NGC 6441: another indication of very high helium content in globular cluster stars

Abstract

Context. The metal-rich bulge globular cluster NGC 6441 shows both a well-developed blue horizontal branch (Rich et al.), and a strong slope upward from the red clump to the blue of the RR Lyrae region. The former corresponds to the well–known second parameter problem. Both features are not explained by conventional evolutionary models. Aims. Helium self-enrichment is proposed as a possible solution to both questions, a mechanism already invoked for the interpretation of the peculiarities in NGC 2808 and M 13. Methods. We make use of horizontal branch simulations that cover a wide range of main sequence helium abundance to investigate whether the main features of the NGC 6441 horizontal branch population, including the RR Lyrae variables period, can be reproduced. Results. To describe the horizontal branch of NGC 6441, the following must be met. The helium content Y in the red clump must reach at least 0.35; values up to $Y\sim0.37$ are necessary to populate the RR Lyr region, also reproducing the observed mean long periods; depending on the dispersion in mass loss assumed in the simulations, values up to $Y\sim0.38{-}0.40$ are necessary to populate the blue HB. The total self-enriched population amounts to ~60% of the whole stellar content. Conclusions. Self-enrichment and multiple star-formation episodes in the early evolution of globular clusters appear more and more able to account for many of the chemical and population peculiarities observed in these systems. The very large helium abundances ($Y\ga0.35$) required for ~14% of the horizontal branch population pose some problems for the enrichment mechanisms.