Twin Jets and Close Binary Formation

The formation of a close binary system is investigated using a three-dimensional resistive magnetohydrodynamic simulation. Starting from a prestellar cloud, the cloud evolution is calculated until ∼400 yr after protostar formation. Fragmentation occurs in the gravitationally collapsing cloud, and two fragments evolve into protostars. The protostars orbit each other and a protobinary system appears. A wide-angle low-velocity outflow emerges from the circumbinary streams that enclose two protostars, while each protostar episodically drives high-velocity jets. Thus, the two high-velocity jets are surrounded by the low-velocity circumbinary outflow. The speed of the jets exceeds ≳100 $\mathrm{km}\,{{\rm{s}}}^{-1}$. Although the jets have a collimated structure, they are swung back on the small scale and are tangled at the large scale due to the binary orbital motion. A circumstellar disk also appears around each protostar. In the early main accretion phase, the binary orbit is complicated, while the binary separation is within <30 au. For the first time, all the characteristics of protobinary systems recently observed with large telescopes are reproduced in a numerical simulation.