Astrometric radial velocities I. Non-spectroscopic methods for measuring stellar radial velocity

High-accuracy astrometry permits the determination of not only stellar tang ential motion, but also the component along the line-of-sight. Such non-spe ctroscopic (i.e. astrometric) radial velocities are independent of stellar atmospheric dynamics, spectral complexity and variability, as well as of gr avitational redshift. Three methods are analysed: (1) changing annual paral lax, (2) changing proper motion and (3) changing angular extent of a moving group of stars. All three have significant potential in planned astrometri c projects. Current accuracies are still inadequate for the first method, w hile the second is marginally feasible and is here applied to 16 stars. The third method reaches high accuracy (< 1 km s(-1)) already with present dat a, although for some clusters an accuracy limit is set by uncertainties in the cluster expansion rate.