Abstract

We have obtained high-resolution, high-signal-to-noise-ratio echelle CCD spectra of three red giants in the metal-poor globular cluster NGC 2298. A detailed analysis of their chemical composition yields [Fe/H] = - 1.91 +/- 0.1 (using a solar Fe abundance of 7.52), where the error includes the dispersion in the observed values and uncertainties in the model-atmosphere parameters. From spectrum synthesis of the [O I] line at 6300 angstrom, we find [O/Fe] = + 0.2 +/- 0.2, with all three giants having the same ratio within the errors. These stars fall somewhat below the mean trend between [O/Fe] and M(V) exhibited by the O-rich sample in Sneden et al. (1991). The alpha elements Mg, Si, and Ca are enhanced more than oxygen, with an average of [alpha/Fe] = - + 0.47, suggesting an oxygen depletion as might be expected from proton burning by the CNO tri-cycle. Al is strongly overabundant in all three stars relative to typical values found in field halo giants of similar metallicity. This Al overabundance could be due to proton burning of the envelope material by the Mg-Al cycle. The apparent dispersion in Al abundance would suggest that the Al overabundance is due to self-enrichment by the individual stars, rather than Al-rich primordial gas, but this conclusion requires verification. Based on our metallicity and composition we apply corrections to the estimated age of NGC 2298 found by previous studies, giving an age range of 14-18 Gyr. Our O and alpha-element abundances for NGC 2298, combined with those of other recent investigations of the most metal-poor globular clusters, indicate substantially lower values than measured by Abia and Rebolo (1989) or assumed O enhancements in the isochrones of McClure et al. (1987), leading to somewhat larger ages for these most ancient Galactic systems.