Astrometric Binaries in the Age of the Next Generation of Large (Space) Telescopes

I analyze several catalogs of known visual and spectroscopic binaries and conclude that a large number of binaries is missing in current catalogs. Samples of the best studied (nearby and bright) stars indicate that the true binary fraction may be as high as 95%. A preliminary analysis indicates that these binaries can affect the astrometry significantly. 1. Binarity: Past Present & Future Duquennoy & Mayor (1991, hereafter referred to as DM1991) established the multiplicity of G-type stars within about 22 pc. Their observational dataset consists of thirteen years of radial velocity monitoring, in combination with astrometric, visual and eclipsing binaries, for 164 G-type stars. However, the pre-Hipparcos lists of wide binaries were rather incomplete. Currently, the number of known binaries is about three times larger. In fact, an analysis of the currently available binarity and multiplicity data of nearby Hipparcos-selected G-type stars indicates that as many as 95% of primaries have a cataloged companion. This binary fraction is almost twice as large as reported by DM1991. In the remainder of this paper I will use the term binarity loosely to mean binarity or multiplicity, where the companion can be a star, a brown dwarf or a planet. 1.1. Binarity: Present After the publications of the Hipparcos catalogs (ESA 1997), several new sources of binarity have become available, which are include in this paper. Specifically, the following (compilation) catalogs are used: HIP [the Hipparcos catalog (ESA 1997)], TY2 [The Tycho-2 catalog (Høg et al. 2000)], TDS [the Tycho Double Star catalog (Fabricius et al. 2002)], SB9 [the 9 catalog of spectroscopic binaries (Pourbaix et al. 2004)], EXOP [the catalog of confirmed extra-solar planets (California/Carnegie Planet Search Compilation 2003)], INT4 [the 4 Catalog of Interferometric Measurements of Binary Stars (Hartkopf et al. 2004)], and GCSN [the Geneva-Copenhagen Solar Neighborhood Radial Velocity Survey (Nordström et al. 2004)]. This combined catalog contains 37,341 binaries, or more than twice the number of Hipparcos multiples. Angular separations are available for 11,101 entries for the Hipparcos-only data, and for almost three times as many (29,769) stars in the combined catalog.