Wiyn open cluster study. VII. NGC 2451A and the Hipparcos distance scale

We provide new evidence that NGC 2451A is undoubtedly a young open cluster, although sparsely populated. New cluster membership has been derived from relative proper motions of 5868 stars. In total, 136 stars down to V simila r to 15 have membership probability P(mu)greater than or equal to2% New CCD BV photometry indicates that about 70 stars are indeed main-sequence stars of NGC 2451A. This is also supported by our measurements of radial velocit ies. A total of 34 very likely cluster members yield a mean heliocentric ra dial velocity for NGC 2451A equal to +22.9 km s(-1). The high quality of ou r BV photometry, a confirmation of cluster membership from proper motions a nd radial velocities, and a recently obtained metallicity estimate for seve ral cluster stars allow us to perform a precise isochrone fit. The Yale iso chrones, updated by the latest available input physics, have been fitted to the cluster's color-magnitude diagram, yielding a distance modulus V-0-M-V in the range 6.35 to 6.38, which is in excellent agreement with the distan ce modulus (m-M)(0)=6.38 derived from the Hipparcos data recently by van Le euwen and Robichon et al. For NGC 2451A the isochrone fit yields an age of 60 +/- 20 Myr; hence, the cluster appears to be somewhat younger than the P leiades. We also present alternative evidence suggesting that the cluster c ould be slightly older than the Pleiades. The most important result of this study is an excellent match between the main-sequence fitting and Hipparco s distances to NGC 2451A. If we consider noticeable similarities between NG C 2451A and Pleiades, then the Hipparcos distance anomaly for Pleiades may not require an explanation of astrophysical nature.