The helium abundance in the metal-poor globular clusters M30 and NGC 6397

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Abstract

We present the helium abundance of the two metal-poor clusters M30 and NGC 6397. Helium estimates have been obtained by using the high-resolution spectrograph FLAMES at the European Southern Observatory Very Large Telescope and by measuring the He I line at 4471 Å in 24 and 35 horizontal branch (HB) stars in M30 and NGC 6397, respectively. This sample represents the largest data set of He abundances collected so far in metal-poor clusters. The He mass fraction turns out to be Y = 0.252 ± 0.003 (σ = 0.021) for M30 and Y = 0.241 ± 0.004 (σ = 0.023) for NGC 6397. These values are fully compatible with the cosmological abundance, thus suggesting that the HB stars are not strongly enriched in He. The small spread of the Y distributions are compatible with those expected from the observed main sequence splitting. Finally, we find a hint of a weak anticorrelation between Y and [O/Fe] in NGC 6397 in agreement with the prediction that O-poor stars are formed by (He-enriched) gas polluted by the products of hot proton-capture reactions. © 2014. The American Astronomical Society. All rights reserved..

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Mucciarelli, A., Lovisi, L., Lanzoni, B., & Ferraro, F. R. (2014). The helium abundance in the metal-poor globular clusters M30 and NGC 6397. Astrophysical Journal, 786(1). https://doi.org/10.1088/0004-637X/786/1/14

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