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(abridged) The Hundred-Thousand-Proper-Motion (HTPM) project will determinethe proper motions of ~113500 stars using a 23-year baseline. The propermotions will use the Hipparcos data, with epoch 1991.25, as first epoch and thefirst intermediate-release Gaia astrometry, with epoch ~2014.5, as secondepoch. The expected HTPM proper-motion standard errors are 30-190 muas/yr,depending on stellar magnitude. Depending on the characteristics of an object,in particular its distance and velocity, its radial velocity can have asignificant impact on the determination of its proper motion. The impact ofthis perspective acceleration is largest for fast-moving, nearby stars. Ourgoal is to determine, for each star in the Hipparcos catalogue, theradial-velocity standard error that is required to guarantee a negligiblecontribution of perspective acceleration to the HTPM proper-motion precision.We employ two evaluation criteria, both based on Monte-Carlo simulations, withwhich we determine which stars need to be spectroscopically (re-)measured. Bothcriteria take the Hipparcos measurement errors into account. For each star inthe Hipparcos catalogue, we determine the confidence level with which theavailable radial velocity and its standard error, taken from the XHIPcompilation catalogue, are acceptable. We find that for 97 stars, the radialvelocities available in the literature are insufficiently precise for a 68.27%confidence level. We also identify 109 stars for which radial velocities arecurrently unknown yet need to be acquired to meet the 68.27% confidence level.To satisfy the radial-velocity requirements coming from our study will be adaunting task consuming a significant amount of spectroscopic telescope time.Fortunately, the follow-up spectroscopy is not time-critical since the HTPMproper motions can be corrected a posteriori once (improved) radial velocitiesbecome available.

Radial velocities for the Hipparcos-Gaia Hundred-Thousand-Proper-Motion project