ABUNDANCES FOR GLOBULAR-CLUSTER GIANTS .1. HOMOGENEOUS METALLICITIES FOR 24 CLUSTERS

We have obtained high resolution, high signal-to-noise ratio CCD echel le spectra of 10 bright red giants in 3 globular clusters (47 Tuc, NGC 6752 and NGC 6397) roughly spanning the whole range of metallicities of the galactic globular cluster system. The analysis of this newly ac quired material reveals no significant evidence of star-to-star variat ion of the [Fe/H] ratio in these three clusters. Moreover, a large set of high quality literature data (equivalent widths from high dispersi on CCD spectral was reanalyzed in an homogeneous and self-consistent w ay to integrate our observations and derive new metal abundances for m ore than 160 bright red giants in 24 globular clusters (i.e. about 16% of the known population of galactic globulars). This set was then use d to define a new metallicity scale for globular clusters which is the result of high quality, direct spectroscopic data, of new and updated model atmospheres from the grid of Kurucz (1992), and of a careful fi ne abundance analysis; this last, in turn, is based on a common set of both atomic and atmospheric parameters for all the stars examined. Gi ven the very high degree of internal homogeneity, our new scale supers edes the offsets and discrepancies existing in previous attempts to ob tain a metallicity scale. The internal uncertainty in [Fe/H] is very s mall: 0.06 dex (24 clusters) on average, and can be interpreted also a s the mean precision of the cluster ranking. Compared to our system, m etallicities on the widely used Zinn and West's scale are about 0.10 d ex higher for [Fe/H]> -1, 0.23 dex lower for -1 <[Fe/H]< -1.9 and 0.11 dex too high for [Fe/H]< -1.9. The non-linearity of the Zinn and West 's scale is significant even at 3 sigma level. A quadratic transformat ion is given to correct older values to the new scale in the range of our calibrating clusters (-2.24 less than or equal to[Fe/H](zw) less t han or equal to -0.51). A minor disagreement is found at low metallici ties between the metallicity scale based on field and cluster RR Lyrae variables (via a new calibration of the Delta S index) and our new cl uster metallicities. It could be tentatively ascribed to non-linearity in the [Fe/H]-Delta S relationship. The impact of new metallicities o n major astrophysical problems is exemplified through a simple exercis e on the Oosterhoff effect in the classical pair M 3 and M 15.