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 HST .
We confirm the multimodal distribution of stars along the HB : 4 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 modeling is possible for three HB groups by assuming different helium abundances in the range 0.24 < \Delta Y < 0.4 that are consistent with the multiple populations observed in the Main Sequence ; ( 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 UV bands and do not depend on specific assumptions about mass loss .