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We report ALMA Cycle 4 observations of the Class I binary protostellar systemL1551 IRS 5 in the 0.9-mm continuum emission, C18O (J=3-2), OCS (J=28-27), andfour 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 areseparated from the circumbinary disk (CBD). The CBD is resolved into two spiralarms, 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 neighboringprotobinary 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 IRS5. Furthermore, the dust and molecular-line brightness temperatures of CSDs andCBD reach >260 K and >100 K, respectively, in L1551 IRS 5, much hotter thanthose in L1551 NE. The gas motions in the spiral arms are characterized byrotation and expansion. Furthermore, the transitions from the CBD to the CSDrotations at around the L2 and L3 Lagrangian points and gas motions around theL1 point are identified. Our numerical simulations reproduce the observed twospiral arms and expanding gas motion as a result of gravitational torques fromthe binary, transitions from the CBD to the CSD rotations, and the gas motionaround the L1 point. The higher temperature in L1551 IRS 5 likely reflects theinferred FU-Ori event.

Circumbinary Disks of the Protostellar Binary Systems in the L1551 Region