Hubble Space Telescope Advanced Camera for Surveys Coronagraphic Imaging of the AU Microscopii Debris Disk
We present Hubble Space Telescope Advanced Camera for Surveys multicolor coronagraphic images of the recently discovered edge-on debris disk around the nearby (~10 pc) M dwarf AU Microscopii. The disk is seen between r=0.75" and 15" (7.5-150 AU) from the star. It has a thin midplane with a projected FWHM thickness of 2.5-3.5 AU within r<50 AU of the star that increases to 6.5-9 AU at r~75 AU. The disk's radial brightness profile is generally flat for r<15 AU, then decreases gradually (I~r-1.8) out to r~43 AU, beyond which it falls rapidly (I~r-4.7). Within 50 AU the midplane is straight and aligned with the star, and beyond that it deviates by ~3°, resulting in a bowed appearance that was also seen in ground-based images. Three-dimensional modeling of the disk shows that the inner region (r<50 AU) is inclined to the line of sight by less than 1° and the outer disk by ~3°. The inclination of the outer disk and moderate forward scattering (g~0.4) can explain the apparent bow. The intrinsic, deprojected FWHM thickness is 1.5-10 AU, increasing with radius. The models indicate that the disk is clear of dust within ~12 AU of the star, in general agreement with the previous prediction of 17 AU based on the infrared spectral energy distribution. The disk is blue, being 60% brighter at B than I relative to the star. One possible explanation for this is that there is a surplus of very small grains compared with other imaged debris disks that have more neutral or red colors. This may be due to the low radiation pressure exerted by the late-type star. Observations at two epochs show that an extended source seen along the midplane is a background galaxy.