CCD Photometry of the Globular Cluster M53. I. Color-Magnitude Data and Blue Straggler Stars

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Abstract
The first large-format CCD color-magnitude diagram (CMD) in the B and V passbands is presented for the Galactic globular cluster M53 (NGC 5024). The observations consist of 40 pairs of B and V CCD frames with FWHM < 12 in a pattern covering a 12' × 13' region of M53. These frames were used to produce color-magnitude data for more than 20,000 stars from the tip of the red giant branch (RGB) to about 2 mag below the main-sequence turnoff (V ~ 22). The CMD reveals a classic example of a metal-poor stellar system, with the horizontal branch (HB) predominantly blueward of the RR Lyrae instability strip and the relatively steep RGB. The blue HB of M53 does not show the blue tail phenomenon that is observed in M15. The relative age dating based on the color difference between the turnoff and the base of RGB reveals no significant age difference (Δt < 1 Gyr) between M53 and M92. We have discovered 114 new blue straggler stars (BSSs) in the field of M53. The analysis of bright (V < 19.39) BSSs clearly shows a bimodal radial distribution 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.