Radiative scale-height and shadows in protoplanetary disks

TL;DR

This paper investigates how radiative pressure influences the vertical structure of protoplanetary disks with embedded planets, revealing effects like shadow casting and high aspect ratios that impact disk observations.

Contribution

It introduces a model incorporating radiative pressure into disk scale-height calculations, providing a more accurate physical framework for interpreting disk features.

Findings

Radiative pressure can dominate near accreting planets, creating vertical gas and dust columns.

Shadow casting by radiative effects explains observed illumination patterns in disks.

High aspect ratios in some disks may result from radiative pressure effects.

Abstract

Planets form in young circumstellar disks called protoplanetary disks. However, it is still difficult to catch planet formation in-situ. Nevertheless, from recent ALMA/SPHERE data, encouraging evidence of the direct and indirect presence of embedded planets has been identified in disks around young stars: co-moving point sources, gravitational perturbations, rings, cavities, and emission dips or shadows cast on disks. The interpretation of these observations needs a robust physical framework to deduce the complex disk geometry. In particular, protoplanetary disk models usually assume the gas pressure scale-height given by the ratio of the sound speed over the azimuthal velocity $H/r = c_{s\rm }/v_{\rm k}$. By doing so, \textit{radiative} pressure fields are often ignored, which could lead to a misinterpretation of the real vertical structure of such disks. We follow the evolution of a gaseous disk with an embedded Jupiter mass planet through hydrodynamical simulations, computing the disk scale-height including radiative pressure, which was derived from a generalization of the stellar atmosphere theory. We focus on the vertical impact of the radiative pressure in the vicinity of circumplanetary disks, where temperatures can reach $\gtrsim 1000$ K for an accreting planet, and radiative forces can overcome gravitational forces from the planet. The radiation-pressure effects create a vertical optically thick column of gas and dust at the proto-planet location, casting a shadow in scattered light. This mechanism could explain the peculiar illumination patterns observed in some disks around young stars such as HD 169142 where a moving shadow has been detected, or the extremely high aspect-ratio $H/r \sim 0.2$ observed in systems like AB Aur and CT Cha.