Modelling shadows in scattered light observations as signals from companions in protoplanetary discs

TL;DR

This study uses 3D radiative transfer simulations to analyze how embedded companions in protoplanetary discs create detectable shadows in scattered light images, linking shadow features to companion properties.

Contribution

It provides an empirical model relating shadow characteristics to companion mass and location, aiding indirect detection of embedded objects in discs.

Findings

Companions ≥14 Jupiter masses cast detectable shadows.

Shadow width and depth correlate with companion mass and position.

Shadow effects influence disc thermal structure and chemistry.

Abstract

Over the past decade, SPHERE scattered light observations of protoplanetary discs have revealed previously unseen features with unprecedented resolution. One such feature are radial streaks of reduced brightness that are commonly interpreted as shadows. A possible cause for these shadows is an embedded companion within the disc. In this work, we use 3D radiative transfer simulations with RADMC-3D to investigate the shadowing effects of embedded companions across a range of orbital distances (5-30 au) and companion masses (0.5-30 Jupiter masses). We model 0.1 $\mu$m dust grains, which are well-coupled to the gas, to produce synthetic scattered light images of the disc. Companions with masses equal to or greater than 14 Jupiter masses consistently cast detectable shadows throughout the disc. We hence derive an empirical solution to describe the width and depth of the shadow as functions of companion mass and location. This scaling suggests that shadow features observed in scattered light images could serve as reliable indicators of companion mass and position, providing an indirect method for identifying and characterising otherwise challenging-to-detect objects within these discs. Additionally, our analysis reveals that companion shadows influence the disc thermal structure, with notable cooling effects that could impact disc chemistry and the dynamics of planet formation.