2016A&A...589A.126V

Context. The color and luminosity distribution of horizontal branch (HB) stars in globular clusters (GCs) are sensitive probes of the original helium abundances of those clusters. In this sense, recently the distributions of HB stars in GC color-magnitude diagrams (CMDs) have been extensively used as indicators of possible variations in the helium content Y among the different generations of stars within individual GCs. However, recent analyses based on visual and near-ultraviolet (UV) CMDs have provided conflicting results.
Aims. To clarify the situation, we address the optimum ranges of applicability (in terms of the T_eff range covered by the HB stars) for visual and near-UV CMDs, as far as application of this "HB Y test" goes.
Methods. We considered both Stromgren and Hubble Space Telescope (HST) bandpasses. In particular, we focus on the F336W filter of the HST, but also discuss several bluer UV bandpasses, such as F160BW, F255W, and F300W. Using the Princeton-Goddard-PUC (PGPUC) code, we computed a large set of zero-age HB (ZAHB) loci and HB evolutionary models for masses ranging from M_HB=0.582 to 0.800M_☉, assuming an initial helium abundance Y=0.246, 0.256, and 0.266, with a global metallicity Z=0.001. The results of these calculations were compared against the observations of M3 (NGC 5272), with special attention on the y vs. (b-y) and F336W vs. (F336W-F555W) CMDs.
Results. Our results indicate that, from an evolutionary perspective, the distributions of HB stars in the y vs. (b - y) plane can be a reliable indicator of the He content in cool blue HB (BHB) stars, particularly when a differential comparison between blue and red HB stars is carried out in the range T_eff <= 8300 K. Conversely, we demonstrate that CMDs using the F336W filter have a much less straightforward interpretation at the cool end of the BHB because the distributions of HB stars in the F336W vs. (F336W-F555W) plane, for instance, are affected by a triple degeneracy effect. In other words, the position of an HB star in such a CMD is exactly the same for a given chemical composition for multiple combinations of the parameters Y, M_HB, and age along the HB evolutionary track. Other HST UV filters do not appear to be as severely affected by this degeneracy effect, to which visual bandpasses are also immune. On the other hand, such near-UV CMDs can be extremely useful for the hottest stars along the cool BHB end.
Conclusions. Based on a reanalysis of the distribution of HB stars in the y vs. (b-y) plane, we find that the coolest BHB stars in M3 (i.e., those with T_eff<8300K) are very likely enhanced in helium by ΔY≃0.01, compared with the red HB stars in the same cluster. Using near-UV HST photometry, on the other hand, we find evidence of a progressive increase in Y with increasing temperature, reaching ΔY≃0.02 at T_eff≃10900K.