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G. H. Kaplan, S. C. Snell (U.S. Naval Observatory)
The positions, proper motions, and parallaxes of 105 stars in the Hipparcos main catalog, derived from satellite data taken in the early 1990s, define the International Celestial Reference Frame at optical wavelengths. The Tycho-2 catalog is a 25x densification of the Hipparcos system, combining data from the Hipparcos satellite with those from a large number of ground-based astrometric catalogs produced over the last century. Tycho-2 proper motions have been previously shown to be comparable, in the aggregate, to those in Hipparcos (Urban et al. 2000) for stars in common.
In this pilot study, we assessed the external accuracy of the proper motions for a selected subset of stars in the Hipparcos and Tycho-2 catalogs, using an independent source of data, the observational measures recorded in the Washington Double Star Catalog (WDS). The selected stars make up 162 wide visual double star systems for which nonlinear orbital motion should be negligible over the almost two centuries for which WDS measures are available. For each of these pairs, we computed relative proper motions (B component minus A) in RA and Dec from the WDS measures of separation and position angle. We then compared these relative proper motions to those derived from the Hipparcos and Tycho-2 catalog data for each component.
The pair-by-pair differences in relative proper motion among the three catalogs indicate that Tycho-2 is significantly better matched to the WDS data than Hipparcos. Furthermore, this kind of three-way catalog comparison can provide unique estimates of the external proper motion errors in each catalog separately. This analysis shows that the underlying mean errors in the proper motions of individual stars are about 2.5 mas/yr for Tycho-2 and 3.5 mas/yr for Hipparcos, which is not inconsistent with the tabulated errors for the sample of stars studied here. A broader survey is planned.