CCD [ITAL][CLC]uvby[/CLC][/ITAL]Hβ Photometry in Clusters. I. The Open Cluster Standard, IC 4651

CCD photometry of the intermediate-age open cluster, IC 4651, on theuvbyHbeta system is presented and analyzed. By using a combination of theinformation from the color-magnitude diagram (CMD) and the color-colordiagrams, a sample of 98 highly probable main sequence cluster members withhigh photometric accuracy is isolated. From this sample, adopting the intrinsiccolor relation of Olsen (1988), 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 andrefer to the internal errors alone. Use of the Nissen (1988) intrinsic colorrelation produces E(b-y) = 0.071 and [Fe/H] = +0.115. Adopting the lowerreddening, a direct main-sequence fit to the Hyades with (m-M) = 3.33 leads to(m-M) = 10.15, while isochrones with convective overshoot and zeroed to theHyades produce an age of 1.7 +/- 0.1 Gyr, with an excellent match to themorphology of the turnoff. The higher reddening produces (m-M) = 10.3 and anage lower by 0.1 Gyr. Comparison with the CMD of NGC 3680 shows that the twoclusters have virtually identical morphology which, in combination with theirsimilar compositions, produces identical ages. Coincidentally, the shifts inthe CMD necessary to superpose the two clusters require that the apparentmoduli of IC 4651 and NGC 3680 be the same, while E(b-y)(4651) = E(b-y)(3680) +0.04.Comment: 28 pages and 3 tables, in latex, 11 postscript figures. Accepted for Astronomical Journa