Old stellar populations. III. The metallicities of M 5, M 71, and M 67.

A moderate-resolution iron index (Delta Fe) has been measued for individual stars in the clusters M5, M71, and M67. At given (V - K)0, the iron lines are significantly stronger in M71 than in M5, consistent with previous moderate-resolution observations. Iron abundances based on (Delta Fe) for the three clusters are: M5: (Fe/H) = -1.11 + or - 0.16 dex, M71: (Fe/H) = -0.56 + or - 0.09 dex, M67: (Fe/H) = -0.10 + 0.12 or - 0.04 dex, where the errors quoted include both measurement error and reddening uncertainty. The data do not seem to permit an abundance for M71 as low as -0.8 dex if (Delta Fe) and the mean color of the giant branch are both taken into account. The present line-strength ranking of M5 and M71 agrees well with earlier measurements of their integrated spectra. This and agreement in previous studies support the idea that the mean metallicity of homogeneous old stellar populations can be accurately determined from either integrated spectra or the spectra of individual giant-branch stars.