Astronomical Applications Department, U.S. Naval Observatory pm AAS poster Page 3
Two Catalogs: Which Has the Better Proper Motions?
The two highest precision astrometric star catalogs yet produced are the Hipparcos main catalog
(ESA 1997) and the Tycho-2 catalog (Høg, et al. 2000), containing 1.2
×
10
5
and 2.5
×
10
6
stars,
respectively. The Hipparcos positions, proper motions, and parallaxes were derived from satellite
data taken in the early 1990s and define the International Celestial Reference Frame (ICRS) at
optical wavelengths. The Tycho-2 catalog combines data from the Hipparcos satellite with those
from 144 ground-based astrometric catalogs produced over the last century.
This paper investigates the proper motion data from these two catalogs. The Hipparcos proper
motions represent a snapshot of stellar motions over about 3 years centered at mid-1991,
whereas the Tycho-2 proper motions represent the average stellar motion over most of the 20th
century. Formal 1
uncertainties in proper motion are typically about 1 milliarcsecond per year
(mas/yr) in Hipparcos and 13 mas/yr in Tycho-2. Urban, Wycoff, & Makarov (2000) showed
that the distribution of the proper motion differences for single stars in common has a 1
width of
1.6 mas/yr. Nevertheless, for many individual stars, especially those in double systems, the
differences in proper motion significantly exceed that expected from the formal errors. For these
stars, is there a way that we can decide which proper motion -- Hipparcos or Tycho-2 -- is
better? Can any generalizations be made regarding the relative quality of the proper motions in
the two catalogs? To answer such questions, a third, independent source of high-precision proper
motion data is needed.
