Spirals in protoplanetary disks from photon travel time

TL;DR

This paper investigates how light travel time effects can create spiral and arc-shaped shadows in protoplanetary disks, offering a new explanation for observed spiral structures beyond traditional mechanisms.

Contribution

It introduces a novel model showing that light travel time delays can produce spiral shadows, providing an alternative explanation for certain disk features.

Findings

Light travel time causes spiral-shaped shadows in disks.

The model fits a variety of observed shadow shapes.

Shadow shapes depend on disk inclination and flaring.

Abstract

Spiral structures are a common feature in scattered-light images of protoplanetary disks, and of great interest as possible tracers of the presence of planets. However, other mechanisms have been put foward to explain them, including self-gravity, disk-envelope interactions, and dead zone boundaries. These mechanisms explain many spirals very well, but are unable to easily account for very loosely wound spirals and single spiral arms. We study the effect of light travel time on the shape of a shadow cast by a clump orbiting close (within ${\sim}1\,$au) of the central star, where there can be significant orbital motion during the light travel time from the clump to the outer disk and then to the sky plane. This delay in light rays reaching the sky plane gives rise to a variety of spiral- and arc-shaped shadows, which we describe with a general fitting formula for a flared, inclined disk.