New Analyses of Star-to-Star Abundance Variations among Bright Giants in the Mildly Metal-poor Globular Cluster M5

Abstract

We present a chemical composition analysis of 36 giant stars in themildly metal-poor ({\lt}[Fe/H]{\gt}=-1.21) globular cluster M5 (NGC 5904).The analysis makes use of high-resolution data acquired for 25 stars atthe Keck I telescope, as well as a reanalysis of the high-resolutionspectra for 13 stars acquired for an earlier study at Lick Observatory.We employed two analysis techniques: (1) adopting standard spectroscopicconstraints, including setting the surface gravity from the ionizationequilibrium of iron, and (2) subsequent to investigating alternativeapproaches, adopting an analysis consistent with the non-LTE precepts asrecently described by Th{é}venin {\amp} Idiart. The abundance ratioswe derive for magnesium, silicon, calcium, scandium, titanium, vanadium,nickel, barium, and europium in M5 show no significant abundancevariations, and the ratios are comparable to those of halo field stars.However, large variations are seen in the abundances of oxygen, sodium,and aluminum, the elements that are sensitive to proton-capturenucleosynthesis. These variations are well-correlated with the CN bandstrength index S(3839). Surprisingly, in M5 the dependence of theabundance variations on logg is in the opposite sense to that discoveredin M13 by the Lick-Texas group where the relationship provided strongevidence in support of the evolutionary scenario. The present analysisof M5 giants does not necessarily rule out an evolutionary scenario, butit provides no support for it either. In comparing the abundances of M5and M4 (NGC 6121), another mildly metal-poor ({\lt}[Fe/H]{\gt}=-1.08)globular cluster, we find that silicon, aluminum, barium, and lanthanumare overabundant in M4 with respect to that seen in M5, confirming andexpanding the results of previous studies. In comparing the abundancesbetween these two clusters and others having comparable metallicities,we find that the anticorrelations observed in M5 are similar to thosefound in more metal-poor clusters, M3, M10, and M13 ({\lt}[Fe/H]{\gt}=-1.5to -1.6), whereas the behavior in M4 is more like that of the moremetal-rich globular cluster M71 ({\lt}[Fe/H]{\gt}~-0.8). We conclude thatamong stars in Galactic globular clusters there is no definitive``single'' value of [X/Fe] at a given [Fe/H] for at least some{α}-capture, odd-Z, and slow neutron-capture process elements, inthis case, silicon, aluminum, barium, and lanthanum. Based in part onobservations obtained with the W. M. Keck Observatory, which is operatedby the California Association for Research in Astronomy, Inc., on behalfof the University of California, the California Institute of Technology,and the National Aeronautics and Space Administration.