Photometry of Faint Stars in Globular Clusters using the Six Meter Telescope

Abstract

Our program is based on photographic and photoelectric UBV photometry of globular cluster stars with the Soviet 6 m telescope. M 10 = NGC 6254 remains, regretfully, the only cluster for which we were able to gain photoelectric observational material sufficient for calibration of the photographic photometry for faint stars. Samus and Shugarov (1983) presented for M 10 a V, (B - V) diagram showing an unusually large magnitude difference between the main sequence turn-off point and the horizontal branch, V(TO) - V(HB) ≍ 3.8 mag. It seemed of interest to compare M 10 and M 12 by their values of V(TO) - V(HB). A preliminary calibration of the faint star photographic photometry in M 12 leads to V(TO) - V(HB) ≍ 4 mag (Mironov et al., 1984). One may notice that in the classification introduced by Mironov and Samus (1974, 79), which is based mainly on the horizontal-branch morphology, both M 10 and M 12 belong to group I, presumably the older group. V, (B-V) diagrams were also published by us for NGC 288 and M 2 = NGC 7089 (Samus and Shugarov 1978, 79).