CN Bimodality at Low Metallicity: The Globular Cluster M53

TL;DR

This study investigates the CN and CH absorption features in red giant stars of the low-metallicity globular cluster M53, revealing a broad CN distribution and anticorrelation with CH, contributing to understanding chemical variations in such clusters.

Contribution

It provides the first detailed analysis of CN and CH bandstrength distributions in M53, highlighting its intermediate bimodality status at low metallicity.

Findings

M53 shows a broad, not strongly bimodal, CN distribution.

CN and CH bandstrengths are anticorrelated in less-evolved stars.

CH bandstrength declines with increasing luminosity.

Abstract

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 angstrom CN bandstrength is common in moderate- and 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.