of Position mode and
Transfer mode observations is an effective means to derive a full orbital solution of a binary system. Wide-angle astrometry from a multi-epoch Position mode program can be used to measure the parallax, proper motion and reflex motion of a binary system. High angular resolution Transfer mode observations can be used to determine the relative orbit and differential photometry of the components. Figure 3.4
illustrates the benefit of this technique as applied to the low mass binary system Wolf 1062 (Benedict et al. 2001). The small inner orbit of the primary star was determined from Position mode measurements of the primary’s position relative to reference field stars. The large orbit of the secondary low mass companion was derived from Transfer mode observations of the binary, which at each epoch yields the system separation and position angle. Combining these data allows one to locate the system barycenter and thereby compute the relative mass of each component. And with the parallax known (from the Position mode data), the total system mass, and hence the mass of each component, can be determined.
shows the orbits of the components of Wolf 1062 about the system’s barycenter. This has been determined from both Transfer mode observations, which yield the relative orbit, and Position mode observations, which map the orbit of the primary relative to reference stars distributed about the FGS FOV (in effect establishing the inertial reference frame). Note that the Position mode data also yield the system’s parallax and proper motion.