Evolved Stars in the Core of the Massive Globular Cluster NGC 2419

TL;DR

This study analyzes the horizontal branch stars in NGC 2419 using Hubble data, introducing a method to correct for lifetime effects, and finds minimal helium enrichment and no evidence of multiple populations.

Contribution

It presents a new method for removing lifetime effects from HB star distributions to improve helium abundance measurements in globular clusters.

Findings

Slight helium enrichment among EHB stars, with dY < 0.05.

No evidence of multiple populations in the upper main sequence.

Identifies the end of the canonical HB and a density change indicating the boundary to blue hook stars.

Abstract

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. 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 EHB stars in the cluster, but that it is likely to be small (dY < 0.05). An examination of the upper main sequence does not reveal any sign of multiple populations. Through comparisons of optical CMDs, we present evidence that the EHB clump in NGC 2419 contains the end of the canonical horizontal branch, 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 omega Cen. The more clearly visible HB gap at V = 23.5 appears to be too bright.(Abridged)