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

S. Cassisi, M. Salaris, J. Anderson, G. Piotto, A. Pietrinferni, A. Milone, A. Bellini, R. L. Bedin

Published 2009-07-21Version 1

We investigate a peculiar feature at the hottest, blue end of the horizontal branch of Galactic globular cluster omega Centauri, using the high-precision and nearly complete catalog that has been constructed from a survey taken with the ACS on board the HST, that covers the inner 10x10 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 $\sim$0.03Mo. By comparing observed and theoretical star counts along both the H- and 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 omega Centauri.