Astrometric Orbit

Definition

When two bodies are in Keplerian orbitsaround a common center of mass, the apparent motion projected onto the plane of the sky defines an astrometric orbit. In some cases, such as the famous example of Sirius A and its white dwarf companion Sirius B, both objects can be spatially resolved, and their absolute positions and motions can be determined. This allows the ratio of the two masses to be determined, which can be combined with the total mass, if the distance is known, to derive individual masses. More often, only a single photocenter can be resolved, and only motions relative to nearby stars can be determined. In this case an orbital solution requires additional information about the relative brightness of the components. In the case of a planetary companion, the contribution of the planet to the total light can almost always be neglected, and the mass of the planet can be estimated from the observed wobble of the parent star if the mass of the star and its distance...