Twin Jets and Close Binary Formation
  The formation of a close binary system is investigated using a
three-dimensional resistive magnetohydrodynamics simulation. Starting from a
prestellar cloud, the cloud evolution is calculated until about 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 encloses 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 $\gtrsim 100 km s^{-1}$. 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.
