THE ASTRONOMICAL JOURNAL      VOLUME 106, NUMBER 1, PAGE 181          JULY 1993

                  CCD PHOTOMETRY OF THE OLD OPEN CLUSTER M67

                              KENT A. MONTGOMERY

    Department of Astronomy, Boston University, Boston, Massachusetts 02215
                      Electronic mail: kent@hyades.bu.edu

                             LAURENCE A. MARSCHALL

   Department of Physics, Gettysburg College, Gettysburg, Pennsylvania 17325
                  Electronic mail: marschal@cc.gettysburg.edu

                               KENNETH A. JANES

    Department of Astronomy, Boston University, Boston, Massachusetts 02215
                      Electronic mail: janes@hyades.bu.edu


                                   ABSTRACT

    We present a CCD photometric survey of the central one-half degree of
    the old open cluster, M67, in U, B, V, and I colors to magnitude V=20.
    Extensive comparison of our photometry with other published datasets
    shows excellent agreement, indicating that CCD photometry is capable of
    producing a uniform set of measurements consistent with the photometric
    system defined primarily by the Landolt standard sequence.  The
    color-magnitude diagram of the cluster shows a well-defined main
    sequence extending at least to the limit of the photometry at
    M(V)=10.55 and a substantial binary sequence.  At least 38% of cluster
    stars are binaries.  The current generation of theoretical isochrones
    cannot be fit to the observed sequences within the observational
    errors.  We find a tendency for more massive members of the cluster to
    be more centrally concentrated, along with a turnover in the cluster
    luminosity function at low masses, which may be due to dynamical
    relaxation of the cluster.  To the limit of our photometry, we find a
    mass of the cluster of 724 solar masses.  In addition, we present a
    sample of stars of well-determined standard magnitudes that are
    suitable as photometric standards for further studies of this cluster
    and for general calibration of the UBVI photometry using CCDs.