Astrometric radial velocities^*,**

III. Hipparcos measurements of nearby star clusters and associations

Lund Observatory, Box 43, 22100 Lund, Sweden e-mail: soren, dainis, lennart@astro.lu.se

Corresponding author: S. Madsen, soren@astro.lu.se

Received: 10 September 2001

Abstract

Radial motions of stars in nearby moving clusters are determined from accurate proper motions and trigonometric parallaxes, without any use of spectroscopy. Assuming that cluster members share the same velocity vector (apart from a random dispersion), we apply a maximum-likelihood method on astrometric data from Hipparcos to compute radial and space velocities (and their dispersions) in the Ursa Major, Hyades, Coma Berenices, Pleiades, and Praesepe clusters, and for the Scorpius-Centaurus, α Persei, and “HIP 98321” associations. The radial motion of the Hyades cluster is determined to within 0.4 km s^-1 (standard error), and that of its individual stars to within 0.6 km s^-1. For other clusters, Hipparcos data yield astrometric radial velocities with typical accuracies of a few km s^-1. A comparison of these astrometric values with spectroscopic radial velocities in the literature shows a good general agreement and, in the case of the best-determined Hyades cluster, also permits searches for subtle astrophysical differences, such as evidence for enhanced convective blueshifts of F-dwarf spectra, and decreased gravitational redshifts in giants. Similar comparisons for the Scorpius OB2 complex indicate some expansion of its associations, albeit slower than expected from their ages. As a by-product from the radial-velocity solutions, kinematically improved parallaxes for individual stars are obtained, enabling Hertzsprung-Russell diagrams with unprecedented accuracy in luminosity. For the Hyades (parallax accuracy 0.3 mas), its main sequence resembles a thin line, possibly with wiggles in it. Although this main sequence has underpopulated regions at certain colours (previously suggested to be “Böhm-Vitense gaps”), such are not visible for other clusters, and are probably spurious. Future space astrometry missions carry a great potential for absolute radial-velocity determinations, insensitive to the complexities of stellar spectra.