Horizontal Branch stars in ω Centauri : clues about their origin from the cluster Color Magnitude Diagram

We investigate a peculiar feature at the hottest, blue end of the horizontal branch of Galactic globular cluster ω Centauri, using the high-precision and nearly complete catalog that has been constructed from a survey taken with the Advanced Camera for Survey on board the Hubble Space Telescope, that covers the inner 10 × 10 arcminutes. It is a densely populated clump of stars with an almost vertical structure in the F435W-(F435W−F625W) plane, that we termed ‘blue clump’. A comparison with theoretical models leads to the conclusion that this feature must necessarily harbor either hot flasher stars, or canonical He-rich stars –progeny of the blue Main Sequence sub-population observed in this cluster– or a mixture of both types, plus possibly a component from the normal-He population hosted by the cluster. A strong constraint coming from theory is that the mass of the objects in the ‘blue clump’ has to be very finely tuned, with a spread of at most only ∼0.03 M⊙. By comparing observed and theoretical star counts along both the Hand He-burning stages we then find that at least 15% of the expected He-rich Horizontal Branch stars are missing from the color-magnitude diagram. This missing population could be the progeny of red giants that failed to ignite central He-burning and have produced He-core White Dwarfs. Our conclusion supports the scenario recently suggested by Calamida et al. (2008) for explaining the observed ratio of White Dwarfs to Main Sequence stars in ω Centauri. Subject headings: galaxies: stellar content – globular clusters: general – stars: abundances – stars: evolution – stars: horizontal-branch – stars: white dwarf