The horizontal branch in the UV colour-magnitude diagrams - II. The case of M3, M13 and M79

We present a detailed comparison between far-ultraviolet (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 (Ymax) values compatible with the data. We find differences ΔYmax ~ 0.02-0.04 between the clusters with M13 showing the largest value (Ymax~0.30) and M3 the smallest (Ymax~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 NaO anticorrelations.