2008AJ....136.2259S

We present an analysis of optical and ultraviolet Hubble Space Telescope photometry for evolved stars in the core of the distant massive globular cluster NGC 2419. We characterize the horizontal branch (HB) population in detail including corrections for incompleteness on the long blue tail. The majority of the HB stars can be identified with two main groups (one slightly bluer than the instability strip, and the other at the extreme end of the HB). We present a method for removing (to first-order) lifetime effects from the distribution of HB stars to facilitate more accurate measurements of helium abundance for clusters with blue HBs and to clarify the distribution of stars reaching the zero-age HB. The population ratio R = N_HB/N_RGB implies there may be slight helium enrichment among the extended HB (EHB) stars in the cluster, but that it is likely to be small (ΔY < 0.05). An examination of the upper main sequence does not reveal any sign of multiple populations indicative of helium enrichment. The stellar distribution allows us to follow how the two main types of stars evolve after the HB. We find that the transition from stars that reach the asymptotic giant branch to stars that remain at high temperatures probably occurs among the EHB stars at a larger temperature than predicted by canonical evolution models, but qualitatively consistent with helium-enriched models. Through comparisons of optical color-magnitude diagrams (CMDs), we present evidence that the EHB clump in NGC 2419 contains the end of the canonical HB, and that the boundary between the normal HB stars and blue hook stars shows up as a change in the density of stars in the CMD. This corresponds to a spectroscopically verified gap in NGC 2808 and an "edge" in ω Cen. The more clearly visible HB gap at V ∼ 23.5 identified by Ripepi et al. appears to be too bright. Once corrected for lifetime effects, we find that NGC 2419 is currently converting about 25-31% of the first-ascent red giant stars in its core into extreme blue HB stars–the largest fraction for any known globular cluster. A comparison of upper red giant branch with theoretical models indicates there is a slight deficiency of bright red giant stars. This deficiency occurs far enough below the tip of the red giant branch that it is unlikely to be associated with the production of EHB stars via strong mass loss before the core helium flash.