Pre-transitional disk nature of the ab aur disk

The disk around AB Aur was imaged and resolved at 24.6 μm using the Cooled Mid-infrared Camera and Spectrometer on the 8.2 m Subaru Telescope. The Gaussian full width at half-maximum of the source size is estimated to be 90 ± 6 AU, indicating that the disk extends further out at 24.6 μm than at shorter wavelengths. In order to interpret the extended 24.6 μm image, we consider a disk with a reduced surface density within a boundary radius R _c, which is motivated by radio observations that suggest a reduced inner region within about 100 AU from the star. Introducing the surface density reduction factor f_c for the inner disk, we determine that the best match with the observed radial intensity profile at 24.6 μm is achieved with R_c = 88 AU and f_c = 0.01. We suggest that the extended emission at 24.6 μm is due to the enhanced emission from a wall-like structure at the boundary radius (the inner edge of the outer disk), which is caused by a jump in the surface density at R_c. Such a reduced inner disk and geometrically thick outer disk structure can also explain the more point-like nature at shorter wavelengths. We also note that this disk geometry is qualitatively similar to a pre-transitional disk, suggesting that the AB Aur disk is in a pre-transitional disk phase.