Astrometric radial velocities (Dravins, Madsen, Lindegren):
Accurate astrometric measurements of stars allow their space motions, including the radial component, to be determined without using the spectroscopic Doppler effect. For a few fast-moving nearby stars accuracies of the order of 10 km/s have been achieved by combining Hipparcos results with old ground-based observations. This method needs Gaia to provide astrophysically interesting accuracies (better than 1 km/s). With currently available (Hipparcos) data, such accuracies are in fact obtained with another method based on the parallel space motions of stars in moving clusters. We have used Hipparcos measurements of nearby clusters like Ursa Major and the Hyades to determine astrometric radial velocities with accuracies of a few hundred m/s. More information
Main references: Dravins, Lindegren & Madsen (A&A 348, 1040, 1999), Lindegren, Madsen & Dravins (A&A 356, 1119, 2000), Madsen, Dravins & Lindegren (A&A 382, 446, 2002).
The fundamental definition of radial velocity (Dravins, Lindegren):
The need to accurately compare astrometric and spectroscopic radial velocities has forced us to examine the fundamental definition of 'radial velocity'. Two resolutions providing stringent definitions of the spectroscopic quantity, called the barycentric radial-velocity measure, and the geometric quantity, called astrometric radial velocity, were adopted by the IAU General Assembly in Manchester, August 2000. More information
Main reference: Lindegren & Dravins (A&A 401, 1185, 2003).
Spectroscopic radial-velocity measures (Dravins, Gullberg, Lindegren):
Measurements of stellar spectra on an absolute wavelength scale were obtained during 16 nights of observing in 1996-97, using the ELODIE spectrometer at Observatoire de Haute-Provence. These provide spectroscopic radial velocity measures which are not equivalent to the astrometric ones, due to spectroscopic wavelength shifts from gravitation, stellar surface convection, etc.
Main reference: Gullberg & Lindegren (A&A 390, 383, 2002).
Stellar surface structure (Dravins et al.):
In a programme aiming at understanding stellar surface structures we compare the astrometric radial velocities with spectroscopic measures, which include the effects of gravitational redshift and convective motions in the stellar atmospheres. The observed wavelength shifts for different atomic species will be compared with synthetic shifts computed from a grid of hydrodynamic models of stellar photospheres across the HR diagram.