The Peculiar Horizontal Branch Morphology of the Galactic Globular Clusters NGC6388 and NGC6441

TL;DR

This study uses multiband Hubble Space Telescope photometry to explore the unusual horizontal branch morphology of NGC6388 and NGC6441, suggesting helium enrichment as a key factor behind their extended blue tails and slopes.

Contribution

It provides evidence that helium enrichment may explain the peculiar horizontal branch features in these globular clusters, challenging previous interpretations.

Findings

Horizontal branch tilt is not caused by differential reddening.

Helium enrichment in a small stellar fraction explains the extended blue tail.

Supports helium enrichment as a key factor in HB morphology.

Abstract

I present multiband optical and UV Hubble Space Telescope photometry of the two Galactic globular clusters NGC6388 and NGC6441, in order to investigate the nature of the physical mechanism(s) responsible for the existence of an extended blue tail and of a slope in the horizontal branch. Further evidence that the horizontal branch tilt cannot be interpreted as an effect of differential reddening is provided, while I show that a possible solution of the puzzle is to assume that a small fraction of the stellar population in the two clusters is strongly helium enriched.