Publications of the Astronomical Society of the Pacific
106: 486, 1994 April

    vbyHbeta CCD Photometry of Melotte 66: A Disk Analog of Omega Centauri?

        BARBARA J. ANTHONY-TWAROG, BRUCE A. TWAROG, AND MARIAN SHEERAN

          Department of Physics and Astronomy, University of Kansas,
                          Lawrence, Kansas 66045-2151
 Electronic mail: anthony@kuphsx.phsx.ukans.edu, twarog@kuphsx.phsx.ukans.edu


    ABSTRACT.  A CCD study of the peculiar open cluster, Melotte 66, on the
    vbyHBeta 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.