CHEMICAL-COMPOSITION OF OPEN CLUSTERS .3. IRON AND CARBON IN F-DWARFS IN COMA, PRAESEPE, AND M67

We have determined iron and carbon abundances for F dwarfs in several open clusters and the field to continue our investigations on the homogeneity of chemical mixing in the Galactic disk. These new data on stars in Coma, Praesepe, M67, and the field complement our previous sample of clusters and moving groups and extend the sample coverage in metallicity and age. The data are high resolution, high signal-to-noise Reticon observations made with the Canada-France-Hawaii Telescope coude. Abundances are based on measurements of eight Fe I lines and six high excitation C I lines in the spectral region lambda-lambda-7065-7155 analyzed with model atmospheres. Our results for these new clusters are consistent with the conclusions drawn from our previous sample. Intrinsic dispersions in [Fe/H] and [C/H] in each cluster are consistent with observational error alone, and are extremely small. The [C/Fe] ratio for all clusters is solar to within observational error over the metallicity range [Fe/H] greater-than-or-equal-to -0.50, and has an intrinsic dispersion of much less than 0.1 dex. Cluster mean values of [Fe/H] and [C/H] show cluster-to-cluster variations well in excess of observational errors, indicating both that the overall metal content of the gas from which these clusters formed preserved small, but significant differences over periods of several billion years, and that the production of carbon closely followed that of iron so that these regions of star formation had the same ratio of C/Fe. The mean value of [C/Fe] for the 84 stars in our three papers with values of [Fe/H] > -0.2 is -0.031 +/- 0.009 (error in the mean). Five metal-poor stars which have -1.3 < [Fe/H] < -0.5 have <[C/Fe]> = +0.140 +/- 0.044 (error in the mean), indicating, in the absence of systematic errors in adopted parameters, that these stars of intermediate metallicity have slightly enhanced values of [C/Fe], as predicted by some models of Galactic chemical evolution.