Formation of giant planets and brown dwarfs on wide orbits

Aims. We numerically studied the formation of giant planet (GP) and brown dwarf (BD) embryos in gravitationally unstable protostellar disks and compared our findings with the directly imaged, wide-orbit (greater than or similar to 50 AU) companions that are known to-date. The viability of the disk fragmentation scenario for the formation of wide-orbit companions in protostellar disks around (sub-)solar mass stars was investigated. We focused on the likelihood of survival of GP/BD embryos formed via disk gravitational fragmentation. Methods. We used numerical hydrodynamics simulations of disk formation and evolution with an accurate treatment of disk thermodynamics. Using the thin-disk limit allowed us to probe the long-term evolution of protostellar disks, starting from the gravitational collapse of a pre-stellar core and ending in the T Tauri phase after at least 1.0 Myr of disk evolution. We focused on models that produced wide-orbit GP/BD embryos that opened a gap in the disk and showed radial migration timescales similar to or longer than the typical disk lifetime. Results. While most models showed disk fragmentation, only 6 models out of 60 revealed the formation of quasi-stable, wide-orbit GP/BD embryos. The low probability for the fragment survival is caused by efficient inward migration/ejection/dispersal mechanisms that operate in the embedded phase of star formation. We found that only massive and extended protostellar disks (greater than or similar to 0.2 M-circle dot), which experience gravitational fragmentation not only in the embedded but also in the T Tauri phases of star formation, can form wide-orbit companions. Disk fragmentation produced GP/BD embryos with masses in the 3.5-43 M-J range, covering the whole mass spectrum of directly imaged, wide-orbit companions to (sub-) solar mass stars. On the other hand, our modeling failed to produce embryos on orbital distances less than or similar to 170 AU, whereas several directly imaged companions were found at smaller orbits down to a few AU. Disk fragmentation also failed to produce wide-orbit companions around stars with mass less than or similar to 0.7 M-circle dot, in disagreement with observations. Conclusions. Disk fragmentation is unlikely to explain the whole observed spectrum of wide-orbit companions to (sub-)solar-mass stars and other formation mechanisms, for instance, dynamical scattering of closely packed companions onto wide orbits, should be invoked to account for companions at orbital distance from a few tens to approximate to 150 AU and wide-orbit companions with masses of the host star <= 0.7 M-circle dot. Definite measurements of orbit eccentricities and a wider sample of numerical models are needed to distinguish between the formation scenarios of GP/BD on wide orbits.