Critical tests of stellar evolution in open clusters -: III.: Stellar population and dynamical evolution of IC 4651

We present multiple-epoch radial-velocity observations for 104 stars in a 10' x 10' field of the intermediate-age open cluster IC 4651 to V similar to 14.5. Only 13 stars (13%) of the full sample are field stars. From the 44 single member stars we find a mean radial velocity of -30.76 +/- 0.20 km s(-1), and the 12 single red-giant members yield a true radial-velocity dispersion of 0.74 km s(-1). Of the 19 giant members, 7 (37%) are spectroscopic binaries with periods up to 5000 days, while 35 (52%) of the 67 main-sequence and turnoff members are binaries with periods less than similar to1000 days. Combined with our deep, accurate CCD Stromgren photometry in a similar to21' x 21' field of IC 4651 (Meibom 2000), these data substantially improve the definition of the cluster locus in the colour-magnitude diagram and the spatial structure of the cluster, although the photometry shows that IC 4651 contains at least twice as many stars on the upper main sequence as was believed when the radial-velocity survey was initiated. The single cluster members define a very tight sequence in the CMD, and two sets of isochrones from stellar models with convective overshooting (d/H-p = 0.2) have been fit to it. Our best estimate for the age of IC 4651 is 1.7 +/- 0.15 Gyr, assuming [Fe/H] = 0.12 (Hyades) and E(b-y) = 0.071. Including the similar to650 stars newly discovered from the photometry, we estimate the present total mass of IC 4651 to be similar to630 M., excluding any undetected stellar remnants. The corresponding tidal cutoff radius is similar to22'. IC 4651 shows evidence of moderate mass segregation: Most of the turn-off stars and nearly all the red giants are located at radii smaller than similar to7', while the lower main-sequence stars are less centrally concentrated. The spatial distributions of cluster and field stars indicate that additional cluster stars are probably still to be found outside the fields studied so far. Comparison of the present mass function of IC 4651 with plausible initial mass functions indicates that the cluster initially contained at least similar to8300 stars with a total mass of similar to5300 M.. Thus, of the original cluster stars only similar to7%, containing similar to12% of the initial mass, remain today. Of the initial cluster mass, similar to35% has been lost due to evolution of the most massive stars into white dwarfs or other remnants while the remaining similar to53%, comprising similar to93% of the original low-mass stars, appear to have migrated out of the observed field or been lost from the cluster altogether. IC 4651 is currently 1 kpc closer to the Galactic center than its "sister" cluster NGC 3680 (Nordstrom et al. 1997), but their Galactic orbital parameters indicate that the mean orbital radius of IC 4651 is in fact larger by 0.7 kpc, providing a plausible reason why it is much less advanced in its dynamical evolution than the coeval cluster NGC 3680.