ON THE METALLICITY AND THE TURNOFF AGE OF NGC-188

High-dispersion echelle spectra of seven stars located within the turnoff region in NGC 188 are analyzed, using the model atmospheres of Kurucz. An average iron abundance of the cluster, relative to the Sun, of [Fe/H] = -0.12 ± 0.16 is derived from the observed equivalent widths of up to six weak Fe II lines in each of the seven spectra and in the solar spectrum. With this known relative iron abundance, the equivalent widths of up to eight weak Fe I lines in these same spectra then require that the effective temperatures of the turnoff stars average 〈Te〉turnoff = 5885 ± 120 K. The uncertain Fe II oscillator strengths were calibrated by requiring that, for the physically very similar Sun, the identical analysis recover the correct effective temperature from the same Fe II and Fe I lines. Finally, a turnoff age of τ = 7.7 ± 1.4 Gyr is derived by comparing the deduced upper limit on these turnoff temperatures to appropriate isochrones in the theoretical H-R diagram, as calculated anew by VandenBerg (1990). A primary advantage of this purely spectroscopic method is that it effectively requires no estimates of the reddening or the intrinsic colors of the cluster stars or of the distance to the cluster; the attendant uncertainties in these cluster parameters are thereby circumvented entirely. A similar, auxiliary investigation of the luminosities of the cluster stars in the H-R diagram favors the young end of the cited range of ages, but it is less intrinsically reliable owing to its sensitivity to possible errors in the assumed cluster parameters.