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Preprint typeset using L ATEX style emulateapj v. 10/09/06 EVOLVED STARS IN THE CORE OF THE MASSIVE GLOBULAR CLUSTER NGC 2419 1

By Eric L. S and Jordan M. Hess


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 horizontal branch 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 = NHB/NRGB implies there may be slight helium enrichment among the EHB stars in the cluster, but that it is likely to be small (∆Y < 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 extreme horizontal branch stars (EHB) at a large

Year: 2008
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