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 & 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. We observed 20 M4 giant stars in 1997 using high resolutionéchelle spectrographs covering the effective wavelength range: λλ 5200 to 8800Å on the 2.7-m at McDonald Observatory (R ≈ 60, 000) and the 3.0-m at Lick Observatory (R ≈ 50, 000). Large and differential interstellar reddening due to the dark nebulosity in Scorpio-Ophiuchus does not permit a careful estimate of reddenings for individual stars; one cannot assume a strict (B−V) versus Teff relationship. Instead, we obtained precise Teffs by employing two different spectroscopic