We analyse complementary HST and SUBARU data for the globular cluster NGCÂ 2419 .
We make a detailed analysis of the horizontal branch ( HB ) , that appears composed by two main groups of stars : the luminous blue HB stars —that extend by evolution into the RR Lyrae and red HB region— and a fainter , extremely blue population .
We examine the possible models for this latter group and conclude that a plausible explanation is that they correspond to a significant ( \sim 30 % ) extreme second generation with a strong helium enhancement ( Y \sim 0.4 ) .
We also show that the color dispersion of the red giant branch is consistent with this hypothesis , while the main sequence data are compatible with it , although the large observational error blurs the possible underlying splitting . While it is common to find an even larger ( 50 – 80 ) percentage of second generation in a globular cluster , the presence of a substantial and extreme fraction of these stars in NGCÂ 2419 might be surprising , as the cluster is at present well inside the radius beyond which the galactic tidal field would be dominant .
If a similar situation had been present in the first stages of the cluster life , the cluster would have retained its initial mass , and the percentage of second generation stars should have been quite small ( up to \sim 10 % ) .
Such a large fraction of extreme second generation stars implies that the system must have been initially much more massive and in different dynamical conditions than today .
We discuss this issue in the light of existing models of the formation of multiple populations in globular clusters .