The Horizontal Branch in the UV Colour Magnitude Diagrams. II. The case of M3, M13 and M79

TL;DR

This study uses Hubble UV/optical data and models to explore the role of initial Helium abundance variations in explaining the different Horizontal Branch morphologies of three globular clusters, M3, M13, and M79.

Contribution

It provides the first constraints on maximum Helium abundance variations in these clusters and links them to Horizontal Branch morphology and chemical anti-correlations.

Findings

Y_{max} varies by 0.02-0.04 among clusters

Y_{max} correlates with Horizontal Branch extension

Y_{max} correlates with Na-O anti-correlation range

Abstract

We present a detailed comparison between far-UV/optical colour Magnitude Diagrams obtained with high-resolution Hubble Space Telescope data and suitable theoretical models for three Galactic Globular Clusters: M3, M13 and M79. These systems represents a classical example of clusters in the intermediate metallicity regime that, even sharing similar metal content and age, show remarkably different Horizontal Branch morphologies. As a consequence, the observed differences in the colour distributions of Horizontal Branch stars cannot be interpreted in terms of either first (metallicity) or a second parameter such as age. We investigate here the possible role of variations of initial Helium abundance (Y). Thanks to the use of a proper setup of far-UV filters, we are able to put strong constraints on the maximum Y (Y_{max}) values compatible with the data. We find differences Delta Y_{max} ~ 0.02-0.04 between the clusters with M13 showing the largest value (Y_{max} ~ 0.30) and M3 the smallest (Y_{max} ~ 0.27). In general we observe that these values are correlated with the colour extensions of their Horizontal Branches and with the range of the observed Na-O anti-correlations.