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Description
The census of stellar and substellar companions of nearby stars is largely incomplete, in particular towards the low mass brown dwarf and long-period exoplanets. It is however of fundamental importance to understand the stellar and planetary formation and evolution mechanisms. Nearby stars are particularly favorable targets for high precision astrometry. We aim at characterizing the presence of physical companions of stellar and substellar mass in orbit around nearby stars. Orbiting secondary bodies influence the proper motion of their parent star through their gravitational reflex motion. Using the Hipparcos and Gaia DR2 catalogs, we determine the long-term proper motion of the stars common to these two catalogs. We then search for a proper motion anomaly (PMa) between the long-term proper motion vector and the Gaia DR2 (or Hipparcos) measurements, indicative of the presence of a perturbing secondary object. We focus our analysis on the 6741 nearby stars located within 50pc, and we also present a catalog of the PMa for >99 percent of the stars of the full Hipparcos catalog. A fraction of 30 percent of the studied stars presents a PMa at a level of more than 3 sigma. The PMa allows us to detect orbiting companions, or set stringent limits to their presence. We present a few illustrations of the PMa analysis to interesting targets. We set upper limits of 0.1-0.3MJ to potential planets orbiting Proxima between 1 and 10au (P_orb_=3 to 100 years). We confirm that Proxima is gravitationally bound to alpha Cen. We recover the masses of the known companions of epsilon Eri, epsilon Ind, Ross 614 and beta Pic. We also detect the signature of a possible planet of a few jovian masses orbiting tau Ceti. Based on only 22 months of Gaia data, the DR2 has limitations, but its combination with the Hipparcos catalog and the very high accuracy of the derived PMa already enables to set valuable constraints on the binarity of nearby objects. The detection of tangential velocity anomalies at a median accuracy of sigma({Delta}v_T_)=1.0m/s per parsec of distance is already possible with the Gaia DR2. This type of analysis opens the possibility to identify long period orbital companions otherwise inaccessible. The complementarity of Gaia (for long orbital periods), radial velocity and transit techniques (for short periods) already appears as remarkably powerful.
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