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

Abstract

We present a detailed chemical abundance study of seven giant stars in M68 including six red giants and one post-AGB star. We find significant differences in the gravities determined using photometry and those obtained from ionization balance, which suggests that non-LTE effects are important for these low-gravity, metal-poor stars. We adopt an iron abundance using photometric gravities 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 ionized lines 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 oxygen abundances. 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 seen among some light elements within individual globular clusters arises from primordial variations, and not from deep mixing. M68, like M15, shows elevated abundances of silicon compared to other globular clusters and comparable metallicity field stars. But M68 deviates even more in showing a relative underabundance of titanium. We speculate that in M68, titanium is behaving like an iron-peak element rather than its more commonly observed adherence to enhancements seen in the "alpha" elements such as magnesium, silicon, and calcium. We interpret this result as implying that the chemical enrichment seen in M68 may have arisen from contributions from supernovae with somewhat more massive progenitors than contribute normally to abundances seen in other globular clusters.