  Combining numerical simulations and analytical modeling, we investigate
whether close binary systems form by the effect of magnetic braking. Using
magnetohydrodynamics simulations, we calculate the cloud evolution with a sink,
for which we do not resolve the binary system or binary orbital motion to
realize long-term time integration. Then, we analytically estimate the binary
separation using the accreted mass and angular momentum obtained from the
simulation. In unmagnetized clouds, wide binary systems with separations of
>100 au form, in which the binary separation continues to increase during the
main accretion phase. In contrast, close binary systems with separations of
<100 au can form in magnetized clouds. Since the efficiency of magnetic braking
strongly depends on both the strength and configuration of the magnetic field,
they also affect the formation conditions of a close binary. In addition, the
protostellar outflow has a negative impact on close binary formation,
especially when the rotation axis of the prestellar cloud is aligned with the
global magnetic field. The outflow interrupts the accretion of gas with small
angular momentum, which is expelled from the cloud, while gas with large
angular momentum preferentially falls from the side of the outflow onto the
binary system and widens the binary separation. This study shows that a cloud
with a magnetic field that is not parallel to the rotation axis is a favorable
environment for the formation of close binary systems.
