Chemical Abundances for Seven Giant Stars in M68 (NGC 4590): A Globular Cluster with Abnormal Silicon and Titanium Abundances

We present a detailed chemical abundance study of seven giant stars in M68including six red giants and one post-AGB star. We find significant differencesin the gravities determined using photometry and those obtained from ionizationbalance, which suggests that non-LTE effects are important for theselow-gravity, metal-poor stars. We adopt an iron abundance using photometricgravities and Fe II lines to minimize those effects, finding [Fe/H] = -2.16 +/-0.02. For element-to-iron ratios,we rely on neutral lines vs. Fe I and ionizedlines vs. FeII (except for [O/Fe]) to also minimize non-LTE effects. We find variations in the abundances of sodium among the program stars.However, there is no correlation (or anti-correlation) with the oxygenabundances. Further, the post-AGB star has a normal (low) abundance of sodium.Both of these facts add further support to the idea that the variations seenamong some light elements within individual globular clusters arises fromprimordial variations, and not from deep mixing. M68, like M15, shows elevated abundances of silicon compared to otherglobular clusters and comparable metallicity field stars. But M68 deviates evenmore in showing a relative underabundance of titanium. We speculate that inM68, titanium is behaving like an iron-peak element rather than its morecommonly observed adherence to enhancements seen in the "alpha" elements suchas magnesium, silicon, and calcium. We interpret this result as implying thatthe chemical enrichment seen in M68 may have arisen from contributions fromsupernovae with somewhat more massive progenitors than contribute normally toabundances seen in other globular clusters.Comment: Accepted for publication in AJ (Jan. 2005