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

ABSTRACT Over the past decade, Spectro-Polarimetric High-contrast Exoplanet REsearch scattered light observations of protoplanetary discs have revealed previously unseen features with unprecedented resolution. One such feature is 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 $M_\text{J}$). 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 characterizing 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.