2011MNRAS.410..694D

We present an accurate analysis of the peculiar horizontal branch (HB) of the massive Galactic globular cluster NGC 2808, based on high-resolution far-UV and optical images of the central region of the cluster obtained with the Hubble Space Telescope. We confirm the multimodal distribution of stars along the HB: four sub-populations separated by gaps are distinguishable. The detailed comparison with suitable theoretical models showed that (i) it is not possible to reproduce the luminosity of the entire HB with a single helium abundance, while an appropriate modelling is possible for three HB groups by assuming different helium abundances in the range 0.24 < ΔY < 0.4 that are consistent with the multiple populations observed in the main sequence; and (ii) canonical HB models are not able to properly match the observational properties of the stars populating the hottest end of the observed HB distribution, the so-called blue hook region. These objects are probably `hot flashers', stars that peel off the red giant branch before reaching the tip and ignite helium at high T_eff. Both of these conclusions are based on the luminosity of the HB in the optical and ultraviolet bands and do not depend on specific assumptions about mass loss.