Evolution of Spin Direction of Accreting Magnetic Protostars and Spin-Orbit Misalignment in Exoplanetary Systems

TL;DR

This paper explores how magnetic interactions between young stars and their discs can cause stellar spin axes to misalign with planetary orbits, offering an explanation for observed spin-orbit misalignments in exoplanetary systems.

Contribution

It introduces a magnetic star-disc interaction mechanism that can naturally produce spin-orbit misalignments before planet-planet interactions occur.

Findings

Magnetosphere-disc interactions can tilt stellar spins away from disc axes.

The model predicts a broad distribution of spin-orbit angles.

Misalignments can occur even in the absence of post-formation dynamical processes.

Abstract

Recent observations have shown that in many exoplanetary systems the spin axis of the parent star is misaligned with the planet's orbital axis. These have been used to argue against the scenario that short-period planets migrated to their present-day locations due to tidal interactions with their natal discs. However, this interpretation is based on the assumption that the spins of young stars are parallel to the rotation axes of protostellar discs around them. We show that the interaction between a magnetic star and its circumstellar disc can (but not always) have the effect of pushing the stellar spin axis away from the disc angular momentum axis toward the perpendicular state and even the retrograde state. Planets formed in the disc may therefore have their orbital axes misaligned with the stellar spin axis, even before any additional planet-planet scatterings or Kozai interactions take place. In general, magnetosphere--disc interactions lead to a broad distribution of the spin--orbit angles, with some systems aligned and other systems misaligned.