Disentangling Planets from Photoelectric Instability in Debris Disks

Structures found in circumstellar disks, such as gaps and rings are usually attributed to planets. Yet, this causation thus far has been difficult to show unequivocally, as other disk processes may also produce these features. In particular, a photoelectric instability (PEI) has been proposed as a possible cause for structures. The instability operates in gas-rich optically thin disks, and generate non-trivial structures, including some predicted by planet-disk interactions. We examine the question of how to disentangle planetary effects on disk structure from those of the photoelectric instability. We use the Pencil Code to perform two-dimensional global hydrodynamic models of the dynamics of gas and dust in a non-magnetized thin disk, with and without planetary perturbers. Photoelectric heating is modeled with an equation of state where the pressure if proportional to the dust surface density (short thermal time approximation). The drag force on grains and its backreaction on the gas are included. Starting without the effect of the photoelectric heating, we find that dust-gas interactions alter the shape of the planetary gap from the dust-free case when the local dust-to-gas ratio ε approaches unity. When photoelectric heating is included, we find that the photoelectric instability obscures structures induced by planets unless the planet's mass is sufficiently large to carve a noticeable gap. More 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 instability.