A Helium-Carbon Correlation on the Extreme Horizontal Branch in $\omega$ Centauri

Abstract

Taking advantage of a recent FORS2/VLT spectroscopic sample of Extreme Horizontal Branch (EHB) stars in $\omega$ Cen, we isolate 38 spectra well suited for detailed atmospheric studies and determine their fundamental parameters ($T_{\mathrm{eff}}$, log $g$, and log $N$(He)/$N$(H)) using NLTE, metal line-blanketed models. We find that our targets can be divided into three groups: 6 stars are hot ($T_{\mathrm{eff}}$$\buildrel>\over\sim\ $ 45,000 K) H-rich subdwarf O stars, 7 stars are typical H-rich sdB stars ($T_{\mathrm{eff}}$$ \buildrel<\over\sim\ $35,000 K), and the remaining 25 targets at intermediate effective temperatures are He-rich (log $N$(He)/$N$(H)$ \buildrel>\over\sim\ $ $-$1.0) subdwarfs. Surprisingly and quite interestingly, these He-rich hot subdwarfs in $\omega$ Cen cluster in a narrow temperature range ($\sim$35,000 K to $\sim$40,000 K). We additionally measure the atmospheric carbon abundance and find a most interesting positive correlation between the carbon and helium atmospheric abundances. This correlation certainly bears the signature of diffusion processes - most likely gravitational settling impeded by stellar winds or internal turbulence - but also constrains possible formation scenarios proposed for EHB stars in $\omega$ Cen. For the He-rich objects in particular, the clear link between helium and carbon enhancement points towards a late hot flasher evolutionary history.

Accepted for publication in ApJ (12-09-2014), 17 pages

References in corpus (10)

• Fast rotating massive stars and the origin of the abundance patterns in galactic globular clusters
• A Grid of NLTE Line-Blanketed Model Atmospheres of Early B-type Stars
• A selection of hot subluminous stars in the GALEX survey II. Subdwarf atmospheric parameters
• Hot subdwarfs from the ESO Supernova Ia Progenitor Survey: II. Atmospheric parameters of subdwarf O stars
• Modeling He-rich subdwarfs through the hot-flasher Scenario
• The Peculiar Horizontal Branch Morphology of the Galactic Globular Clusters NGC 6388 and NGC 6441: new insights from UV observations
• On the Origin of the High Helium Sequence in $\omega$ Centauri
• 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
• Discovery of a rapidly pulsating subdwarf B star candidate in omega Cen

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• Hot subdwarf stars identified in Gaia DR2 with spectra of LAMOST DR6 and DR7 I. Single-lined spectra
• The enigmatic He-sdB pulsator LS IV$-$14${}^{\circ }$116: new insights from the VLT
• A NLTE analysis of the hot subdwarf O star BD+28 4211. II. The optical spectrum
• A possible formation channel for blue hook stars in globular cluster
• A possible formation channel for blue hook stars in globular cluster - II. Effects of metallicity, mass ratio, tidal enhancement efficiency and helium abundance
• A Wide View of the Galactic Globular Cluster NGC 2808: Red Giant and Horizontal Branch Star Spatial Distributions
• Hot subdwarfs from the surviving companions of the white dwarf + main-sequence channel of Type Ia supernovae