An HST/WFC3 view of stellar populations on the Horizontal Branch of NGC
  2419
  We use images acquired with the Hubble Space Telescope Wide Field Camera 3
and new models to probe the Horizontal Branch (HB) population of the We use
images acquired with the Hubble Space Telescope Wide Field Camera 3 and new
models to probe the horizontal branch (HB) population of the Galactic globular
cluster (GC) NGC 2419. A detailed analysis of the composite HB highlights three
populations:(1) the blue luminous HB, hosting standard helium stars (Y=0.25)
with a very small spread of mass, (2) a small population of stars with
intermediate helium content (0.26<Y<=0.29), and (3) the well-populated extreme
HB. We can fit the last group with models having high helium abundance (Y \sim
0.36), half of which (the hottest part, 'blue hook' stars) are identified as
possible 'late flash mixed stars'. The initial helium abundance of this extreme
population is in nice agreement with the predicted helium abundance in the
ejecta of massive asymptotic giant branch (AGB) stars of the same metallicity
as NGC 2419. This result further supports the hypothesis that second-generation
stars in GCs formed from the ashes of intermediate-mass AGB stars. We find that
the distribution in magnitude of the blue hook stars is larger than that
predicted by theoretical models. We discuss the possible uncertainties in the
magnitude scales and different attempts to model this group of stars. Finally,
we suggest that consistency can be better achieved if we assume core masses
larger than predicted by our models. This may be possible if the progenitors
were fast rotators on the main sequence. If further study confirms this
interpretation, a fast initial rotation would be a strong signature of the
peculiarity of extreme second-generation stars in GCs.
