The abundance of oxygen in M92 giant stars

The abundance of oxygen has been determined for six giant stars in the metal-poor globular cluster M92 from high-resolution, CCD echelle spectra. The abundance of oxygen varies from star to star and is anti-correlated with the abundance of nitrogen determined by Carbon et al. (1982), in such a way that the sum of the carbon, nitrogen, and oxygen abundances remains constant at + 0.03 + or - 0.05, despite large variations in the abundance of nitrogen from star to star. The constancy of C + N + O suggests that the abundance differences are due to variation in the degree of mixing, and not to initial differences in the amount of C + N + O. Furthermore, the anticorrelation of oxygen and nitrogen suggests that the ON cycle has been active to modify the surface C + N + O abundances in M92 giants. M92 giants also contain only a small excess of oxygen relative to iron; an oxygen excess may not be cited to reduce the derived age of the cluster.