CCD Stromgren studies in NGC 6397
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CCD vby photometry has been obtained in six fields of NGC 6397 and used to delineate the principal sequences for this metal-poor globular cluster. Based upon the distribution in the m1, (b - y) diagram, a sample of highly probable members has been identified. From this sample, photometric binary candidates have been selected from the main sequence by comparison of their location in the color-magnitude diagram to the well-defined fiducial relation in V, (b - y). It is found that 7% of the main-sequence members exhibit evidence for a composite nature. Though the photometric discrepancies appear real in that the fields are well away from the cluster core and the binary images do not suffer from any identifiable abnormalities, the binary fraction is consistent with an optical rather than a physical association of the superposed pair. For a reddening value of E(b - y) = 0.14 and [Fe/H] = -2.0 an apparent distance modulus of (m - M) = 12.1 is derived if the parallax-calibrated main sequence of Laird et al. [AJ, 95, 1843 (1988)] is compared to the fiducial cluster main sequence, leading to an age greater than 18 109 yr from the luminosity of the turnoff. In contrast, a modulus of (m - M) = 12.4 is derived if the oxygen-enhanced isochrones detailed in McClure et al. [AJ, 93, 1144 (1987)] are adopted as the standard main-sequence relation; the corresponding age can range between 14 and 18 109 yr for only minor adjustments in the luminosity of the turnoff. Use of the differential approach of Sarajedini & King [AJ, 98, 1624 (1989)] places NGC 6397 in the old tail of a broad globular cluster age distribution.
