Stellar Obliquity Excitation via Disk Dispersal-Driven Resonances in Binaries

TL;DR

This paper investigates how the dispersal of protoplanetary disks in binary star systems can cause large stellar obliquities, providing a mechanism for initial star-planet misalignments that influence planetary system evolution.

Contribution

The study analytically demonstrates how disk dispersal-driven resonances can produce a wide range of stellar obliquities, including high angles, in systems with distant binary companions, accounting for observed misalignments.

Findings

Stellar obliquities are broadly distributed between 60° and 180° for most planetary systems.

Non-homologous disk dissipation can help maintain orbital alignment of warm planets.

The mechanism explains primordial obliquities and misalignments in systems without current binary companions.

Abstract

The stellar obliquity of a planetary system is often used to help constrain the system's formation and evolution. One of the mechanisms to reorient the stellar spin involves a secular resonance crossing due to the dissipation of the protoplanetary disk when the system also has an inclined, distant ($\sim 300\;\mathrm{AU}$) binary companion. This mechanism is likely to operate broadly due to the $\sim 50\%$ binary fraction of FGK dwarfs and can play an important role in setting the initial stellar obliquities prior to any dynamical evolution. In this work, we revisit this mechanism analytically for idealized, homologously evolving disk models and show that the resulting stellar obliquities are broadly distributed between $60^\circ$ and $180^\circ$ for most warm and cold planets. We further show that non-homologus disk dissipation, such as the development of a photoevaporatively-opened gap at $\sim 2\;\mathrm{AU}$, can help maintain orbital alignment of warm planets, in agreement with observations. Our results represent the proper primordial obliquities for planetary systems with distant binary companions. They also represent the obliquities of stars with no present-day binary companions if these companions are dynamically unbound during the birth cluster phase of evolution, a process that occurs on a comparable timescale as the disk-driven obliquity excitation.