Effects of Disk Warping on the Inclination Evolution of Star-Disk-Binary Systems

TL;DR

This study investigates how disk warping influences the inclination evolution in star-disk-binary systems, finding that small warps generally lead to alignment unless external torques are strong, with implications for observed spin-orbit alignments.

Contribution

The paper provides a detailed calculation of disk warp profiles considering bending waves and viscosity, and assesses the conditions under which disk alignment occurs in binary systems.

Findings

Disk warps are typically small, validating the flat-disk approximation.

Viscous dissipation tends to align disks with the binary or star.

Alignment is effective mainly for cold disks with strong external torques.

Abstract

Several recent studies have suggested that circumstellar disks in young stellar binaries may be driven into misalignement with their host stars due to secular gravitational interactions between the star, disk and the binary companion. The disk in such systems is twisted/warped due to the gravitational torques from the oblate central star and the external companion. We calculate the disk warp profile, taking into account of bending wave propagation and viscosity in the disk. We show that for typical protostellar disk parameters, the disk warp is small, thereby justifying the "flat-disk" approximation adopted in previous theoretical studies. However, the viscous dissipation associated with the small disk warp/twist tends to drive the disk toward alignment with the binary or the central star. We calculate the relevant timescales for the alignment. We find the alignment is effective for sufficiently cold disks with strong external torques, especially for systems with rapidly rotating stars, but is ineffective for the majority of star-disk-binary systems. Viscous warp driven alignment may be necessary to account for the observed spin-orbit alignment in multi-planet systems if these systems are accompanied by an inclined binary companion.