An HST/WFC3 view of stellar populations on the Horizontal Branch of NGC 2419

TL;DR

This study uses Hubble Space Telescope data and new models to analyze the Horizontal Branch populations in NGC 2419, revealing multiple helium-enriched groups and suggesting a link to AGB star ejecta and potential stellar rotation effects.

Contribution

It provides a detailed characterization of multiple HB populations in NGC 2419, including the identification of helium-enriched groups and implications for stellar evolution and formation scenarios.

Findings

Identification of three distinct HB populations with varying helium content.

The extreme HB stars' properties align with predictions from AGB star ejecta.

Discrepancies in blue hook star magnitudes suggest possible effects of stellar rotation.

Abstract

We use images acquired with the Hubble Space Telescope Wide Field Camera 3 and new models to probe the Horizontal Branch (HB) population of the We use images acquired with the Hubble Space Telescope Wide Field Camera 3 and new models to probe the horizontal branch (HB) population of the Galactic globular cluster (GC) NGC 2419. A detailed analysis of the composite HB highlights three populations:(1) the blue luminous HB, hosting standard helium stars (Y=0.25) with a very small spread of mass, (2) a small population of stars with intermediate helium content (0.26<Y<=0.29), and (3) the well-populated extreme HB. We can fit the last group with models having high helium abundance (Y \sim 0.36), half of which (the hottest part, 'blue hook' stars) are identified as possible 'late flash mixed stars'. The initial helium abundance of this extreme population is in nice agreement with the predicted helium abundance in the ejecta of massive asymptotic giant branch (AGB) stars of the same metallicity as NGC 2419. This result further supports the hypothesis that second-generation stars in GCs formed from the ashes of intermediate-mass AGB stars. We find that the distribution in magnitude of the blue hook stars is larger than that predicted by theoretical models. We discuss the possible uncertainties in the magnitude scales and different attempts to model this group of stars. Finally, we suggest that consistency can be better achieved if we assume core masses larger than predicted by our models. This may be possible if the progenitors were fast rotators on the main sequence. If further study confirms this interpretation, a fast initial rotation would be a strong signature of the peculiarity of extreme second-generation stars in GCs.