Star-to-Star Abundance Variations among Bright Giants in the Mildly Metal-poor Globular Cluster M4

We present a chemical composition analysis of 36 giants in the nearby mildly metal-poor ( = -1.18) "CN-bimodal" globular cluster M4. The stars were observed at the Lick and McDonald Observatories using high-resolution échelle spectrographs and at the Cerro Tololo Inter-American Observatory using the multiobject spectrometer. Confronted with a cluster having interstellar extinction that is large and variable across the cluster face, we combined traditional spectroscopic abundance methods with modifications to the line depth ratio technique pioneered by Gray to determine the atmospheric parameters of our stars. We derive a total-to-selective extinction ratio of 3.4 ± 0.4 and an average ⟨E(B-V)⟩ reddening of 0.33 ± 0.01, which is significantly lower than that estimated by using the dust maps made by Schlegel and coworkers. We determine abundance ratios typical of halo field and cluster stars for scandium, titanium, vanadium, nickel, and europium with star-to-star variations in these elements of less than ±0.1. Silicon, aluminum, barium, and lanthanum are overabundant with respect to what is seen in other globular clusters of similar metallicity. These overabundances confirm the results of an earlier study by Brown & Wallerstein based on a much smaller sample of M4 giants. Superposed on the primordial abundance distribution is evidence for the existence of proton capture synthesis of carbon, oxygen, neon, and magnesium. We recover some of the C, N, O, Na, Mg, and Al abundance swings and correlations found in other more metal-poor globular clusters, but the range of variation is muted. In the case of Mg and Al, this is compatible with the idea that the Al enhancements are derived from the destruction of 25,26Mg, not 24Mg. We determine that the C + N + O abundance sum is constant to within the observational errors and agrees with the C + N + O total that might be expected for M4 stars at birth. The asymptotic giant branch (AGB) stars in M4 have C, N, and O abundances that show less evidence for proton capture nucleosynthesis than is found in the less evolved stars of the red giant branch (RGB). Deeply mixed stars of the RGB, subsequent to the helium core flash, might take up residence on the blue end of the horizontal branch and thus fail to evolve back to the AGB, but reasons for skepticism concerning this scenario are noted.