Cn Bimodality at Low Metallicity: the Globular Cluster M53

We present low resolution UV-blue spectroscopic observations of red giant stars in the globular cluster M53 ([Fe/H]= −1.84), obtained to study primordial abundance variations and deep mixing via the CN and CH absorption bands. The metallicity of M53 makes it an attractive target: a bimodal distribution of 3883 Å CN bandstrength is common in moderateand high-metallicity globular clusters ([Fe/H] ≥ −1.6) but unusual in those of lower metallicity ([Fe/H] ≤ −2.0). We find that M53 is an intermediate case, and has a broad but not strongly bimodal distribution of CN bandstrength, with CN and CH bandstrengths anticorrelated in the less-evolved stars. Like many other globular clusters, M53 also exhibits a general decline in CH bandstrength and [C/Fe] abundance with rising luminosity on the red giant branch. Subject headings: star clusters and associations