# Structure of a protobinary system: an asymmetric circumbinary disk and spiral arms

## TL;DR

This study uses 3D simulations to explore gas structures around young binary stars, revealing asymmetric circumbinary disks, spiral arms, and their dependence on temperature and angular momentum, with implications for accretion processes.

## Contribution

It provides new insights into the morphology and dynamics of circumbinary disks, including asymmetry, vortex formation, and accretion behavior, influenced by temperature and angular momentum.

## Key findings

- Circumbinary disks exhibit asymmetry and vortex features.
- Asymmetry rotation speed varies with disk temperature.
- Accretion rates depend on envelope angular momentum and star position.

## Abstract

We investigate the gas structures around young binary stars by using three-dimensional numerical simulations. Each model exhibits circumstellar disks, spiral arms, and a circumbinary disk with an inner gap or cavity. The circumbinary disk has an asymmetric pattern rotating at an angular velocity of approximately one-fourth of the binary orbit of the moderate-temperature models. Because of this asymmetry, the circumbinary disk has a density bump and a vortex, both of which continue to exist until the end of our calculation. The density bump and vortex are attributed to enhanced angular momentum, which is promoted by the gravitational torque of the stars. In a hot model ($c \ge 2.0$), the asymmetry rotates considerably more slowly than in the moderate-temperature models. The cold models ($c \le 0.02$) exhibit eccentric circumbinary disks, the precession of which is approximated by a secular motion of the ballistic particles. The asymmetry in the circumbinary disk does not depend on the mass ratio, but it becomes less clear as the specific angular momentum of the infalling envelope increases. The relative accretion rate onto the stars is sensitive to the angular momentum of the infalling envelope. For envelopes with constant angular momentum, the secondary tends to have a higher accretion rate than the primary, except in very low angular momentum cases. For envelopes with a constant angular velocity, the primary has a higher accretion rate than the secondary because gas with low specific angular momentum falls along the polar directions.