A New Approach to Zero-Age Main Sequence Fitting.

A computerized method of zero-age main sequence fitting and its applications to open clusters are presented here. Individual judgment is minimized in this method thereby guaranteeing the homogeneity of the derived results. In this approach, stars are individually dereddened by tracing them back to the intrinsic colour-colour curve (U-B vs. B -V) along reddening lines with slopes determined by both the spectral type and the colour excess of the particular star. Magnitudes are dereddened by using total-to-selective ratios which are also dependent on the spectral type and the colour excess. A zero-age main sequence is adopted to determine absolute magnitudes and individual distance moduli for cluster stars. With the aid of the colour excess vs. distance modulus diagram the mean colour excess and true distance modulus are determined for each cluster. At the same time, the selection of cluster members and binary candidates is made. Throughout this process the presence of photometric errors, differential reddening (when applicable), and binary stars is accounted for. A visual inspection of the final fits on both (or more if available) (U-B) and (B-V) colour-magnitude diagrams is made for assessing the goodness of the fit. In order to estimate the uncertainties involved in the determination of the parameters, our method is applied to simulated clusters (with pre-selected parameters). Then, applications on published photoelectric UBV data and on our own photographic and photoelectric photometry are carried out. In the light of our results, an analysis of the dependence of binary frequency with spectral type is made. A special application of our method on clusters containing Cepheids is carried out along with a discussion of the implications of our results on the Cepheid period-luminosity relation. Derivation of the turn-off colours for clusters in our sample are made; these, in turn, are linked to cluster ages. The implications of this cluster dating on cluster survival are discussed. Finally, an analysis of our results in the framework of galactic structure is made.