The Circumbinary Outflow: A Protostellar Outflow Driven by a Circumbinary Disk

TL;DR

This study demonstrates that circumbinary disks can drive outflows during binary star formation, with magnetic flux removal enabling angular momentum transfer and outflow generation, explaining observed bipolar outflows in young stellar objects.

Contribution

It reveals that circumbinary disks are capable of driving outflows in binary star formation, highlighting the role of magnetic flux removal and magnetic field accumulation in this process.

Findings

Circumbinary outflows are driven by magnetic fields in the disk.

Magnetic flux removal via Ohmic dissipation facilitates binary formation.

Most observed bipolar outflows may originate from circumbinary or multiple systems.

Abstract

The protostellar outflows have indispensable role in the formation of single stars, because they carry off the excess angular momentum from the centre of the shrinking gas cloud, and permits further collapse to form a star. On the other hand, a significant fraction of stars is supposed to be born as binaries with circumbinary disk that are frequently observed. Here, we investigate the evolution of a magnetized rotating cloud using three-dimensional resistive MHD nested-grid code, and show that the outflow is driven by the circumbinary disk and has an important role even in the binary formation. After the adiabatic core formation in the collapsing cloud core, the magnetic flux is significantly removed from the centre of the cloud by the Ohmic dissipation. Since this removal makes the magnetic braking ineffective, the adiabatic core continuously acquires the angular momentum to induce fragmentation and subsequent binary formation. The magnetic field accumulates in the circumbinary disk where the removal and accretion of magnetic field are balanced, and finally drives circumbinary outflow. This result explains the spectacular morphology of some specific young stellar objects such as L1551 IRS5. We can infer that most of the bipolar molecular outflows observed by low density tracers (i.e., CO) would correspond to circumbinary or circum-multiple outflows found in this report, since most of the young stellar objects are supposed to be binaries or multiples.