CN Abundance Inhomogeneities in the Globular Cluster Messier 4 (NGC 6121): Results Based on Merged Data Sets from the Literature

Abstract

ABSTRACTIn this paper, we investigate the behavior of the λ3883 CN absorption bands in the spectra of red giants in the globular cluster Messier 4. Data from three literature sources, including the original survey of Norris, have been combined to produce a picture of the CN band strength over an absolute magnitude range of − 2.5<MV < + 0.5. In addition, measurements of carbon and oxygen abundances, as well as radial velocities, have been compiled from the literature and used to compare the properties of the CN‐strong and CN‐weak groups of giants evident in the cluster. Large CN differences are present among both red giant branch and asymptotic giant branch stars. Although the CN‐weak giants have higher [C/Fe] and [O/Fe] abundances on average than the CN‐strong giants, there is a considerable range in the abundances of both elements among the CN‐strong stars. The radial velocity dispersion of the available sample of CN‐strong stars is identical within the observational errors to that of the CN‐weak stars. A small fraction of cluster giants have CN bands of intermediate strength, and there is a curious trend for these red giants to be concentrated within the inner five core radii of the cluster, although this is still within the uncertainties of small‐sample statistics. The observations are consistent with a hybrid picture for the origin of CNO element inhomogeneities in M4. Deep mixing within stars on the upper red giant branch (MV < - 1.0) is indicated by the relatively low [C/Fe] or [O/Fe] abundances of these giants. However, the large scatter in [C/Fe] and [N/Fe] among fainter stars (at MV≳0) suggests the possibility of a pre‐RGB component to the abundance inhomogeneities. Accretion of material from a central reservoir of stellar ejecta, or mass transfer within binary star systems, are discussed as possible origins for the enrichment of the CN‐strong stars. In the case of M4, these processes must have occurred at sufficiently early times that dynamical relaxation of the cluster has largely erased any initial gradient in the relative space densities of CN‐strong and CN‐weak stars, at least within the volume of the current stellar sample that extends to three half‐mass radii from the cluster center.