A new formation scenario of a counter-rotating circumstellar disk: spiral-arm accretion from a circumbinary disk in a triple protostar system

TL;DR

This study reveals a novel formation pathway for long-lived counter-rotating circumstellar disks in triple protostar systems, driven by spiral-arm accretion from a circumbinary disk, which may explain recent ALMA observations.

Contribution

It introduces a new formation scenario for counter-rotating disks in triple systems through spiral-arm accretion, supported by hydrodynamical simulations.

Findings

Counter-rotating disks can form via spiral-arm accretion in triple systems.

The counter-rotation persists for over 6.4×10^4 years after formation.

This mechanism may explain observed counter-rotating disks in young stellar objects.

Abstract

We present the evolution of rotational directions of circumstellar disks in a triple protostar system simulated from a turbulent molecular cloud core with no magnetic field. We find a new formation pathway of a counter-rotating circumstellar disk in such triple systems. The tertiary protostar forms via the circumbinary disk fragmentation and the initial rotational directions of all the three circumstellar disks are almost parallel to that of the orbital motion of the binary system. Their mutual gravito-hydrodynamical interaction for the subsequent $\sim10^4\thinspace\rm{yr}$ greatly disturbs the orbit of the tertiary, and the rotational directions of the tertiary disk and star are reversed due to the spiral-arm accretion of the circumbinary disk. The counter-rotation of the tertiary circumstellar disk continues to the end of the simulation ($\sim6.4\times10^4\thinspace\rm{yr}$ after its formation), implying that the counter-rotating disk is long-lived. This new formation pathway during the disk evolution in Class 0/I Young Stellar Objects possibly explains the counter-rotating disks recently discovered by ALMA.