Abstract
The color-magnitude diagram and the main-sequence luminosity function of the globular cluster M13 have been investigated. While those clusters observed by the Hubble Space Telescope (HST) show a shallow luminosity function for low-mass stars, M13 has been known to have a very steep gradient toward the faint end. To understand this seemingly unique characteristic of M13, we carried out deep BV CCD observations. The observed field of nearly 56 arcmin2 is located approximately 12' from the cluster center. Our (B-V)-V color-magnitude diagram has the main sequence extended to the detection limit at V ≈ 24.5 mag. It is apparent that there is a significant population of unexpected field stars below V ≈ 22.5, which cannot be accounted for by photometric errors. When these field stars are eliminated, the derived luminosity function of M13 shows a much shallower slope at the faint end, just like other Galactic globular clusters studied by HST observations.
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