NGC 2419: a large and extreme second generation in a currently undisturbed cluster

TL;DR

This study analyzes NGC 2419's stellar populations, revealing a surprisingly large and extreme second-generation star fraction with high helium content, challenging current models of globular cluster evolution.

Contribution

It provides evidence for a significant (~30%) extreme second-generation population in NGC 2419, suggesting a more massive initial cluster and informing formation models of multiple stellar populations.

Findings

Approximately 30% of stars are extreme second-generation with high helium.

Color dispersion and main sequence data support the multiple population hypothesis.

The cluster's large second-generation fraction implies a more massive initial system.

Abstract

We analyse complementary HST and SUBARU data for the globular cluster NGC 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 (~30 %) extreme second generation with a strong helium enhancement (Y~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 NGC 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 ~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.