CCD PHOTOMETRY OF THE GLOBULAR-CLUSTER M53 - I - COLOR-MAGNITUDE DATAAND BLUE STRAGGLER STARS

The first large-format CCD color-magnitude diagram (CMD) in the B and V passbands is presented for the Galactic globular cluster M53 (NGC 50 24). The observations consist of 40 pairs of B and V CCD frames with F WHM < 1.'' 2 in a pattern covering a 12' x 13' region of M53. These fr ames were used to produce color-magnitude data for more than 20,000 st ars from the tip of the red giant branch (RGB) to about 2 mag below th e main-sequence turnoff (V similar to 22). The CMD reveals a classic e xample of a metal-poor stellar system, with the horizontal branch (HB) predominantly blueward of the RR Lyrae instability strip and the rela tively steep RGB. The blue HE of M53 does not show the blue tail pheno menon that is observed in M15. The relative age dating based on the co lor difference between the turnoff and the base of RGB reveals no sign ificant age difference (Delta t < 1 Gyr) between M53 and M92. We have discovered 114 new blue straggler stars (BSSs) in the field of M53. Th e analysis of bright (V < 19.39) BSSs clearly shows a bimodal radial d istribution with a high frequency in the inner and outer regions but a distinct dip in the intermediate region. The distribution is similar to that found in M3, a globular cluster with a similar central density and concentration.