Circumbinary Disks of the Protostellar Binary Systems in the L1551 Region

TL;DR

This study uses high-resolution ALMA observations to analyze the structure, temperature, and gas dynamics of the circumbinary and circumstellar disks in the L1551 IRS 5 protostellar binary system, revealing spiral arms, hot regions, and complex gas motions.

Contribution

First detailed ALMA imaging of L1551 IRS 5's circumbinary and circumstellar disks, including spiral structures and gas dynamics, with numerical simulations supporting gravitational torque effects.

Findings

The circumbinary disk is more compact (~150 au) than in similar systems.

The disks exhibit high temperatures (>260 K in CSDs, >100 K in CBD).

Gas motions include rotation, expansion, and transitions at Lagrangian points.

Abstract

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.