Unveiling wide-orbit companions to K-type stars in Sco-Cen with Gaia EDR3

Context. Detection of low-mass companions to stellar hosts is important to test formation scenarios of these systems. Companions at wide separations are particularly intriguing objects as they are easily accessible for variability studies towards rotational dynamics and cloud coverage of these brown dwarfs or planetary-mass objects.Aims. We aim to identify new low-mass companions to young pre-main sequence stars using the astrometric measurements provided by the Gaia space mission and high-contrast imaging data collected with VLT/SPHERE.Methods. We present an algorithm that enables detection of co-moving companions to arbitrary samples of stars using the second data release of the Gaia mission. Based on the provided positions, proper motions, and magnitudes, we identify all objects within a pre- defined radius, whose differential proper motions are consistent with a gravitationally bound system. If the system age is known, the masses of the companions can be determined by comparison to evolutionary tracks. For a brown dwarf companion of the debris disk host TYC 8252-533-1 we use additional data collected with VLT/SPHERE and VLT/NACO to assess the accuracy of the properties of the companion based on Gaia photometry alone.Results. Applying this algorithm to a sample of K-type, pre-main sequence stars in the Scorpius Centaurus association, we identify62 co-moving companions that have a dynamical companionship likelihood of more than 95%. Further color-magnitude analysisconfirms strong compliance with Sco-Cen membership. With Gaia DR2 5860803696599969280, Gaia DR2 6073106193460538240,Gaia DR2 6083750638540951552 (TYC 8252-533-1 B), Gaia DR2 5984404849554176256, and Gaia DR2 6245781131640479360we identify five especially intriguing companions that have masses in the brown dwarf regime down to 40 MJup. Our high-contrastimaging data confirm that TYC 8252-533-1 B is indeed a co-moving companion to its solar-type host star as suggested by Gaia alone.It is likely to be located outside the debris disk around its primary star and SED modeling of Gaia, SPHERE, and NACO photometryprovides a companion mass of 44+9 MJup. This measurement does not only corroborate the brown dwarf nature of the companion but −6also confirms the mass estimate derived from individual Gaia photometry.Conclusions. We show that the Gaia database is well suited for identifying low-mass companions at wide separations from their host stars. For K-type Sco-Cen members Gaia can detect sub-stellar objects at projected separations larger than 300 au and is sensitivity limited beyond 1,000 au with a lower mass limit down to 20 MJup . As the presented algorithm can be easily applied to members in other star-forming regions, it can be used to significantly enlarge the sample size of such objects and test formation and evolution theories of planetary systems. Future data releases of the Gaia mission will enable the identification of additional fainter and lower mass companions and facilitate dynamical analyses of these systems.