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

## TL;DR

This study uses high-resolution ALMA observations and simulations to reveal spiral arms, infall, and misaligned disks in the protostellar binary system L1551 NE, providing insights into disk dynamics and chemical differentiation.

## Contribution

First high-resolution imaging of the spiral structure and gas motions in L1551 NE, combined with simulations explaining the gravitational torque effects and disk misalignments.

## Key findings

- Spiral arms connect to circumstellar disks, showing m=1 mode.
- Gas motions indicate infall and outward flows in the spiral arms.
- Disks around individual protostars are misaligned with the circumbinary disk.

## Abstract

We report the ALMA Cycle 2 observations of the Class I binary protostellar system L1551 NE in the 0.9-mm continuum, C18O (3-2), 13CO (3-2), SO (7_8-6_7), and the CS (7-6) emission. At 0.18" (= 25 AU) resolution, ~4-times higher than that of our Cycle 0 observations, the circumbinary disk as seen in the 0.9-mm emission is shown to be comprised of a northern and a southern spiral arm, with the southern arm connecting to the circumstellar disk 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 C18O emission, the infall gas motions in the inter-arm 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 inter-arm regions. Chemical differentiation of the circumbinary disk is seen in the four molecular species. Our Cycle 2 observations have also resolved the circumstellar disks around the individual protostars, and the beam-deconvolved sizes are 0.29" X 0.19" (= 40 X 26 AU) (P.A. = 144 deg) and 0.26" X 0.20" (= 36 X 27 AU) (P.A. = 147 deg) for Sources A and B, respectively. The position and inclination angles of these circumstellar disks are misaligned with that of the circumbinary disk. The C18O emission traces the Keplerian rotation of the misaligned disk around Source A.