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We report follow-up observations of the Class I binary protostellar systemL1551 NE in the C18O (3--2) line with the SMA in its compact and subcompactconfigurations. Our previous observations at a higher angular resolution in theextended configuration revealed a circumbinary disk exhibiting Keplerianmotion. The combined data having more extensive spatial coverage (~140 - 2000AU) verify the presence of a Keplerian circumbinary disk, and reveals for thefirst 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 thecircumbinary disk. Our simple model that reproduces the main features seen inthe Position-Velocity diagrams comprises a circumbinary disk exhibitingKeplerian motion out to a radius of ~300 AU, beyond which the gas exhibits pureinfall at a constant velocity of ~0.6 km s-1. The latter is significantlysmaller than the expected free-fall velocity of ~2.2 km s-1 onto the L1551 NEprotostellar mass of ~0.8 Msun at ~300 AU, suggesting that the infalling gas isdecelerated as it moves into regions of high gas pressure in the circumbinarydisk. The discontinuity in angular momenta between the outer infalling gas andinner Keplerian circumbinary disk implies an abrupt transition in theeffectiveness at which magnetic braking is able to transfer angular momentumoutwards, a result perhaps of the different plasma beta and ionizationfractions between the outer and inner regions of the circumbinary disk.

Evidence for Infalling Gas of Low Angular Momentum towards the L1551 NE Keplerian Circumbinary Disk