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I.I. Ivans, C. Sneden (UT-Austin), R.P. Kraft (UCSC), N.B. Suntzeff (CTIO), V.V. Smith (UTEP), G.E. Langer (Colorado C)
We present a chemical composition analysis of a sample of 37 giant branch stars in the ``CN-bimodal'' globular cluster M4 observed using high resolution échelle spectrographs at Lick & McDonald Observatories and the multi-object spectrometer instrument at CTIO. M4, a mildly metal-poor ([Fe/H] = --1.18) globular cluster, is the nearest, brightest, and one of the most accessible targets to study the CN-bimodal phenomena. In M4 we find anti-correlated abundances of N/Na/Al with C/O, similar to the anti-correlations found in other classical northern-hemisphere clusters spanning a range of metallicities, from --0.8 \le [Fe/H] \le --2.24, including M3, M5, M10, M13, M15, M71, M92, & NGC7006. The anti-correlations, observed to be a function of giant branch position, are consistent with material having undergone high-temperature proton-capture nucleosynthesis, presumably taking place in regions where the CNO cycle operates, and brought to the surface by a deep-mixing mechanism.\\
We also find elements Na, Mg, Si, & Ca all enhanced with respect to the iron abundance, in approximate agreement with previous high resolution studies. Extremely low carbon isotope ratios indicate onset of mixing of CNO-cycled material occurs at a luminosity lower than that of our observed stars. Al & Ba show uniform enhancements indicative of primordial enrichment. Increasing O-depletion/N-enhancement is observed as stars ascend RGB but combined (C+N+O) remains constant. Our analysis also confirms anomalous absorption properties of dust along the line of sight to M4 that deviate from the normal law of interstellar extinction. \\ \\ \\
The author(s) of this abstract have provided an email address for comments about the abstract: iivans@astro.as.utexas.edu