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.