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 obta ined, with the Birmingham stellar spectrometer, of the F5 IV-V star Pr ocyon 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 b arycentric velocity of the star is equal to the barycentric velocity o f the instrument, which can be calculated to a high accuracy. Seven su ch velocities have been determined from five observing runs during the years 1986 to 1990 inclusive. The internal precision and external err ors (e.g. our zero-point) cannot be uniquely determined from so few po ints but the former appears to be in the range similar to 10 to simila r to 40 m s(-1). We have compared our data with the orbit of Procyon r ecently derived by Irwin et al. The mean annual acceleration of Procyo n A over the two years 1988 to 1990, according to the orbit of Irwin e t 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 agreemen t with the value obtained by Irwin et al. A detailed comparison will b e possible when several more years of data have been obtained.