Astronomical Applications Department, U.S. Naval Observatory pm AAS poster Page 16
Furthermore, if we assume equipartition of the errors in the relative proper motions between the
two components of each pair, then the mean error estimates computed above for the relative
proper motions should simply be
2 times the mean error of the individual star proper motions.
The estimates of the external mean errors of the individual star proper motions in each catalog
therefore are:
µ
(RA):
WDS
= 1.73
TYC
= 2.50
HIP
= 3.45
µ
(Dec):
WDS
= 0.95
TYC
= 2.40
HIP
= 3.61
However, there are many caveats! The scheme depends critically upon the independence of the
data in the three catalogs, and also requires that the differences all have near-Gaussian distri-
butions. Neither of these conditions is strictly true for the catalogs analyzed here. For example,
Hipparcos and Tycho-2 are not totally independent, since Hipparcos positions and proper motions
were used to align the ground-based catalogs that were used in constructing Tycho-2. (Note also
that the most recent update to the WDS, not used here, added data from Hipparcos and Tycho-2.)
Two of the three distributions shown above appear to have broader wings than for a true
Gaussian. The assumption of equipartition of errors between the two components does not quite
hold for the HipparcosTycho-2 comparison (see TYCHIP graphs above for BA and individual
stars), probably because of small systematic errors in one or both catalogs that affect the proper
motions of both stellar components but not their difference. Therefore, we must be very cautious
before taking the above numbers too literally. However, they probably do indicate the relative
ranking of the quality of the proper motions from the three data sources, at least for the sample of
stars investigated here.
(units are mas/yr)
