An Inclined, Eccentric Planet and an Inner Debris Disk Could Reproduce AU Mic Structure

Abstract

The debris disk orbiting the M star AU Microscopii has a series of large-scale clumps that move away from the star at high velocities above the mid-plane on the southeast side. Two more bright features lie on the northwest side of the disk, localized below the mid-plane and moving toward the star. These clumps are only observed in scattered light indicating that they affect small 0.2$\mu$m-sized grains. We present a mechanism for emitting periodic dust clumps by appealing to stellar forces and an inclined, eccentric planet interacting with an exterior debris disk. In our best-matching simulations, the planet exerts an impulse on the disk every orbital period, generating periodic enhancements in dust above the mid-plane. We assume that the stellar wind only acts on grains once they reach a height above the mid-planet that exceeds a threshold value (a free parameter in our model), at which point they are accelerated outward. The behavior of periodic particle ejections and trajectories depends significantly on the planet's mass, eccentricity, and inclination; separation between the planet and disk; and the ratio of stellar wind force to the star's gravitational force ($\beta$). We find a promising qualitative match to observations with simulations that include an as-yet-undiscovered and observationally allowed planet with mass $2 M_J$, semi-major axis between 3-4 au, eccentricity of 0.37, and inclination of 30$^\circ$, a ring of particles between 5-6 au, and a stellar wind height threshold of $z_h = hr$, where $h \approx 0.02$. We visualize our simulation with surface brightness maps to compare with existing observations of AU Mic. We find that a value of $\beta \approx 1.8$ accelerates the clumps radially outward at velocities that are comparable to the clumps seen in the AU Mic disk and produces features similar to those observed.