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

摘要

We present a detailed chemical abundance study of seven giant stars in M68, including six red giants and one post–asymptotic giant branch (AGB) star. We find significant differences in the gravities determined using photometry and those obtained from ionization balance, which suggests that non-LTE (NLTE) affects 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 (σ = 0.04). For element-to-iron ratios, we rely on neutral lines versus Fe I and ionized lines versus Fe II (except for [O/Fe]) to also minimize NLTE effects. We find variations in the abundances of sodium among the program stars. However, there is no correlation (or anticorrelation) with the oxygen abundances. Furthermore, 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 arise from primordial variations and not from deep mixing. M68, like M15, shows elevated abundances of silicon compared with 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 so-called α-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 those that contribute to abundances normally seen in other globular clusters. The neutron capture elements barium and europium vary among the stars in M15, according to earlier work by Sneden et al., but [Ba/Eu] is relatively constant, suggesting that both elements arise in the same nucleosynthesis events. M68 shares the same [Ba/Eu] ratio as the stars in M15, but the average abundance ratio of these elements, as well as lanthanum, are lower in M68 relative to iron than in M15, implying a slightly weaker contribution of r-process nucleosynthesis in M68.