Vol. 531In section 5. Galactic structure, stellar clusters and populations

Measuring helium abundance difference in giants of NGC 2808

One explanation for the presence of multiple populations in globular clusters is a spread in the helium abundance within these clusters. Helium is notoriously difficult to detect in evolved stars. There is a weak He I line at 10830 Angstroms, which has recently become accessible with new high-resolution infrared spectrographs, such as CRIRES at the VLT. The authors used VLT/CRIRES and VLT/UVES to observe two giant stars in globular cluster NGC2808 with similar stellar parameters, but different Na and O abundances, hence presumed to belong to different subpopulations. These two stars show very similar Ca II and Hα chromospheric lines, but only the Na- rich star shows any He I detection. This leads to inferring an abundance difference of ΔY ≥ 0.17. This is the first tentative direct detection of an He abundance variation in a classical globular cluster, and it is consistent with expectations from stellar evolution theory.