Star-to-Star Abundance Variations among Bright Giants in the Mildly Metal-poor Globular Cluster M4
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Abstract
We present a chemical composition analysis of 36 giants in the nearby mildly metal-poor (S[Fe/H]T \\[1.18) “ “ CN-bimodal ÏÏ globular cluster M4. The stars were observed at the Lick and McDonald Observatories using high-resolution e � chelle 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 modiÐcations 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 SE(B[V)T reddening of 0.33 ^ 0.01, which is signiÐcantly lower than that estimated by using the dust maps made by Schlegel and coworkers. We determine abundance ratios typical of halo Ðeld 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 conÐrm 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,
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