# Four stellar populations and extreme helium variation in the massive outer-halo globular cluster NGC 2419

## TL;DR

This study uses Hubble data to analyze multiple stellar populations in the massive, distant globular cluster NGC 2419, revealing detailed helium variations and star fractions that challenge existing formation models.

## Contribution

It provides the first detailed helium abundance measurements for multiple populations in NGC 2419, highlighting its unique stellar composition and implications for globular cluster formation theories.

## Key findings

- NGC 2419 has a higher fraction of first-generation stars (~37%) than other massive GCs.
- Three second-generation populations show helium enhancements of deltaY ~0.01, 0.06, and 0.19.
- High helium enrichment in the most He-rich population challenges current multiple population formation scenarios.

## Abstract

Recent work revealed that both the helium variation within globular clusters (GCs) and the relative numbers of first and second-generation stars (1G, 2G) depend on the mass of the host cluster. Precise determination of the internal helium variations and of the fraction of 1G stars are crucial constraints to the formation scenarios of multiple populations (MPs). We exploit multi-band Hubble Space Telescope photometry to investigate MPs in NGC 2419, which is one of the most-massive and distant GCs of the Galaxy, almost isolated from its tidal influence. We find that the 1G hosts the ~37% of the analyzed stars, and identified three populations of 2G stars, namely 2GA, 2GB, and 2GC, which comprise the ~20%, ~31% and ~12% of stars, respectively. We compare the observed colors of these four populations with the colors derived from appropriate synthetic spectra to infer the relative helium abundances. We find that 2GA, 2GB, and 2GC stars are enhanced in helium mass fraction by deltaY ~0.01, 0.06, and 0.19 with respectto 1G stars that have primordial helium (Y=0.246). The high He enrichment of 2GC stars is hardly reconcilable with most of the current scenarios for MPs. Furthermore, the relatively larger fraction of 1G stars (~37%) compared to other massive GCs is noticeable. By exploiting literature results, we find that the fractions of 1G stars of GCs with large perigalactic distance are typically higher than in the other GCs with similar masses. This suggests that NGC 2419, similarly to other distant GCs, lost a lower fraction of 1G stars.