VBY H-BETA CCD PHOTOMETRY OF MELOTTE-66 - A DISK ANALOG OF OMEGA-CENTAURI

A CCD study of the peculiar open cluster, Melotte 66, on the nu by Hbeta system is presented. The V,(b-y) color-magnitude diagram of this metal-deficient, old disk cluster confirms the anomalous features found in photographic BV studies and extends the main sequence to V=20. The main sequence exhibits a (b - y) color range significantly larger than expected from photometric errors alone; the (b - y) colors are correlated with Hbeta implying that the dispersion is real and is not the result of variable reddening across the cluster. The m1 indices for the turnoff stars imply a spread in [Fe/H] too small to explain the color range. Though the subgiant region is poorly defined, the region above the turnoff in the Hertzsprung gap is richly populated by a group of yellow stragglers whose membership is confirmed by radial star counts and radial velocities. The giant branch is richly populated but shows a smaller spread in color at a given V than one would expect from the turnoff region. More important, the giants exhibit a large scatter in m1, in contradiction with the turnoff observations. Various explanations for these observations are discussed, including CN variations among the giants triggered by either binary evolution or rapid rotation among the main-sequence stars.