High-Resolution Infrared Spectroscopy of the Old Open Cluster NGC 6791

We report abundance analysis for six M giant members of the old open cluster NGC 6791, based on infrared spectroscopy (1.5-1.8 mum) at R=25,000, using the NIRSPEC spectrograph at the Keck II telescope. We find the iron abundance =+0.35+/-0.02, confirming the supersolar metallicity of this cluster as derived from optical medium-high resolution spectroscopy. We also measure C, O, and other alpha-element abundances, finding a roughly solar value of [alpha/Fe] and =-0.35. Our approach constrains [O/Fe] especially well, on the basis of the measurement of a number of OH lines near 1.6 mum we find [O/Fe]=-0.07+/-0.03. The solar value of [alpha/Fe] is in contrast to the composition of similar stars in the Galactic bulge. We also find a low value of 12C/13C~10, confirming the presence of extramixing processes during the red giant phase of evolution, up to supersolar metallicities.