Unresolved Systems

Abstract

Position measurements of a visual binary occasionally show systematic periodic variations which may indicate that one of the components forms an orbiting sub-system with an unseen third body. The observed relative motion will be the sum of two Keplerian motions, and the other visible star provides a convenient point of reference. This line of investigation commenced with Seeliger’s studies of ζ Cnc. In the case of an apparently single star the presumed orbital motion is superposed on the proper motion. Ninety years after Bessel had successfully analysed the transit circle observations of Procyon and Sirius, Reuyl initiated the use of photographic parallax observations, for these plate series are suitable sources of the accurate relative motion of a target star with respect to some field stars. The problem of separating a constant and a periodic term in the motion is the same as in the single-lined spectroscopic binaries, yet the realm of astrometric analysis is narrowed: the condition that an amplitude of measurable size occurs in a limited time interval leaves a narrow range of periods; the parallax enters as an additional restricting factor, and the confirmation of a suspected period through recurrence usually takes longer.