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

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.

17 pages, 3 figures, in press on The Astrophysical Journal

References in corpus (12)

• The ACS Survey of Galactic Globular Clusters. I. Overview and Clusters Without Previous HST Photometry
• The ACS Survey of Galactic Globular Clusters. III. The Double Subgiant Branch of NGC 1851
• Modeling He-rich subdwarfs through the hot-flasher Scenario
• Deep FORS1 observations of the double Main Sequence of omega Centauri
• The Peculiar Horizontal Branch Morphology of the Galactic Globular Clusters NGC 6388 and NGC 6441: new insights from UV observations
• Giants in the globular cluster omega Centauri: dust production, mass loss and distance
• First full evolutionary computation of the He-flash induced mixing in Population II stars
• The Hottest Horizontal-Branch Stars in omega Centauri - Late Hot Flasher vs. Helium Enrichment
• The Horizontal Branch of NGC 1851: Constraints on the Cluster Subpopulations
• On the white dwarf cooling sequence of the globular cluster Omega Centauri
• On the effect of helium enhancement on bolometric corrections and Teff-colour relations
• Blue Hook Stars in Globular Clusters

Cited by in corpus (32)

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• New HST WFC3/UVIS observations augment the stellar-population complexity of omega Centauri
• The C+N+O abundance of Omega Centauri giant stars: implications on the chemical enrichment scenario and the relative ages of different stellar populations
• Formation of Multiple Populations in Globular Clusters: Another Possible Scenario
• Helium in first and second-generation stars in Globular Clusters from spectroscopy of red giants
• The hot horizontal-branch stars in omega Centauri
• New insights on the Galactic Bulge Initial Mass Function
• UV Properties of Galactic Globular Clusters with GALEX II. Integrated colors
• The mosaic multiple stellar populations in $\omega$ Centauri : the Horizontal Branch and the Main Sequence
• Rapidly rotating second-generation progenitors for the blue hook stars of ω Cen
• A Double White-Dwarf Cooling Sequence in ω Centauri
• An empirical mass-loss law for Population II giants from the Spitzer-IRAC survey of Galactic globular clusters
• Spectroscopy of horizontal branch stars in Omega Centauri
• On the period distribution of cluster RR Lyrae stars to constrain their helium content: the case of omega Centauri
• An HST/WFC3 view of stellar populations on the Horizontal Branch of NGC 2419
• The evolution of white dwarfs resulting from helium-enhanced, low-metallicity progenitor stars
• From the CMD of Omega Centauri and (super-)AGB stellar models to a Galactic plane passage gas purging chemical evolution scenario
• On the separation between RR Lyrae and Type II Cepheids and their importance for distance determination: the case of $\omega$ Cen
• A Helium-Carbon Correlation on the Extreme Horizontal Branch in $\omega$ Centauri
• The HST large programme on $\omega$ Centauri -- V. Exploring the Ultracool Dwarf Population with Stellar Atmosphere and Evolutionary Modelling
• The evolutionary status of the blue hook stars in Omega Centauri
• The rise and fall of the UV upturn: $z=0.3,0.55$ and $0.7$
• Spectroscopy of blue horizontal branch stars in NGC 6656 (M22)
• Helium Enhancement in the Metal Rich Red Giants of Omega Centauri
• A Universal Transition in Atmospheric Diffusion for Hot Subdwarfs Near 18,000 K
• Evolution of long-lived globular cluster stars IV. Initial helium content and white-dwarf properties
• Globular clusters as laboratories for stellar evolution
• A Wide View of the Galactic Globular Cluster NGC 2808: Red Giant and Horizontal Branch Star Spatial Distributions
• WFPC2 UV survey of Galactic Globular Clusters. The Horizontal Branch temperature distribution