Stirring up the dust: a dynamical model for halo-like dust clouds in transitional disks

Context. A small number of young stellar objects show signs of a halo-like structure of optically thin dust, in addition to a circumstellar disk. This halo or torus is located within a few AU of the star, but its origin has not yet been understood. Aims. A dynamically excited cloud of planetesimals colliding to eventually form dust could produce such a structure. The cause of the dynamical excitation could be one or more planets, perhaps on eccentric orbits, or a migrating planet. This work investigates an inwardly migrating planet that is dynamically scattering planetesimals as a possible cause for the observed structures. If this mechanism is responsible, the observed halo-like structure could be used to infer the existence of planets in these systems. Methods. We present analytical estimates on the maximum inclination reached owing to dynamical interactions between planetesimals and a migrating planet. In addition, a symplectic integrator is used to simulate the effect of a migrating planet on a population of planetesimals. Collision time scales are estimated for the resulting population of planetesimals and the size distribution of the dust created in catastrophic collisions is determined. Results. It is found that an inwardly migrating planet is only able to scatter the material it encounters to highly-inclined orbits if the material is on an eccentric orbit. Such eccentric orbits can be the result of resonance trapping and eccentricity pumping. Simulations show that for a certain range of migration rates and planet masses, resonance capture combined with planetary migration indeed causes the planetesimals to reach eccentric orbits and subsequently get scattered to highly-inclined orbits. The size distribution of the resulting dust is calculated determined to find the total mass and optical depth, which are found to compare reasonably well with the observed structures. Conclusions. Dynamical scattering of planetesimals caused by a planet migrating in, followed by the grinding down of these planetesimals to dust grains, appears to be a promising explanation for the inferred circumstellar dust clouds. Further study is needed to see if the haloes can be used to infer the presence of planets.