# Disentangling Planets from Photoelectric Instability in Gas-Rich Optically Thin Dusty Disks

## TL;DR

This study uses hydrodynamical models to distinguish between planetary influences and photoelectric instability in shaping structures within gas-rich, optically thin circumstellar disks, revealing conditions under which planetary effects can be identified.

## Contribution

It demonstrates how PEI can obscure planetary signatures in disk structures and identifies criteria for detecting planets despite PEI effects.

## Key findings

- PEI can mask planetary structures unless the planet's gap is sufficiently large.
- High dust-to-gas ratios lead to dust accumulation and Rossby wave instability.
- Planetary gaps wider than PEI wavelength reveal planetary presence.

## Abstract

Structures in circumstellar disks such as gaps and rings are often attributed to planets. This connection has been difficult to show unequivocally, as other processes may also produce these features. Particularly, a photoelectric instability (PEI) has been proposed, operating in gas-rich optically thin disks, that generates structures predicted by planet--disk interactions. We examine the question of how to disentangle planetary effects on disk structure from the effects of the PEI. We use the Pencil Code to perform 2D global hydrodynamical models of the dynamics of gas and dust in a thin disk, with and without planetary perturbers. Photoelectric heating is modeled with an equation of state where pressure is proportional to dust surface density. The drag force on grains and its backreaction on the gas are included. Analyzing the situation without PEI, we find that gas--dust interactions alter the shape of the planetary gap from the dust-free case when the local dust-to-gas ratio $\varepsilon$ approaches unity. This result applies also to primordial disks, because dust drifting inwards accumulates at the edge of the planetary gap, and any initial dust-to-gas ratio eventually achieves $\varepsilon=1$ if the dust reservoir is sufficient. We find a result particular to high dust-to-gas ratio disks as well: as dust drifts inwards, the dust front becomes a sharp transition, and the backreaction triggers the Rossby wave instability. When PEI is included, we find that it obscures structures induced by planets unless the planet's mass is sufficiently large to carve a noticeable gap. Specifically, the instability generates arcs and rings of regular spacing: a planet is discernible when it carves a dust gap wider than the wavelength of the PEI.