Leading & Trailing Spiral Arms in a Nearly Broken Protoplanetary Disc

TL;DR

This study uses 3D simulations to explore how misaligned inner and outer regions of a protoplanetary disc create unique leading and trailing spiral arms, influenced by disc misalignment and shadows, independent of gravitational instability.

Contribution

It reveals that misalignment-driven interactions produce leading spirals and shadows can generate trailing spirals, expanding understanding of spiral formation in low-mass, stable discs.

Findings

Leading spirals form at disc nodes due to misalignment.

Shadows from the inner disc can launch trailing spirals at high misalignments.

Both leading and trailing spirals can coexist and interact in strongly misaligned discs.

Abstract

We perform three-dimensional smoothed particle hydrodynamics simulations to investigate the formation of spiral arms in misaligned circumbinary discs. In a nearly broken disc the misaligned inner and outer discs interact at two nodes, launching leading spiral arms that do not rotate with the disc. These spirals vanish when the disc is fully broken or aligned. Our results show that the formation of leading spirals is driven by the relative misalignment of the inner and outer disc, and does not depend on the disc physics. With live radiative transfer, the shadows cast by the misaligned inner disc are also able to launch trailing spiral arms that only appear at high misalignments when the discs are disconnected. When the disc is strongly misaligned, leading and trailing spiral arms can both appear and interact with each other. At lower misalignments, the impact of shadows is negligible and leading spiral arms are seen instead. The presence of both leading and trailing spiral arms implies that the rotation of the disc cannot be assumed based on the orientation of the spiral arms alone. Unlike spirals formed by gravitational instability, the spirals in this work can also form in low-mass, gravitationally stable discs.