Light Element Chemistry and the Double Red Giant Branch in the Galactic Globular Cluster NGC 288

TL;DR

This study investigates the double red giant branch in NGC 288, revealing that CN contamination in the Ca filter causes the split, with light element differences but no significant iron spread.

Contribution

It demonstrates that CN contamination explains the RGB split and introduces a new method to identify CN-weak and CN-strong stars using the Ca filter.

Findings

RGB split driven by CN contamination in Ca filter

Light element chemistry differs between the two RGBs

No significant [Fe/H] spread observed

Abstract

The globular cluster NGC 288 was previously reported to exhibit two distinct red giant branches (RGBs) in the narrow-band Calcium (HK) and Str\"omgren b and y band passes. In order to investigate this phenomenon further, we obtained moderate resolution (R$\sim$18,000) spectra of 27 RGB stars in NGC 288 with the Hydra multifiber spectrograph on the Blanco 4m telescope at Cerro Tololo Inter-American Observatory. From these data we derive iron ($\langle$[Fe/H]$\rangle$=-1.19; $\sigma$=0.12), oxygen ($\langle$[O/Fe]$\rangle$=$+$0.25; $\sigma$=0.13), and sodium ($\langle$[Na/Fe]$\rangle$=$+$0.15; $\sigma$=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.