Multiple stars: Anathemas or friends?

Abstract

Multiple systems are generally divided into two groups: Trapezium systems and hierarchical systems. Trapezium systems consist of three or more stars whose separations are roughly equal. These systems are expected to be dynamically unstable. In hierarchical systems successive separations increase by large factors. The multiplicity of such hierarchies may be detected by a number of different techniques including astrometry, spectroscopy, photometry and speckle interferometry. Photometrically, an eclipsing system orbiting a third star will have periodic changes in eclipse timings as a result of light travel time effects. This manifests itself as an apparent change of the eclipse period. Another detectable effect is the precession of the line of nodes, resulting in a change of the inclination of the eclipsing pair. Such nodal precession may also be detected spectroscopically as a change in the semiamplitude of the orbit. Also, unexpectedly large and/or systematic radial velocity residuals often indicate a change in the center-of-mass velocity of the short period system due to orbital motion about a third component. Observational examples of such occurences are discussed. Finally, preliminary theoretical work on multiple star stability and formation is discussed and compared in some cases with observations.