Light-element chemistry and the double red giant branch in the galactic globular cluster NGC 288

The globular cluster NGC 288 was previously reported to exhibit two distinct red giant branches (RGBs) in the narrowband calcium (HK) and Strömgren b- and y-band passes. In order to investigate this phenomenon further, we obtained moderate resolution (R ~ 18, 000) spectra of 27 RGB stars in NGC 288 with the Hydra multifiber spectrograph on the Blanco 4 m telescope at Cerro Tololo Inter-American Observatory (CTIO). From these data we derive iron (〈[Fe/H]〉 = -1.19; σ = 0.12), oxygen (〈[O/Fe]〉 = +0.25; σ = 0.13), and sodium (〈[Na/Fe]〉 = +0.15; σ = 0.26) abundances using standard equivalent width and spectrum synthesis techniques. Combining these data with those available in the literature indicates that the two giant branches have distinctly different light-element chemistry but do not exhibit a significant spread in [Fe/H]. A new transmission tracing for the CTIO Ca filter, obtained for this project, shows that CN contamination is the primary spectral feature driving the split RGB. Interestingly, the CN leak in the current CTIO Ca filter may be used as an efficient means to search for CN-weak and CN-strong stars in systems with otherwise small Ca abundance variations.