  The NGC 1333 IRAS 4A protobinary was observed in the ammonia (2, 2) and (3,
3) lines with an angular resolution of 0.3 arcsec. The ammonia emission source
of IRAS 4A2 is elongated in the direction perpendicular to the bipolar jet and
has a size of 0.55 arcsec or 130 AU. This emission structure was interpreted as
a circumstellar disk around the IRAS 4A2 protostar, and the rotation kinematics
of the disk was investigated by making a position-velocity diagram along the
major axis. Assuming a Keplerian rotation, the disk has a rotation velocity of
1.8 km/s at a radius of 20 AU, which implies a central object of about 0.08
solar masses. The collapse age of the protostar is about 50,000 yr. The mass,
accretion rate, and age are consistent with what are expected from the standard
theory of low-mass star formation. If IRAS 4A2 grows at this rate, it may
become a star similar to the Sun.
