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Astronomical Applications Department, U.S. Naval Observatory - pm AAS poster (Page 3)

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Astronomical Applications Department, U.S. Naval Observatory - pm AAS poster
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
and 2.5
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.

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