Generating Stellar Obliquity in Systems with Broken Protoplanetary Disks

TL;DR

This paper proposes a new mechanism where broken protoplanetary disks can torque the star's spin axis, leading to stellar obliquities, supported by analytical models and relevant for understanding spin-orbit misalignments.

Contribution

It introduces a novel pathway for stellar obliquity generation via disk-induced stellar spin precession crossing frequencies, supported by analytical derivations.

Findings

Broken disks can torque stellar spin axes.

Frequency crossing leads to obliquity excitation.

Supports link between disk structure and spin-orbit misalignments.

Abstract

Recent advances in sub-millimeter observations of young circumstellar nebulae have opened an unprecedented window into the structure of protoplanetary disks, which has revealed the surprising ubiquity of broken and misaligned disks. In this work, we demonstrate that such disks are capable of torquing the spin axis of their host star, representing a hitherto unexplored pathway by which stellar obliquities may be generated. The basis of this mechanism is a crossing of the stellar spin precession and inner disk regression frequencies, resulting in adiabatic excitation of the stellar obliquity. We derive analytical expressions for the characteristic frequencies of the inner disk and star as a function of the disk gap boundaries, and place an approximate limit on the disk architectures for which frequency crossing and resulting obliquity excitation are expected, thereby illustrating the efficacy of this model. Cumulatively, our results support the emerging concensus that significant spin-orbit misalignments are an expected outcome of planet formation.