Proper Motion Comparison: Three Data Sources
For the 162 selected double star systems, relative proper motions -- that is, the motions of the B
components relative to A -- were computed from the observed data recorded in the WDS. For
each system, WDS measurements of separation and position angle, which had been made at
various epochs with respect to the equator of date, were transformed to
Dec values on
the J2000 equator. Then, a least-squares solution for BA proper motion (as well as relative
position at epoch J2000) was computed. The computed relative proper motions were quite good
for these solutions, with typical standard errors of 0.52 mas/yr.
Relative proper motions were also computed for the same systems based on the proper motions
for the individual components recorded in the Hipparcos and Tycho-2 catalogs.
The graphs on the right compare these three sets of data, as three two-way difference histograms.
widths of each distribution of differences are indicated (not counting the few outliers with
|differences| > 20 mas/yr). The 1
width of the WDSTycho-2 distribution (3
graph on right)
is significantly less than that of the WDSHipparcos distribution (2
graph on right). On the
surface, this indicates that the WDS data is better matched to the relative proper motions derived
from Tycho-2 -- and, by implication, to the individual star proper motions from Tycho-2 -- than
to the corresponding Hipparcos data. It appears that the difference may be due mainly to fewer
widely discrepant values in Tycho-2; that is, the WDSTycho-2 distribution has smaller "wings".
This needs to be verified.