HIGH-PRECISION VELOCITY OBSERVATIONS OF PROCYON-A .2. MEASUREMENT OF THE ORBITAL MOTION OF PROCYON-A DURING 1986-90

Radial velocity observations of high internal precision have been obtained, with the Birmingham stellar spectrometer, of the F5 IV-V star Procyon A. The velocities have been derived using a 'null technique' by determining the time when the topocentric velocity of Procyon A passes through zero (which occurs in January each year). At this time, the barycentric velocity of the star is equal to the barycentric velocity of the instrument, which can be calculated to a high accuracy. Seven such velocities have been determined from five observing runs during the years 1986 to 1990 inclusive. The internal precision and external errors (e.g. our zero-point) cannot be uniquely determined from so few points but the former appears to be in the range similar to 10 to similar to 40 m s(-1). We have compared our data with the orbit of Procyon recently derived by Irwin et al. The mean annual acceleration of Procyon A over the two years 1988 to 1990, according to the orbit of Irwin et al., is 133 m s(-1) yr(-1). Our measured value is 122 m s(-1) yr(-1), with an error of the order of +/- 10 m s(-1), in reasonable agreement with the value obtained by Irwin et al. A detailed comparison will be possible when several more years of data have been obtained.