Astronomical Applications Department, U.S. Naval Observatory pm AAS poster Page 4
The Independent Data Set
Observational measures recorded in the Washington Double Star (WDS) catalog (Mason et al.
2000) were used as an independent source of proper motion data for evaluating Hipparcos and
Tycho-2 proper motions. The WDS is the largest repository available of double star obser-
vations. We identified WDS pairs in which both A and B components had separate proper
motion determinations in both Hipparcos and Tycho-2 catalogs. We computed relative proper
motions (B minus A) in RA and Dec from the individual WDS measures, then compared these
relative proper motions to those derived from the Hipparcos and Tycho-2 catalog data for each
component.
The WDS may seem to be a rather odd source of information on proper motions. Obviously such
data are relevant only to stars that are members of double systems, and it provides data only on
relative proper motions. Furthermore, one must be careful in selecting the WDS pairs for such
analysis, since it is necessary to choose either optical doubles or wide binaries with very long
periods so that nonlinear motion is negligible. For this pilot study, we selected 162 pairs (based
on somewhat subjective criteria, listed below), although many more pairs could undoubtedly be
used. All that having been said, the WDS data is of quite high quality and in many cases spans a
century or more. In cases where the Hipparcos and Tycho-2 proper motions are significantly
different for the stars in a pair, the WDS data can often be used to select the better values.
