NGC 6441 : another indication for a very high helium content in Globular Cluster stars

Context. The metal-rich bulge globular cluster NGC 6441 shows a well developed blue horizontal branch (Rich et al.), together with a strong slope upward from the red clump to the blue of the RR Lyrae region. Both features, the former corresponding to the well-known second parameter problem, 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 M13. Methods. We make use of horizontal branch simulations, covering a wide range in main sequence helium abundance, to investigate whether the main features of NGC 6441 horizontal branch population, including the RR Lyrae variables period, can be reproduced. Results. To describe the horizontal branch of NGC 6441, the helium content Y in the red clump must reach at least 0.35; values up to Y∼0.37 are necessary to populate the RR Lyr region, reproducing also the observed mean long periods; depending on the dispersion in mass loss assumed in the simulations, values up to Y∼0.38–0.40 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 > ∼ 0.35) required for ∼14% of the horizontal branch population pose some problem on the enrichment mechanisms.