Evidence for Infalling Gas of Low Angular Momentum towards the L1551 NE
  Keplerian Circumbinary Disk
  We report follow-up observations of the Class I binary protostellar system
L1551 NE in the C18O (3--2) line with the SMA in its compact and subcompact
configurations. Our previous observations at a higher angular resolution in the
extended configuration revealed a circumbinary disk exhibiting Keplerian
motion. The combined data having more extensive spatial coverage (~140 - 2000
AU) verify the presence of a Keplerian circumbinary disk, and reveals for the
first time a distinct low-velocity (~< +-0.5 km s-1 from the systemic velocity)
component that displays a velocity gradient along the minor axis of the
circumbinary disk. Our simple model that reproduces the main features seen in
the Position-Velocity diagrams comprises a circumbinary disk exhibiting
Keplerian motion out to a radius of ~300 AU, beyond which the gas exhibits pure
infall at a constant velocity of ~0.6 km s-1. The latter is significantly
smaller than the expected free-fall velocity of ~2.2 km s-1 onto the L1551 NE
protostellar mass of ~0.8 Msun at ~300 AU, suggesting that the infalling gas is
decelerated as it moves into regions of high gas pressure in the circumbinary
disk. The discontinuity in angular momenta between the outer infalling gas and
inner Keplerian circumbinary disk implies an abrupt transition in the
effectiveness at which magnetic braking is able to transfer angular momentum
outwards, a result perhaps of the different plasma beta and ionization
fractions between the outer and inner regions of the circumbinary disk.
