Circumbinary Disks of the Protostellar Binary Systems in the L1551
  Region
  We report ALMA Cycle 4 observations of the Class I binary protostellar system
L1551 IRS 5 in the 0.9-mm continuum emission, C18O (J=3-2), OCS (J=28-27), and
four other Band 7 lines. At ~0.07" (= 10 au) resolution in the 0.9 mm emission,
two circumstellar disks (CSDs) associated with the binary protostars are
separated from the circumbinary disk (CBD). The CBD is resolved into two spiral
arms, one connecting to the CSD around the northern binary source, Source N,
and the other to Source S. As compared to the CBD in the neighboring
protobinary system L1551 NE, the CBD in L1551 IRS 5 is more compact (r ~150 au)
and the m=1 mode of the spirals found in L1551 NE is less obvious in L1551 IRS
5. Furthermore, the dust and molecular-line brightness temperatures of CSDs and
CBD reach >260 K and >100 K, respectively, in L1551 IRS 5, much hotter than
those in L1551 NE. The gas motions in the spiral arms are characterized by
rotation and expansion. Furthermore, the transitions from the CBD to the CSD
rotations at around the L2 and L3 Lagrangian points and gas motions around the
L1 point are identified. Our numerical simulations reproduce the observed two
spiral arms and expanding gas motion as a result of gravitational torques from
the binary, transitions from the CBD to the CSD rotations, and the gas motion
around the L1 point. The higher temperature in L1551 IRS 5 likely reflects the
inferred FU-Ori event.
