| Abstract
| - Aims. We analyze the reddening, surface helium abundance and spectroscopic mass of 115 blue horizontal branch (HB) and blue hook (BH) stars in ω Centauri, spanning the cluster HB from the blue edge of the instability strip ( Teff = 8000 K) to BH objects with Teff ≈ 50 000 K. Methods. The temperatures, gravities, and surface helium abundances were measured on low-resolution spectra fitting the Balmer and helium lines with a grid of synthetic spectra. From these parameters, the mass and reddening were estimated. Results. 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 mag. We find that the helium abundances measured on low-resolution spectra are systematically higher by 0.20−0.25 dex than the measurements based on higher resolution. No difference in surface helium abundance is detected between HB stars in ω Centauri and in three comparison clusters, and the stars in the range 11 500−20 000 K 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) cluster stars ( Teff ≥ 20 000 K) 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 30 000 K 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 across the cluster, but this asymmetric distribution is only marginally significative. We also find that EHB stars with anomalously high spectroscopic mass could be present in ω Centauri, as previously found in other clusters. The derived temperature-color relation reveals that the HB stars hotter than ~11 000 K 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 NGC 6752, is a new peculiarity of ω Centauri HB stars. More investigation is needed to reach a full comprehension of this complex observational picture.
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