The hot horizontal-branch stars in ω Centauri^⋆

¹ European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching, Germany
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² Georg-August-Universität, Institut für Astrophysik, Friedrich-Hund-Platz 1, 37077 Göttingen, Germany
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³ Department of Astronomy, University of Maryland, College Park, MD 20742-2421, USA
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⁴ Dipartimento di Fisica, Universitá di Roma Tor Vergata, via della Ricerca Scientifica 1, 00133 Roma, Italy
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⁵ INAF - Rome Astronomical Observatory, via Frascati 33, 00040 Monte Porzio Catone, Italy
⁶ NASA Goddard Space Flight Center, Code 667, Greenbelt, MD 20771, USA
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⁷ INAF - Trieste Astronomical Observatory, via G.B. Tiepolo 11, 40131 Trieste, Italy
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Received: 19 May 2010
Accepted: 28 August 2010

Abstract

Context. 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 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 and N enrichment cannot be verified.

Aims. We compare the observed effective temperatures, surface gravities, helium abundances, and carbon line strengths (where detectable) of our targets stars with the predictions of the two aforementioned scenarios.

Methods. Moderately high resolution spectra of hot HB stars in the globular cluster ω Cen were analysed for radial velocity variations, atmospheric parameters, and abundances using LTE and non-LTE model atmospheres.

Results. We find no evidence of 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 30 000 K to 50 000 K, we find that 28% of our stars are helium-poor ($\log \frac{{\mathit{n}}_{\mathrm{He}}}{{\mathit{n}}_{\mathrm{H}}}$ < −1.6), while 72% have roughly solar or super-solar helium abundance ($\log \frac{{\mathit{n}}_{\mathrm{He}}}{{\mathit{n}}_{\mathrm{H}}}$  ≥  −1.5). We also find that carbon enrichment is strongly correlated with helium enrichment, with a maximum carbon enrichment of 3% by mass.

Conclusions. A 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 ω Cen cannot be explained solely by the helium-enrichment scenario invoked to explain the blue main sequence.