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

Details The Mildly Metal-Poor Globular Cluster M4 (NGC6121): Star-to-Star Abundance Variations in 37 Giants The Mildly Metal-Poor Globular Cluster M4 (NGC6121): Star-to-Star Abundance Variations in 37 Giants

Dec 1998

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998aas...193.6811i&link_type=abstract

American Astronomical Society, 193rd AAS Meeting, #68.11; Bulletin of the American Astronomical Society, Vol. 30, p.1348

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Scientific paper

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 echelle 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 <= [Fe/H] <= --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.

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