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

Abstract

Context. The detection of low-mass companions to stellar hosts is important for testing the formation scenarios of these systems. Companions at wide separations are particularly intriguing objects as they are easily accessible for variability studies of the rotational dynamics and cloud coverage of these brown dwarfs or planetary-mass objects. Aims. We aim to identify new low-mass companions to young stars using the astrometric measurements provided by the Gaia space mission. When possible, we use high-contrast imaging data collected with VLT/SPHERE. Methods. We identified companion candidates from a sample of K-type, pre-main-sequence stars in the Scorpius Centaurus association using the early version of the third data release of the Gaia space mission. Based on the provided positions, proper motions, and magnitudes, we identified all objects within a predefined radius, whose differential proper motions are consistent with a gravitationally bound system. As the ages of our systems are known, we derived companion masses through comparison with evolutionary tracks. For seven identified companion candidates we used additional data collected with VLT/SPHERE and VLT/NACO to assess the accuracy of the properties of the companions based on Gaia photometry alone. Results. We identify 110 comoving companions that have a companionship likelihood of more than 95%. Further color-magnitude analysis confirms their Sco-Cen membership. We identify ten especially intriguing companions that have masses in the brown dwarf regime down to 20 MJup. Our high-contrast imaging data confirm both astrometry and photometric masses derived from Gaia alone. We discovered a new brown dwarf companion, TYC 8252-533-1 B, with a projected separation of approximately 570 au from its Sun-like primary. It is likely to be located outside the debris disk around its primary star and SED modeling of Gaia, SPHERE, and NACO photometry provides a companion mass of 52₋₁₁⁺¹⁷ M_(Jup). Conclusions. We show that the Gaia database can identify 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 with a sensitivity limit beyond 1000 au and a lower mass limit down to 20 M_(Jup). A similar analysis of other star-forming regions could significantly enlarge the sample size of such objects and facilitate testing of the formation and evolution theories of planetary systems.

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