Dispersing Envelope around the Keplerian Circumbinary Disk in L1551 NE
  and its Implications for the Binary Growth
  We performed mapping observations of the Class I protostellar binary system
L1551 NE in the C$^{18}$O ($J$=3-2), $^{13}$CO ($J$=3-2), CS ($J$=7-6), and SO
($J_N$=7$_8$-6$_7$) lines with Atacama Submillimeter Telescope Experiment
(ASTE). The ASTE C$^{18}$O data are combined with our previous SMA C$^{18}$O
data, which show a $r \sim$300-AU scale Keplerian disk around the protostellar
binary system. The C$^{18}$O maps show a $\sim$20000-AU scale protostellar
envelope surrounding the central Keplerian circumbinary disk. The envelope
exhibits a northeast (blue) - southwest (red) velocity gradient along the minor
axis, which can be interpreted as a dispersing gas motion with an outward
velocity of 0.3 km s$^{-1}$, while no rotational motion in the envelope is
seen. In addition to the envelope, two $\lesssim$4000 AU scale, high-velocity
($\gtrsim$1.3 km s$^{-1}$) redshifted $^{13}$CO and CS emission components are
found to $\sim$40$^{\prime\prime}$ southwest and $\sim$20$^{\prime\prime}$ west
of the protostellar binary. These redshifted components are most likely outflow
components driven from the neighboring protostellar source L1551 IRS 5, and are
colliding with the envelope in L1551 NE. The net momentum, kinetic and internal
energies of the L1551 IRS 5 outflow components are comparable to those of the
L1551 NE envelope, and the interactions between the outflows and the envelope
are likely to cause the dissipation of the envelope and thus suppression of the
further growth of the mass and mass ratio of the central protostellar binary in
L1551 NE.
