THE AU MICROSCOPII DEBRIS DISK: MULTIWAVELENGTH IMAGING AND MODELING

摘要

Debris disks around main-sequence stars are produced by the destruction of unseen parent bodies. AU Microscopii (GJ 803) is a compelling object to study in the context of disk evolution across different spectral types, as it is an M dwarf whose nearly edge-on disk may be directly compared to that of its A5 V sibling β Pic. We resolve the disk from 8-60 AU in the near-IR JHK′ bands at high resolution with the Keck Ⅱ Telescope and adaptive optics, and develop a data reduction technique for the removal of the stellar point-spread function. We measure a blue color across the near-IR bands, and confirm the presence of substructure in the inner disk. Some of the structural features exhibit wavelength-dependent positions. Recent measurements of the scattered-light polarization indicate the presence of porous grains. The scattering properties of these porous grains have a strong effect on the inferred structure of the disk relative to the majority of previously modeled grain types. Complementing prior work, we use a Monte Carlo radiative transfer code to compare a relatively simple model of the distribution of porous grains to a broad data set, simultaneously fitting midplane surface brightness profiles and the spectral energy distribution. Our model confirms that the large-scale architecture of the disk is consistent with detailed models of steady state grain dynamics. A belt of parent bodies from 35-40 AU produces dust that is then swept outward by stellar wind and radiation. We infer the presence of very small grains in the region exterior to the belt, down to sizes of ～0.05 μm. These sizes are consistent with stellar mass-loss rates M_* 《 10~2 M_☉.