Abundances in globular cluster red giants. III - M71, M67, and NGC 2420

High-dispersion spectra of four stars in the metal-rich globular cluster M71, four giants in the old open cluster M67, and two members of the metal-poor old open cluster NGC 2420 are analyzed with the aid of model atmospheres. The derived abundances are Fe/H = -1.27, -0.39, and -0.61 dex with regard to the sun for M71, M67, and NGC 2420, respectively. The pattern of elemental abundances is very similar over the full range from M92 to M67, with the most obvious differences being the strong overdepletion of copper, and perhaps scandium, with progressively lower metallicities. The metallicity derived for M71 is much lower than previous estimates, and the consequences of this for the abundance scale of globular clusters and the formation of the halo and disk of the Galaxy are discussed.