Signatures of Young Planets in the Continuum Emission From Protostellar Disks

TL;DR

This paper models how embedded planets influence the temperature and structure of protostellar disks, revealing that shadows and brightness variations caused by temperature changes can mimic other features, affecting interpretations of disk observations.

Contribution

It introduces models that incorporate radiative heating, cooling, and hydrostatic equilibrium to better understand disk features caused by embedded planets.

Findings

Shadows caused by temperature variations can mimic gaps in disks.

Outer walls of planet-carved gaps puff up and cast significant shadows.

Temperature perturbations influence observable features at millimeter and centimeter wavelengths.

Abstract

Many protostellar disks show central cavities, rings, or spiral arms likely caused by low-mass stellar or planetary companions, yet few such features are conclusively tied to bodies embedded in the disks. We note that even small features on the disk's surface cast shadows, because the starlight grazes the surface. We therefore focus on accurately computing the disk's thickness, which depends on its temperature. We present models with temperatures set by the balance between starlight heating and radiative cooling, that are also in vertical hydrostatic equilibrium. The planet has 20, 100, or 1000~M$_\oplus$, ranging from barely enough to perturb the disk significantly, to clearing a deep tidal gap. The hydrostatic balance strikingly alters the model disk's appearance. The planet-carved gap's outer wall puffs up under starlight heating, throwing a shadow across the disk beyond. The shadow appears in scattered light as a dark ring that could be mistaken for a gap opened by another more distant planet. The surface brightness contrast between outer wall and shadow for the 1000-M$_\oplus$ planet is an order of magnitude greater than a model neglecting the temperature disturbances. The shadow is so deep it largely hides the planet-launched spiral wave's outer arm. Temperature gradients are such that outer low-mass planets undergoing orbital migration will converge within the shadow. Furthermore the temperature perturbations affect the shape, size, and contrast of features at millimeter and centimeter wavelengths. Thus radiative heating and cooling are key to the appearance of protostellar disks with embedded planets.