On the AU Mic Debris Disk Density Profiles , Grain Properties and Dust Dynamics

Context. AU Mic is a young M-type star surrounded by an edge-on optically thin debris disk that shares many common observational properties with the disk around β Pictoris. In particular, the scattered light surface brightness profile falls off as ∼ r −5 outside 120 AU for β Pictoris and 35 AU for AU Mic. In both cases, the disk color raises with increasing distance beyond these reference radii. Aims. We present the first comprehensive analysis of the AU Mic disk properties since the system was discovered by Kalas et al. (2004). We explore whether the dynamical model, successful to reproduce the β Pic brightness profile (e.g. Augereau et al. 2001), could apply to AU Mic. Methods. We calculate the surface density profile of the AU Mic disk by performing the inversion of the near-IR and visible scattered light brightness profiles measured by Liu (2004a) and Krist et al. (2005), respectively. We discuss the grain properties by analysing the blue color of the disk in the visible (Krist et al. 2005) and by fitting the disk spectral energy distribution. We finally evaluate the radiation and wind forces on the grains. The impact of the recurrent X-ray and UV-flares on the dust dynamics is also discussed. Results. We show that irrespective of the mean scattering asymmetry factor of the grains, most of the emission arises from an asymmetric, collisionally-dominated region that peaks close to the surface brightness break around 35 AU. The elementary scatterers at visible wavelengths are found to be sub-micronic, but the inferred size distribution underestimates the amount of large grains, resulting in too low sub-millimeter emissions compared to the observations. From our inversion procedure, we find that the V-to H-band scattering cross sections ratio increases outside 40 AU, in line with the observed color gradient of the disk. This behaviour is expected if the grains have not been produced locally but placed in orbits of high eccentricity by a size-dependent pressure force, resulting in a paucity of large grains beyond the outer edge of the parent bodies disk. Because of the low luminosity of AU Mic, radiation pressure is inefficient to diffuse the smallest grains in the outer disk, even when the flares are taken into account. Conversely, we show that a standard, solar-like stellar wind generates a pressure force onto the dust particles that behaves much like a radiation pressure force. With an assumed ˙ M ≃ 3 …