Double Stars in the Tycho-2 Catalogue

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Abstract
The Tycho-2 Catalogue provides astrometric and photometric information for 2.5 million stars. Within the Tycho-2 Catalogue, while there is no separate listing of double-star parameters, there is a wealth of double-star data. For all measured double-star systems, each component has its own entry in Tycho-2 (this corresponds to the Hipparcos "double-entry systems"), and the classical double-star parameters of separation, position angle, and magnitude difference can be calculated for these systems. Most double stars in the Tycho-2 Catalogue are identified by a code indicating a double, failed double, or photocentric solution. Stars flagged in this manner include 6251 known double stars and 1234 new double-star systems (designated in the Washington Double Star Catalog [WDS] with a "TDS" discovery code). Also, 4726 systems solved via Tycho-2 "single-star treatment" not only matched one component of a WDS pair but also had a nearby star that closely approximated the position and magnitude of the other WDS component. Finally, 1133 WDS systems were seen as single in Tycho-2 whose historical double-star parameters would indicate that Tycho-2 should have measured them. The observational statistics of these various classes of objects are presented together with catalog statistics for those with single-star solutions. A quantitative assessment of Tycho-2 double-star measures is provided via a comparison of Tycho-2 O-C residuals to known orbits. Finally, a subset of systems measured by Tycho-2 have been verified via speckle interferometry using the 82 inch (2 m) telescope of McDonald Observatory. These results are presented here.