Spiral Arms, Infall, and Misalignment of the Circumbinary Disk from the Circumstellar Disks in the Protostellar Binary System L1551 NE

Abstract

We report the ALMA Cycle 2 observations of the Class I binary protostellar system L1551 NE in the 0.9 mm continuum, C 18 O (3–2), 13 CO (3–2), SO (7 8 –6 7 ), and CS (7–6) emission. At 0.″18 (=25 au) resolution, ∼4 times higher than that of our Cycle 0 observations, the circumbinary disk (CBD) as seen in the 0.9 mm emission is shown to be composed of a northern and a southern spiral arm, with the southern arm connecting to the circumstellar disk (CSD) around Source B. The western parts of the spiral arms are brighter than the eastern parts, suggesting the presence of an m  = 1 spiral mode. In the C 18 O emission, the infall gas motions in the interarm regions and the outward gas motions in the arms are identified. These observed features are well reproduced with our numerical simulations, where gravitational torques from the binary system impart angular momenta to the spiral-arm regions and extract angular momenta from the interarm regions. Chemical differentiation of the CBD is seen in the four molecular species. Our Cycle 2 observations have also resolved the CSDs around the individual protostars, and the beam-deconvolved sizes are 0.″29 × 0.″19 (=40 × 26 au) (P.A. = 144°) and 0.″26 × 0.″20 (=36 × 27 au) (P.A. = 147°) for Sources A and B, respectively. The position and inclination angles of these CSDs are misaligned with those of the CBD. The C 18 O emission traces the Keplerian rotation of the misaligned disk around Source A.