_H}}} > -0.4). We also find carbon enrichment strongly correlated with helium enrichment, with a maximum carbon enrichment of 3% by mass.
Conclusions: At least 14% of the hottest HB stars in ω Cen show helium abundances well above the highest predictions from the helium enrichment scenario (Y = 0.42 corresponding to log{{n_He}/{n_H}} ≈ -0.74). In addition, the most helium-rich stars show strong carbon enrichment, as predicted by the late hot flasher scenario. We conclude that the helium-rich HB stars in ω Cen cannot be explained solely by the helium-enrichment scenario invoked to explain the blue main sequence.

Based on observations

with the ESO Very Large Telescope at Paranal Observatory, Chile

(proposal IDs 075.D-0280(A) and 077.D-0021(A)).

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