Reverse Rotation of the Accretion Disk in RW Aur A: Observations and a Physical Model

TL;DR

This paper reports observations of the RW Aur A binary system showing a reverse rotation of its accretion disk, and proposes a physical model explaining how such a counter-rotation can form in young binary star systems.

Contribution

It introduces a physical model that explains the formation of circumstellar disks rotating opposite to the binary orbital motion in young stellar systems.

Findings

Binary components' separation and position angle change over 70 years.

The disk rotation is opposite to the orbital motion.

The model explains high accretion rates and small disk size.

Abstract

Speckle interferometry of the young binary system RW Aur was performed with the 6-m telescope of the Special Astrophysical Observatory of the Russian Academy of Sciences using filters with central wavelengths of 550 nm and 800 nm and pass-band halfwidths of 20 nm and 100 nm, respectively. The angular separation of the binary components was 1.448"{\pm}0.005 and the position angle of the system was 255.9{\deg}{\pm}0.3{\deg} at the observation epoch (JD 2 454 255.9). We find using published data that these values have been changing with mean rates of +0.002"/yr and +0.02{\deg}/yr, respectively, over the past 70 years. This implies that the direction of the orbital motion of the binary system is opposite to the direction of the disk rotation in RW Aur A. We propose a physical model to explain the formation of circumstellar accretion disks rotating in the reverse direction relative to young binary stars surrounded by protoplanetary disks. Our model can explain the characteristic features of the matter flow in RW Aur A: the high accretion rate, small size of the disk around the massive component, and reverse direction of rotation.