DIRECT EVALUATION OF THE HELIUM ABUNDANCES IN OMEGA CENTAURI

A direct measure of the helium abundances from the near-infrared transition of He I at 1.08 mu m is obtained for two nearly identical red giant stars in the globular cluster Omega Centauri. One star exhibits the He I line; the line is weak or absent in the other star. Detailed non-local thermal equilibrium semi-empirical models including expansion in spherical geometry are developed to match the chromospheric H alpha, H beta, and Ca Pi K lines, in order to predict the helium profile and derive a helium abundance. The red giant spectra suggest a helium abundance of Y <= 0.22 (LEID 54064) and Y = 0.39-0.44 (LEID 54084) corresponding to a difference in the abundance Delta Y >= 0.17. Helium is enhanced in the giant star (LEID 54084) that also contains enhanced aluminum and magnesium. This direct evaluation of the helium abundances gives observational support to the theoretical conjecture that multiple populations harbor enhanced helium in addition to light elements that are products of high-temperature hydrogen burning. We demonstrate that the 1.08 mu m He I line can yield a helium abundance in cool stars when constraints on the semi-empirical chromospheric model are provided by other spectroscopic features.