CCD uvbyHβ photometry in clusters.: I.: The open cluster standard, IC 4651

CCD photometry of the intermediate-age open cluster IC 4651 on the uvbyH beta system is presented and analyzed. By using a combination of the information from the color-magnitude diagram (CMD) and the color-color diagrams, a sample of 98 highly probable main-sequence cluster members with high photometric accuracy is isolated. From this sample, adopting the intrinsic color relation of Olsen, E(b - y) = 0.062 +/- 0.003 and [Fe/H] = +0.077 +/- 0.012, where the errors quoted are the standard errors of the mean and refer to the internal errors alone. Use of the Nissen intrinsic color relation produces E(b - y) = 0.071 and [Fe/H] = +0.115. Adopting the lower reddening, a direct main-sequence fit to the Hyades with (m - M) = 3.33 leads to (In - M) = 10.15, while isochrones with convective overshoot and zeroed to the Hyades produce an age of 1.7 +/- 0.1 Gyr, with an excellent match to the morphology of the turnoff. The higher reddening produces (m - M)= 10.3 and an age lower by 0.1 Gyr. Comparison with the CMD of NGC 3680 shows that the two clusters have virtually identical morphology, which in combination with their similar compositions produces identical ages. Coincidentally, the shifts in the CMD necessary to superpose the two clusters require that the apparent moduli of IC 4651 and NGC 3680 be the same, while E(b - y)(4651) = E(b - y)(3680) + 0.04.