UVES radial velocity accuracy from asteroid observations - I. Implications for fine structure constant variability

Context. High resolution observations of the asteroids Iris and Juno have been performed by means of the UV-Visual Echelle Spectrograph (UVES) at the European Southern Observatory (ESO) Very Large Telescope (VLT) to obtain the effective accuracy of the spectrograph's radial velocity. The knowledge of this quantity has important bearings on studies searching for a variability in the fine structure constant carried out with this instrument. Aims. Asteroids provide a precise radial velocity reference at the level of 1 m s(-1), which allows for instrumental calibration and the recognition of small instrumental drifts and calibration systematics. In particular, radial velocity drifts due to nonuniform slit illumination and slit optical misalignment in the two UVES spectrograph arms can be investigated. Methods. We compare the position of the solar spectrum reflected by the asteroids with the solar wavelength positions and with that of asteroid and twilight observations at other epochs to asses the UVES instrumental accuracy. Results. We observe radial velocities offsets in the range of approximate to 10-50 m s(-1), likely due to a nonuniform slit illumination. However, no radial velocity patterns with wavelengths are detected and the two UVES arms provide consistent radial velocities. These results suggest that the detected Delta alpha/alpha variability by Levshakov et al. (2007) deduced from a drift of -180 +/- 85 m s(-1) at z(abs) = 1.84, between two sets of Fe II lines falling in the two UVES arms, may be real or induced by other kinds of systematics than those investigated here. The proposed technique allows us to make a real time quality check of the spectrograph and should be followed for very accurate measurements.