The hottest horizontal-branch stars in ω Centauri.: Late hot flasher vs. helium enrichment

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 omega 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. 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 (nHe)/(nH) < -2), 51% have solar helium abundance within a factor of 3 (- 1.5 less than or similar to log (nHe)/(nH) less than or similar to -0.5) and 14% are helium-rich (log (nHe)/(nH) > -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. Cen show helium abundances well above the highest predictions from the helium enrichment scenario (Y = 0.42 corresponding to log (nHe)/(nH) approximate to -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. Cen cannot be explained solely by the helium-enrichment scenario invoked to explain the blue main sequence.