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Sharp eccentric rings in planetless hydrodynamical models of debris disks

Debris disks should not be completely gas-free, since there is second generation gas from outgassing of planetesimals and dust grains via sublimation, photodesorption, or collisions, generating a system of dust-to-gas ratio close to unity, where hydrodynamics cannot be ignored. A clumping instability exists in this configuration, that has been hitherto explored only in one-dimensional, incompressible models. We performed 2D numerical compressible models of a disk with comparable amounts of gas and dust to study the growth and development of this instability. Our model solves the momentum equation for the gas and dust, together with energy and continuity equations. We uncover that the backreaction of the drag force from the gas onto the dust shepherds rings, similar to those observed in debris disks and usually attributed to the presence of hypothetical undetected planets. We also uncover that the eccentricity of these rings, usually presented as convincing evidence for the presence of a planet, can actually be simply explained by a standing wave propagating along the ring. The rings support a spectrum of oscillations, with one particular mode representing epicyclic motion. The apparent eccentricity matches the eccentricity in observed systems. This suggests that the planet possibility, though thrilling, is not necessarily required to explain these systems.