The Mildly Metal-Poor Globular Cluster M4: Star-to-Star Abundance Variations in 20 Giants

Many low-metallicity globular clusters exhibit large star-to-star variations of C., N, O, Na, Mg, & Al abundances. In higher-metallicity clusters, the abundance swings are muted. Evidence of anticorrelations of O versus Na and Mg versus Al exists in classical northern-hemisphere clusters spanning a range of metallicities, −0.8 < [Fe/H] < −2.2. These and other anticorrelations, observed to be a function of giant branch position, are consistent with material having undergone proton-capture nucleosynthesis, presumably taking place in regions where the CNO-cycle is in operation, and brought to the surface by a deep-mixing mechanism. Distinctly bimodal distributions of cyanogen strengths at nearly all giant branch positions are observed in some clusters, including M4 (Norris, 1981). The metallicity of M4 ([Fe/H] ~ − 1) places it among clusters in which the O versus Na and Mg versus Al anticorrelations are expected to be suppressed. Indeed, not much abundance variation has been observed in small samples of M4 giant stars (see e.g. Brown and Wallerstein, 1992). This puzzle led us to consider an abundance study of a large sample of bright giants in the mildly metal-poor globular cluster M4, the nearest, brightest, and one of the most accessible targets in which to study the CN-bimodal phenomenon.