Helium enhancement among horizontal-branch stars and the near-UV color-magnitude diagrams

Abstract

The distribution of horizontal branch stars in globular cluster color-magnitude diagrams is a sensitive probe of their original helium abundances. In this sense, they have been extensively used recently as indicators of possible variations in the helium content Y among the different populations of stars within individual globular clusters. However, recent analyses based on visual and near-ultraviolet color-magnitude diagrams have provided conflicting results. In order to clarify the situation, in this presentation we discuss the optimum ranges of applicability of this “HB Y test” in different filter systems, in terms of the temperature range covered by the horizontal branch stars. In particular, we discuss the case of M3 (NGC 5272), based on multi-band photometry (HST, Strömgren, Johnson), thus setting new and tight constraints on the internal range in Y within this cluster. We also provide an application of this "HB Y test" to other selected clusters....