Are isolated planetary-mass objects really isolated? A brown dwarf-exoplanet system candidate in the sigma Orionis cluster

摘要

The recent detection by direct imaging of three giant planets at wideseparation (50-250 AU) from their primaries has raised the question about the``true isolation'' of planetary-mass objects in clusters. Our goal was to testthe possibility that some free-floating planetary-mass object could in fact bepart of wide planetary systems. We searched in the literature for stellar andbrown-dwarf member candidates of the sigma Orionis cluster at small angularseparations from published candidate planetary-mass objects. We found onecandidate planetary system, SE 70, composed of an X-ray source and aplanetary-mass object, namely S Ori 68, separated by only 4.6 arcsec. In orderto assess the cluster membership of the X-ray source, we obtainedmid-resolution optical spectroscopy using ISIS on the William HerschelTelescope. We also compiled additional data on the target from availableastronomical catalogues. We have found that SE 70 follows thespectrophotometric sequence of the cluster and displays spectroscopic featuresof youth, such as lithium in absorption and chromospheric Halpha emission. Theradial velocity is consistent with cluster membership. Hence, SE 70 is veryprobably a sigma Orionis cluster member. The projected physical separationbetween SE 70 and S Ori 68 is 1700+/-300 AU at the distance of the cluster. Ifthe common proper motion is confirmed in the near future, the system would becomposed of an M5-6 brown dwarf with an estimated mass of ~45 M_Jup and anL5+/-2 giant planet with an estimated mass of ~5 M_Jup. It would be the widestand one of the lowest-mass planetary systems known so far.