Gaps in Protoplanetary Disks as Signatures of Planets: II. Inclined Disks

TL;DR

This paper studies how the inclination of protoplanetary disks affects the appearance of gaps caused by planets, providing formulas to infer disk properties and applying models to observed disks like LkCa 15.

Contribution

It introduces a geometric framework to interpret inclined disk images and demonstrates how to estimate disk scale height and inclination from brightness asymmetries.

Findings

Inclination causes near side brightness to differ from far side.

A formula to recover disk scale height and inclination from images.

A planet >0.5 Jupiter mass explains the observed gap in LkCa 15.

Abstract

We examine the observational appearance of partial gaps being opened by planets in protoplanetary disks, considering the effects of the inclination relative to the line of sight. The gap's trough is darkened by both shadowing and cooling, relative to the uninterrupted disk. The gap's outer wall is brightened by direct illumination and also by heating, which puffs it up so that it intercepts more starlight. In this paper, we examine the effects of inclination on resolved images of disks with and without gaps at a wide range of wavelengths. The scattering surface's offset from the disk midplane creates a brightness asymmetry along the axis of inclination, making the disk's near side appear brighter than the far side in scattered light. Finite disk thickness also causes the projected distances of equidistant points on the disk surface to be smaller on the near side of the disk as compared to the far side. Consequently, the gap shoulder on the near side of the disk should appear brighter and closer to the star than on the far side. However, if the angular resolution of the observation is coarser than the width of the brightened gap shoulder, then the gap shoulder on the far side may appear brighter because of its larger apparent size. We present a formula to recover the scale height and inclination angle of an imaged disk using simple geometric arguments and measuring disk asymmetries. Resolved images of circumstellar disks have revealed clearings and gaps, such as the transitional disk in LkCa 15. Models created using our synthetic imaging attempting to match the morphology of observed scattered light images of LkCa 15 indicate that the H-band flux deficit in the inner $\sim0.5\arcsec$ of the disk can be explained with a planet of mass greater than 0.5 Jupiter mass.