The Hipparcos-Gaia Catalog of Accelerations: Gaia EDR3 Edition
  We present a cross-calibration of Hipparcos and Gaia EDR3 intended to
identify astrometrically accelerating stars and to fit orbits to stars with
faint, massive companions. The resulting catalog, the EDR3 edition of the
Hipparcos-Gaia Catalog of Accelerations (HGCA), provides three proper motions
with calibrated uncertainties on the EDR3 reference frame: the Hipparcos proper
motion, the Gaia EDR3 proper motion, and the long-term proper motion given by
the difference in position between Hipparcos and Gaia EDR3. Our approach is
similar to that for the Gaia DR2 edition of the HGCA, but offers a factor of ~3
improvement in precision thanks to the longer time baseline and improved data
processing of Gaia EDR3. We again find that a 60/40 mixture of the two
Hipparcos reductions outperforms either reduction individually, and we find
strong evidence for locally variable frame rotations between all pairs of
proper motion measurements. The substantial global frame rotation seen in DR2
proper motions has been removed in EDR3. We also correct for color- and
magnitude-dependent frame rotations at a level of up to ~50 $\mu$as/yr in Gaia
EDR3. We calibrate the Gaia EDR3 uncertainties using a sample of radial
velocity standard stars without binary companions; we find an error inflation
factor (a ratio of total to formal uncertainty) of 1.37. This is substantially
lower than the position dependent factor of ~1.7 found for Gaia DR2 and
reflects the improved data processing in EDR3. While the catalog should be used
with caution, its proper motion residuals provide a powerful tool to measure
the masses and orbits of faint, massive companions to nearby stars.
