Imants Platais

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 ~ 15 have membership probability Pμ ≥ 2%. 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 velocities. A total of 34 very likely cluster members yield a mean heliocentric radial velocity for NGC 2451A equal to +22.9 km s-1. The high quality of our BV photometry, a confirmation of cluster membership from proper motions and radial velocities, and a recently obtained metallicity estimate for several cluster stars allow us to perform a precise isochrone fit. The Yale isochrones, updated by the latest available input physics, have been fitted to the clusters color-magnitude diagram, yielding a distance modulus V0 - MV in the range 6.35 to 6.38, which is in excellent agreement with the distance modulus (m - M)0 = 6.38 derived from the Hipparcos data recently by van Leeuwen 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 Pleiades. We also present alternative evidence suggesting that the cluster could be slightly older than the Pleiades. The most important result of this study is an excellent match between the main-sequence fitting and Hipparcos distances to NGC 2451A. If we consider noticeable similarities between NGC 2451A and Pleiades, then the Hipparcos distance anomaly for Pleiades may not require an explanation of astrophysical nature.

Division I / Commission 8 / Working Group Densification of the Optical Reference Frame

A continuation of this WG was voted for at the IAU GA 2006 in Prague. The International Celestial Reference Frame (ICRF) is defined by the positions of 212 distant quasars at radio wavelengths. The primary, optical reference frame is the Hipparcos Celestial Reference Frame (HCRF), which is the Hipparcos Catalog without astrometric ‘problem stars (in: H. Rickman (ed.) 2001, Proceedings IAU XXIV General Assembly, Transactions IAU XXIVB (San Francisco: ASP), Resolution B1.2). The Tycho-2 catalog with its 2.5 million brightest stars forms the first step in the densification of the optical reference frame. However, the limiting magnitude of about V = 12 of the Tycho-2 catalog is not sufficient for most applications in astronomy and the goal of this IAU Working Group is to further extend the grid of highly accurate positions and motions toward more and fainter stars. The web site of this WG is at .