Discovery of planetary-mass binaries in the Lower Centaurus-Crux association

Context. The recent discovery of free-floating planets (FFPs) in nearby young stellar associations suggests that these objects might be common in the Galaxy. Our search for FFPs in the young Lower Centaurus-Crux (LCC) association using the VISTA Variables in the Vía Láctea VVV and VVVX surveys revealed several candidates with distances d < 200 pc. Aims. The main goal of the paper is to identify binary FFPs among this sample. The presence of such binaries is useful to contrast two different main formation scenarios: the formation in the circumstellar disk of the parent star with subsequent ejection by dynamical interactions and the in situ formation by gravitational collapse of a protostellar cloud. Methods. We used the Gaia, VVV, VVVX, and DECaPS databases to identify pairs of low-mass objects in the LCC association sharing common proper motions. We examined the optical and near-IR color-magnitude and color-color diagrams, and visually confirmed the detections in the available optical and near-IR images. Results. We find 17 young low-mass binaries in the LCC association, with distances starting from 68 pc and projected separations ranging from 88 to 6742 au. A couple of candidates have additional faint companions that need confirmation to secure them as triple systems. Adopting an age of 17 Myr for the LCC association, we find that 14 of the components are faint enough to have planetary masses. Conclusions. Our results indicate that binaries represent ≳2% of the population of FFPs in the LCC association, and this suggests that their preferred formation mechanism is the gravitational collapse independent of a star. Also, many of the recently discovered FFPs in LCC may be unresolved giant binary planets. The wide range of colors and flux reversals observed suggests that the existence of clouds in their atmospheres is important and points to continuity with the BD populations.