AAS Meeting #193 - Austin, Texas, January 1999
Session 68. Globular Clusters
Display, Friday, January 8, 1999, 9:20am-6:30pm, Exhibit Hall 1

## [68.11] The Mildly Metal-Poor Globular Cluster M4 (NGC6121): Star-to-Star Abundance Variations in 37 Giants

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. \\ \\ \\