Stellar and substellar companions from Gaia EDR3 -- Proper motion
  anomaly and resolved common proper motion pairs
  The multiplicity fraction of stars, down to the substellar regime, is a
parameter of fundamental importance for stellar formation, evolution, and
planetology. The census of multiple stars in the solar neighborhood is however
incomplete. Our study is aimed at detecting companions of Hipparcos catalog
stars from the proper motion anomaly (PMa) they induce on their host star,
namely, the difference between their long-term Hipparcos-Gaia and short-term
Gaia proper motion vectors. We also aim to detect resolved, gravitationally
bound companions of the Hipparcos stars (117,955 stars) and of the Gaia EDR3
stars closer than 100 pc (542,232 stars). In order to identify gravitationally
bound visual companions of our sample, we searched the EDR3 catalog for common
proper-motion (CPM) candidates. The detection of tangential velocity anomalies
with a median accuracy of 26 cm/s per parsec of distance is demonstrated. This
improvement by a factor 2.5 in accuracy, as compared to DR2, results in PMa
detection limits on companions that are well into the planetary mass regime for
many targets. We identify 37,515 Hipparcos stars with a significant PMa
(S/N>3), namely, a fraction of 32% and 12,914 (11%) hosting CPM bound candidate
companions. After including the EDR3 renormalised unit weight error (RUWE>1.4)
as an additional indicator, 50,720 stars of the Hipparcos catalog (43%) exhibit
at least one signal of binarity. Among the EDR3 stars within 100 pc, we find
CPM bound candidate companions for 39,490 stars (7.3%). The search for
companions using a combination of the PMa, CPM, and RUWE indicators improves
the exhaustivity of the multiplicity survey. The detection of CPM companions of
very bright stars (heavily saturated on the Gaia detectors) that are classical
benchmark objects for stellar physics provides a useful proxy for estimating
their distance with a higher accuracy than with Hipparcos.
