Spectroscopic comparison of open clusters. I. The reddening, blanketing, and metallicity of M67

Equivalent widths are determined for 13 hydrogen and metallic features in the spectra of M67 dwarfs and subgiants using 1 -mm widened image-tube spectrograms having 48 A mm -1 dispersion. These are directly compared with equivalent widths of the same features in F and G dwarfs in the Hyades and Coma clusters and nearby field dwarfs and subgiants. Assuming that equal hydrogen-line strengths indicate equal effective temperatures for dwarfs in the various clusters, relative metallicities, differential blanketing, and interstellar reddening values are determined. Comparisons with values available in the literature empirically establish the validity of the method. Eight equivalent-width ratios comparing metallic features in the M67 spectra to those in the Coma are in agreement with those comparing the mean of some nearby field stars to the Coma to within a rms scatter of only 21 percent. The averages of these eight ratios for the Hyades and M67 dwarfs indicate that the metallicity of the Hyades dwarfs is 54 percent greater than that of the Coma dwarfs, while the metallicity of the M67 dwarfs is only 14 percent greater. Thus the M67 dwarfs are not super-metal-rich. Examination of the present and previous reddening determinations for M67 leads to the adoption of E(B - V) = 0.04 mag. Considering both the internal and systematic error, the estimated mean uncertainty in the color excess is about +0.01 mag. Subject headings: abundances, stellar - interstellar reddening - open clusters