ABSTRACT
Based on a large sample of stars with accurate spectroscopic abundance determinations, a reexamination of the abundance calibrations of the Washington system for late-type giants with Fe/H abundance ratios of less than -0.5 is presented. The Fe abundance sensitivity of the Delta(M-T1) index is found to be limited for stars more metal-poor than Fe/H abundances of about -1, while the Delta (C-M) index is shown to be sensitive to Fe abundances throughout the entire range of known stellar abundances. Observations of a number of giants in the 47 Tuc and M 4 globular clusters demonstrate that the system can differentiate CN-strong or CH-strong giants from normal giants most effectively for giants more metal-rich than 0.1 solar, and the Delta(C-M) index is found to be much more sensitive to Fe abundance than CN or CH strength. An Fe/H abundance value of -0.85 + or - 0.1 for 47 Tuc is derived which is in good agreement with previous results and which contradicts the suggestion of a significantly lower Fe abundance.