2000A&A...358..923M

The Hyades stars have highly coherent motions in space which reflect in the convergence of their astrometric proper motions in or about a common convergent point. The internal dispersion of the Hyades velocities is as low as about 0.3km/s. This is comparable with the expected uncertainty of the Tycho-2 proper motion at the distance of the Hyades. The classical approach of using the proper motion components normal to the convergent point direction is employed for some 200 bona fide cluster members located in an area of some 2000deg² and ranging in V magnitude 3.4 to 12.1. It is demonstrated that with the most precise Tycho-2 proper motions for some 50 stars, the intrinsic dispersion is measurable at the level of 0.32km/s, in agreement with previous estimations obtained from the Hipparcos data and radial velocities. When only 30 stars out of the 50 are considered, which are not known to be spectroscopic binaries, the observed scatter suggests a dispersion of only 0.22km/s. The approach in use is model independent, and sets an upper limit on the intrinsic dispersion of velocities. It is shown 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 based 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 external error to the formal error of the Tycho-2 proper motions may be up to 1.3 for stars fainter than V_T=8.5mag. An alternative explanation for the extra scatter in low-precision subsets is discussed, that less massive stars may indeed have larger observed velocity dispersion due to the binarity effect.