The main-sequence luminosity function of M13

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.