The Hottest Horizontal-Branch Stars in omega Centauri - Late Hot Flasher vs. Helium Enrichment

Abstract

UV observations of some massive globular clusters uncovered a significant population of very hot stars below the hot end of the horizontal branch (HB), the so-called blue hook stars. This feature might be explained either as results of the late hot flasher scenario where stars experience the helium flash while on the white dwarf cooling curve or by the progeny of the helium-enriched sub-population recently postulated to exist in some clusters. Moderately high resolution spectra of stars at the hot end of the blue HB in omega Cen were analysed for atmospheric parameters and abundances using LTE and Non-LTE model atmospheres. In the temperature range 30,000K to 50,000K we find that 35% of our stars are helium-poor (log(n_He/n_H) < -2), 51% have solar helium abundance within a factor of 3 (-1.5 <= log(n_He/n_H) <= -0.5) and 14% are helium-rich (log(n_He/n_H)> -0.4). We also find carbon enrichment in step with helium enrichment, with a maximum carbon enrichment of 3% by mass. At least 14% of the hottest HB stars in omega Cen show helium abundances well above the highest predictions from the helium enrichment scenario (Y = 0.42 corresponding to log(n_He/n_H) ~ -0.74). In addition, the most helium-rich stars show strong carbon enrichment as predicted by the late hot flasher scenario. We conclude that the helium-rich HB stars in omega Cen cannot be explained solely by the helium-enrichment scenario invoked to explain the blue main sequence. (Abridged)

4 pages, 3 figures, uses aa.cls (enclosed), accepted as A&A Letter

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Cited by in corpus (16)

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• The Swift UVOT Stars Survey: I. Methods and Test Clusters
• A Helium-Carbon Correlation on the Extreme Horizontal Branch in $\omega$ Centauri
• A possible formation channel for blue hook stars in globular cluster
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• Compact stellar systems in the Fornax cluster: a UV perspective
• UV-Bright Stellar Populations and Their Evolutionary Implications in the Collapsed-Core Cluster M15
• A possible formation channel for blue hook stars in globular cluster - II. Effects of metallicity, mass ratio, tidal enhancement efficiency and helium abundance
• SHOTGLAS II. MUSE spectroscopy of blue horizontal branch stars in the core of $\omega$ Centauri and NGC 6752
• A Photometric Study of the Outer Halo Globular Cluster NGC 5824
• Modeling He-rich subdwarfs through the hot-flasher scenario (brief version)
• A Wide View of the Galactic Globular Cluster NGC 2808: Red Giant and Horizontal Branch Star Spatial Distributions