Hot Horizontal Branch stars in omega 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 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.
