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 moderate- and high-metallicity globular clusters ( [ Fe/H ] \geq - 1.6 ) but unusual in those of lower metallicity ( [ Fe/H ] \leq - 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 .