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.

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

References in corpus (38)

• A Grid of NLTE Line-Blanketed Model Atmospheres of O-type Stars
• 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 large stellar evolution database for population synthesis studies. II. Stellar models and isochrones for an alpha-enhanced metal distribution
• Omega Centauri: The Population Puzzle Goes Deeper
• Properties of stellar generations in Globular Clusters and relations with global parameters
• Predictions for self-pollution in globular cluster stars
• Metallicities on the Double Main Sequence of omega Centauri Imply Large Helium Enhancement
• Ultraviolet Radiation from Evolved Stellar Populations -- I. Models
• Chemical Abundances for 855 Giants in the Globular Cluster Omega Centauri (NGC 5139)
• Helium variation due to self-pollution among Globular Cluster stars: consequences on the horizontal branch morphology
• The Origin of Extreme Horizontal Branch Stars
• The multiplicity of the subgiant branch of Omega Centauri: Evidence for prolonged star formation
• Flash Mixing on the White Dwarf Cooling Curve: Understanding Hot Horizontal Branch Anomalies in NGC 2808
• 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
• Spectral analysis of sdB stars from the Hamburg Quasar Survey
• 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
• Slowing down atomic diffusion in subdwarf B stars: mass loss or turbulence?
• Horizontal Branch evolution, metallicity and sdB stars
• Flash Mixing on the White Dwarf Cooling Curve: FUSE Observations of three He-rich sdB Stars
• Spectroscopic Analyses of the Blue Hook Stars in NGC 2808: A More Stringent Test of the Late Hot Flasher Scenario
• Stromgren photometry of Galactic Globular Clusters. II Metallicity distribution of red giants in Omega Centauri
• The hot horizontal-branch stars in omega Centauri
• A dynamical study of Galactic globular clusters under different relaxation conditions
• Star Counts in the Globular Cluster Omega Centauri. I. Bright Stellar Components
• The Hottest Horizontal-Branch Stars in omega Centauri - Late Hot Flasher vs. Helium Enrichment
• Hot Horizontal Branch stars in omega Centauri: clues about their origin from the cluster Color Magnitude Diagram
• Spectroscopic Analyses of the "Blue Hook" Stars in omega Centauri: A Test of the Late Hot Flasher Scenario
• Spectroscopy of horizontal branch stars in Omega Centauri
• Flash Mixing on the White Dwarf Cooling Curve: Spectroscopic Confirmation in NGC 2808
• The evolutionary status of the blue hook stars in Omega Centauri
• The Discovery of Pulsating Hot Subdwarfs in NGC 2808
• On the density profile of the globular cluster M92
• Discovery of a rapidly pulsating subdwarf B star candidate in omega Cen

Cited by in corpus (21)

• Hot subluminous stars
• The catalogue of radial velocity variable hot subluminous stars from the MUCHFUSS project
• On the RR Lyrae stars in globulars: IV. $\omega$ Centauri Optical UBVRI Photometry
• A Chemical Composition Survey of the Iron-Complex Globular Cluster NGC 6273 (M 19)
• Rapidly rotating second-generation progenitors for the blue hook stars of ω Cen
• GeMS MCAO observations of the Galactic globular cluster NGC 2808: the absolute age
• New hot subdwarf stars identified in Gaia DR2 with LAMOST DR5 spectra -- II
• 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
• Evolutionary and pulsation properties of Type II Cepheids
• SHOTGLAS I: The ultimate spectroscopic census of extreme horizontal branch stars in $\omega$ Centauri
• 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
• Heavy-metal enrichment of intermediate He-sdOB stars: the pulsators Feige 46 and LS IV -14 116 revisited
• Pulsating hot O subdwarfs in Omega Cen: mapping a unique instability strip on the Extreme Horizontal Branch
• 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 500 pc volume-limited sample of hot subluminous stars II. Atmospheric parameters, mass distribution, and kinematics
• Just how hot are the $\omega$ Centauri extreme horizontal branch pulsators?
• 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