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.

8 pages, 5 figures

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Cited by in corpus (12)

• Nitrogen abundances and multiple stellar populations in the globular clusters of the Fornax dSph
• Detection of solar-like oscillations in relics of the Milky Way: asteroseismology of K giants in M4 using data from the NASA K2 mission
• Radial distributions of sub-populations in the globular cluster M15: a more centrally concentrated primordial population
• Observing multiple stellar populations with FORS2@VLT - Main sequence photometry in outer regions of NGC 6752, NGC 6397, and NGC 6121 (M 4)
• An extreme paucity of second population AGB stars in the normal globular cluster M4
• Second Generation Stars in Globular Clusters from Rapid Radiative Cooling of Pre-Supernova Massive Star Winds
• Multiple populations along the asymptotic giant branch of the globular cluster M 4
• The Kapteyn moving group is not tidal debris from $\omega$ Centauri
• Level of helium enhancement among M3's horizontal-branch stars
• On the AGB stars of M 4: A robust disagreement between spectroscopic observations and theory
• A combined study of thermohaline mixing and envelope overshooting with PARSEC: Calibration to NGC 6397 and M4
• Matching seismic masses for RR Lyrae-type and oscillating red horizontal-branch stars in M4