Internal velocity dispersion in the Hyades as a test for Tycho-2 proper motions

The Hyades stars have highly coherent motions in space which reflect in the convergence of their astrometric proper motions in or about a common conve rgent point. The internal dispersion of the Hyades velocities is as low as about 0.3 km s(-1). This is comparable with the expected uncertainty of the Tycho-3, proper motion at the distance of the Hyades. The classical approa ch of using the proper motion components normal to the convergent point dir ection is employed for some 200 bona fide cluster members located in an are a of some 2 000 deg(2) and ranging in V magnitude 3.4 to 12.1. It is demons trated that with the most precise Tycho-2 proper motions for some 50 stars, the intrinsic dispersion is measurable at the level of 0.32 km s(-1), in a greement with previous estimations obtained from the Hipparcos data and rad ial velocities. When only 30 stars out of the 50 are considered, which are not known to be spectroscopic binaries, the observed scatter suggests a dis persion of only 0.22 km s(-1). The approach in use is model independent, an d sets an upper limit on the intrinsic dispersion of velocities. It is show n for some 180 stars in common, that the Tycho-2 proper motions have a very similar precision to that of Hipparcos, but reveal fewer extreme deviants from the common convergent point direction. Tycho-2 proper motions are base d on long series of observations, up to a 100 years, and are therefore less subject to the orbital motion effects in binaries. The ratio of the extern al error to the formal error of the Tycho-2 proper motions may be up to 1.3 for stars fainter than V-T = 8.5 mag. An alternative explanation for the e xtra scatter in low-precision subsets is discussed, that less massive stars may indeed have larger observed velocity dispersion due to the binarity ef fect.