Formation of the Unequal-Mass Binary Protostars in L1551 NE by
  Rotationally-Driven Fragmentation
  We present observations at 7 mm that fully resolve the two circumstellar
disks, and a reanalyses of archival observations at 3.5 cm that resolve along
their major axes the two ionized jets, of the class I binary protostellar
system L1551 NE. We show that the two circumstellar disks are better fit by a
shallow inner and steep outer power-law than a truncated power-law. The two
disks have very different transition radii between their inner and outer
regions of $\sim$18.6 AU and $\sim$8.9 AU respectively. Assuming that they are
intrinsically circular and geometrically thin, we find that the two
circumstellar disks are parallel with each other and orthogonal in projection
to their respective ionized jets. Furthermore, the two disks are closely
aligned if not parallel with their circumbinary disk. Over an interval of
$\sim$10 yr, source B (possessing the circumsecondary disk) has moved
northwards with respect to and likely away from source A, indicating an orbital
motion in the same direction as the rotational motion of their circumbinary
disk. All the aforementioned elements therefore share the same axis for their
angular momentum, indicating that L1551 NE is a product of rotationally-driven
fragmentation of its parental core. Assuming a circular orbit, the relative
disk sizes are compatible with theoretical predictions for tidal truncation by
a binary system having a mass ratio of $\sim$0.2, in agreement with the
reported relative separations of the two protostars from the center of their
circumbinary disk. The transition radii of both disks, however, are a factor of
$\gtrsim$1.5 smaller than their predicted tidally-truncated radii.
