Constraints on Helium Enhancement in the Globular Cluster M4 (NGC 6121): The Horizontal Branch Test

Abstract: Recent pieces of evidence have revealed that most, and possibly all, globular star clusters are composed of groups of stars that formed in multiple episodes with different chemical compositions. In this sense, it has also been argued that variations in the initial helium abundance ($Y$) from one population to the next are also the rule, rather than the exception. In the case of the metal-intermediate globular cluster M4 (NGC 6121), recent high-resolution spectroscopic observations of blue horizontal branch (HB) stars (i.e., HB stars hotter than the RR Lyrae instability strip) suggest that a large fraction of blue HB stars are second-generation stars formed with high helium abundances. In this paper, we test this scenario by using recent photometric and spectroscopic data together with theoretical evolutionary computations for different $Y$ values. Comparing the photometric data with the theoretically-derived color-magnitude diagrams, we find that the bulk of the blue HB stars in M4 have $\Delta Y \lesssim 0.01$ with respect to the cluster's red HB stars (i.e., HB stars cooler than the RR Lyrae strip)-- a result which is corroborated by comparison with spectroscopically derived gravities and temperatures, which also favor little He enhancement. However, the possible existence of a minority population on the blue HB of the cluster with a significant He enhancement level is also discussed.