The globular cluster system of the galaxy. III - Measurements of radial velocity and metallicity for 60 clusters and a compilation of metallicities for 121 clusters

Abstract

Spectrograms (121 A mm `)have been obtained of 60 galactic globular clusters and of three globular clusters in the Small Magellanic Cloud. The radial velocities measured from these spectrograms by the cross-correlation technique have an accuracy of +20 km and are in good agreement with most previous measurements. They support Shawl, Hesser, and Meyer's conclusion that some of Mayall's measurements for southern clusters are substantially in error. The pseudoequivalent widths of the hydrogen lines H , Hy, and H and the Ca ii K line (K), the Ci band (Ci) and the Mg I b lines (Mg) have been measured. Most galactic globular clusters have very similar H line strengths, but the lines of four clusters (NOC 6535, 6642, 6717, and 7099 [M30]) are unusually strong. This peculiarity of NCiC 7099 had been noted earlier by Rabin, who speculated that it is related to the color gradient in the cluster. Our spectroscopic observations of two regions offset from the center of NCiC 7099 confirm Rabin's suggestion. It is not known whether the other strong-lined clusters possess similar color and H line gradients or are more homogeneous. The K, Ci, and Mg pseudo-equivalent widths correlate very well with each other and' with the photometric integrated light index Q39. The relationships between Q39 and these widths have been used to transform them into equivalent values of Q39. For the 14 clusters that had not been included in Paper I, values of E( B - V) have been estimated from their apparent B - V colors and the relationship between Q39 and (B-V)0. The metallicity ranking provided by Q39 has been compared with ones in the literature that are based on observations of individual cluster stars (i.e., the analyses of high-dispersion echelle spectrograms of red giants, measures of giant-branch effective temperature, S measurements of RR Lyrae variables, and spectrophotometry or photometry of red giants). These comparisons, which involve 50 clusters, indicate that Q39 is able to rank clusters with a precision of +0.15 in [Fe/H], which is close to that attained by the best of these other methods. It appears that Q39 is not affected much by the variation in horizontal-branch morphology that exists among clusters of similar metallicity. For 121 galactic globular clusters, the metallicities derived from observations of individual stars, the integrated light, or both, have been placed on Cohen's new metallicity scale. The values obtained for the most metal-rich and metal-poor clusters by extrapolating the scale are [Fe/H] = +0.24 and -2.58, respectively. Subject headings: clusters: globular - galaxies: Magellanic Clouds - galaxies: Milky Way - radial velocities - stars: abundances