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

TL;DR

This study examines the peculiar blue clump at the hot end of omega Centauri's horizontal branch, suggesting it contains hot flasher and He-rich stars, with implications for stellar evolution and white dwarf populations.

Contribution

It identifies the nature of the blue clump stars and constrains their masses, providing insights into the evolutionary pathways of helium-rich and normal stars in omega Centauri.

Findings

The blue clump hosts hot flasher and He-rich stars.

At least 15% of He-rich HB stars are missing from the CMD.

The missing stars may be He-core white dwarf progenitors.

Abstract

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.