Effect of Magnetic Misalignment on Protobinary Evolution

TL;DR

This study reveals that magnetic field misalignment in protobinary systems enhances inward orbital migration by weakening magnetic braking and outflows, leading to more efficient binary tightening and circumbinary disk formation.

Contribution

It demonstrates that magnetic misalignment can promote binary tightening and disk formation even in strongly magnetized cores, contrasting with previous aligned field models.

Findings

Misaligned magnetic fields strengthen binary inward migration.

Weaker magnetic fields are needed for the same migration in tilted configurations.

Misalignment facilitates circumbinary disk formation in strong magnetic environments.

Abstract

The majority of solar-type stars reside in multiple systems, especially binaries. They form in dense cores of molecular clouds that are observed to be significantly magnetized. Our previous study shows that magnetic braking can tighten the binary separation during the protostellar mass accretion phase by removing the angular momentum of the accreting material. Recent numerical calculations of single star formation have shown that misalignment between the magnetic field and rotation axis may weaken both magnetic braking and the associated magnetically driven outflows. These two effects allow for disk formation even in strongly magnetized cores. Here we investigate the effects of magnetic field misalignment on the properties of protobinaries. Somewhat surprisingly, the misaligned magnetic field is more efficient at tightening the binary orbit compared to the aligned field. The main reason is that the misalignment weakens the magnetically-driven outflow, which allows more material to accrete onto the binary. Even though the specific angular momentum of this inner material is higher than in the aligned case, it is insufficient to compensate for the additional mass. A corollary of this result is that a weaker field is required to achieve the same degree of inward migration when the field is tilted relative to the rotation axis. Large field misalignment also helps to produce rotationally-supported circumbinary disks even for relatively strong magnetic fields, by weakening the magnetically-dominated structure close to the binary. Our result may provide an explanation for the circumbinary disks detected in recent SMA and ALMA observations.