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Usually, the determination of radial velocities of stars relies on the shiftof spectral lines by the Doppler effect. Russel & Atkinson (1931) and Oort(1932) already noted that due to the large proper motion and parallax of thewhite dwarf (WD) van Maanen 2, a determination of the perspective accelerationof the proper motion would provide a direct astrometric determination of theradial velocity which is independent of the gravitational redshift. Ifspectroscopic redshift measurements of Halpha and Hbeta NLTE cores exist, apurely astrometric determination would allow disentangling the gravitationalredshift from the Doppler shift.  The best instrument for measuring the tiny perspective acceleration is theGaia satellite of the European Space Agency, aiming at absolute astrometricmeasurements of one billion stars down to 20th magnitude with unprecedentedaccuracy. At 15th magnitude, the predicted angular accuracy of Gaia is about 20micro-arcseconds. In this article, we estimate whether it is possible tomeasure the radial velocity of WDs astrometrically by the exchange of propermotion into radial velocity during the 5-year mission of the satellite or bycombining Hipparcos data with the position measurements at the beginning of theGaia mission (the Hundred-Thousand-Proper-Motion project HTPM).

Astrometric determination of white dwarf radial velocities with Gaia?