Spectroscopy of horizontal branch stars in Omega Centauri

Abstract

We analyze the reddening, surface helium abundance and mass of 115 horizontal branch (HB) and blue hook (BH) stars in OmegaCentauri, spanning the HB from the blue edge of the instability strip to Teff~50000K. The mean cluster reddening is E(B-V)=0.115+-0.004, in good agreement with previous estimates, but we evidence a pattern of differential reddening in the cluster area. The stars in the western half are more reddened than in the southwest quadrant by 0.03-0.04 magnitudes. We find that the helium abundances measured on low-resolution spectra are systematically lower by ~0.25 dex than the measurements based on higher resolution. No difference in helium abundance is detected between OmegaCentauri and three comparison clusters, and the stars in the range 11500-20000K follow a trend with temperature, which probably reflects a variable efficiency of the diffusion processes. There is mild evidence that two families of extreme HB (EHB) stars (Teff>20000K) could exist, as observed in the field, with ~15% of the objects being helium depleted by a factor of ten with respect to the main population. The distribution of helium abundance above 30000K is bimodal, but we detect a fraction of He-poor objects lower than previous investigations. The observations are consistent with these being stars evolving off the HB. Their spatial distribution is not uniform, but this asymmetric distribution is only marginally significative. We also find that EHB stars with anomalously high spectroscopic mass could be present in OmegaCentauri, as previously found in other clusters. The derived temperature-color relation reveals that stars hotter than 11000K are fainter than the expectations of the canonical models in the U band, while no anomaly is detected in B and V. This behavior, not observed in NGC6752, is a new peculiarity of OmegaCentauri HB stars.

Accepted for publication in A&A

References in corpus (8)

• A Grid of NLTE Line-Blanketed Model Atmospheres of Early B-type Stars
• The Peculiar Horizontal Branch Morphology of the Galactic Globular Clusters NGC 6388 and NGC 6441: new insights from UV observations
• Hubble Space Telescope Observations of an Outer Field in Omega Centauri: A Definitive Helium Abundance
• First full evolutionary computation of the He-flash induced mixing in Population II stars
• The lack of close binaries among hot horizontal branch stars in NGC6752
• On the remote Galactic globular cluster NGC 2419
• Spectroscopy of horizontal branch stars in NGC6752 - Anomalous results on atmospheric parameters and masses
• The environment of formation as a second parameter for globular cluster classification

Cited by in corpus (28)

• Hot subluminous stars
• The population of hot subdwarf stars studied with Gaia -- IV. Catalogues of hot subluminous stars based on Gaia EDR3
• On the RR Lyrae stars in globulars: IV. $\omega$ Centauri Optical UBVRI Photometry
• Direct Evaluation of the Helium Abundances in Omega Centauri
• The mosaic multiple stellar populations in $\omega$ Centauri : the Horizontal Branch and the Main Sequence
• New hot subdwarf stars identified in Gaia DR2 with LAMOST DR5 spectra -- II
• SHOTGLAS I: The ultimate spectroscopic census of extreme horizontal branch stars in $\omega$ Centauri
• Recent Star Formation in the Leading Arm of the Magellanic Stream
• A Helium-Carbon Correlation on the Extreme Horizontal Branch in $\omega$ Centauri
• The Hubble Space Telescope UV Legacy Survey of Galactic Globular Clusters -- XI. The horizontal branch in NGC\,6388 and NGC\,6441
• A possible formation channel for blue hook stars in globular cluster
• A Plague of Magnetic Spots Among the Hot Stars of Globular Clusters
• Blue straggler masses from pulsation properties. II. Topology of the Instability Strip
• Anchors for the Cosmic Distance Scale: the Cepheids U Sgr, CF Cas and CEab Cas
• G2C2 - IV: A novel approach to study the radial distributions of multiple populations in Galactic globular clusters
• Investigating potential planetary nebula/cluster pairs
• Galactic structure in the outer disk: the field in the line of sight to the intermediate-age open cluster Tombaugh 1
• Hot horizontal branch stars in NGC 288 - effects of diffusion and stratification on their atmospheric parameters
• 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
• Spectroscopy of blue horizontal branch stars in NGC 6656 (M22)
• Stellar-Encounter Driven Red-Giant Star Mass-Loss in Globular Clusters
• Spectral analysis and abundances of the post-HB star HD 76431
• Chemical Composition of Young Stars in the Leading Arm of the Magellanic System
• The temperature distribution of horizontal branch stars: methods and first results
• Globular Cluster UVIT legacy Survey (GlobUleS) III. Omega Centauri in Far-Ultraviolet
• Just how hot are the $\omega$ Centauri extreme horizontal branch pulsators?
• The Origins of Young Stars in the Direction of the Leading Arm of the Magellanic Stream: Abundances, Kinematics, and Orbits