THE ASTRONOMICAL JOURNAL    VOLUME 108, NUMBER 2, PAGE 555          AUGUST 1994

           BVI CCD PHOTOMETRY OF GALACTIC GLOBULAR CLUSTERS. II. M68

                              ALISTAIR R. WALKER

    Cerro Tololo Inter-American Observatory, Casilla 603, La Serena, Chile
                       Electronic mail: awalker@noao.edu


                                   ABSTRACT
    
    A total of 363 CCD frames in the B, V, and I passbands have been used
    to study the color-magnitude diagram (CMD) and the variable-star
    population of the metal-poor galactic globular cluster M68 (=NGC
    4590,=C1236-264).  Light curves have been prepared for 40 RR Lyrae
    variables, five of which are new discoveries, and for two probable SX
    Phe stars.  The RR Lyraes have <V>=15.64 +/- 0.01, and via several
    methods E(B-V)=0.07 +/- 0.01.  [Fe/H]=-2.1 is adopted.  From a
    comparison with Sandage's data for M3 (=NGC 5272,=C1339+286), a period
    shift of Delta(log P)/Delta([Fe/H])=-0.11 +/- 0.02 is derived, and the
    mass-to-light parameter is found to be <A>=1.90.  RR Lyrae masses are
    derived from Fourier fits to the light curves, from the RRd variables,
    and by comparison with HB (horizontal branch) evolutionary models
    calculated by Dorman.  The light-curve structure agrees qualitatively
    with recent theoretical models.  From short exposures, CMDs have been
    prepared that are complete from the tip of the red giant branch (RGB)
    to below the level of the HB, while from the long exposures a CMD to
    V ~ 23 is provided for 7298 stars more than 158 arcsec distant from the
    cluster center.  The HB is skewed to the blue with the eight bluest
    stars clearly separated from the remainder of the HB stars.  This HB
    gap is at the identical color to that in M15 (=NGC 7078,C2127+119), but
    M68 lacks the extensive population of the very blue HB stars present in
    M15.  The total number of HB stars is deficient with respect to the RGB
    population, with the ratio of RGB to HB stars R=0.99.  There are
    several likely blue stragglers identified on CMDs of inner annuli, but
    near to the cluster center photometric errors scatter stars off the
    main sequence (MS) into this region.  The deep CMDs are in excellent
    agreement with existing CCD CMDs for M68, with the MS turnoff
    Delta(V)=3.42 +/- 0.10 mag below the HB.  Matching the M68 CMD to that
    for M15 in the subgiant-MS turnoff region shows that the age of M68 is
    identical to that for M15 to within +/-0.4 Gyr.