Determination of accurate stellar radial-velocity measures

Wavelength measurements in stellar spectra cannot readily be interpreted as true stellar motion on the sub-km s-1 accuracy level due to the presence of many other effects, such as gravitational redshift and stellar convection, which also produce line shifts. Following a recommendation by the IAU, the result of an accurate spectroscopic radial-velocity observation should therefore be given as the “barycentric radial-velocity measure”, i.e. the absolute spectral shift as measured by an observer at zero gravitational potential located at the solar-system barycentre. Standard procedures for reducing accurate radial-velocity observations should be reviewed to take into account this recommendation. We describe a procedure to determine accurate barycentric radial-velocity measures of bright stars, based on digital cross-correlation of spectra obtained with the ELODIE spectrometer (Observatoire de Haute-Provence) with a synthetic template of lines. The absolute zero point of the radial-velocity measures is linked to the wavelength scale of the Kurucz (1984) Solar Flux Atlas via ELODIE observations of the Moon. Results are given for the Sun and 42 stars, most of them members of the Hyades and Ursa Major clusters. The median internal standard error is 27 m s-1. The external error is estimated at around 120 m s-1, mainly reflecting the uncertainty in the wavelength scale of the Solar Flux Atlas. For the Sun we find a radial-velocity measure of m s-1 referring to the full-disk spectrum of the selected lines.