Deep Spitzer Spectroscopy of the "Flying Saucer" Edge-on Disk: Large Grains beyond 50 AU

Abstract

We present deep Spitzer IRS low-resolution () 5-35 mm spectroscopy of the edge-on disk the l/Dl ∼ 100 "Flying Saucer" (2MASS J162813702431391) in the Ophiuchus molecular cloud. The SED exhibits the characteristic two-peak shape predicted for a circumstellar disk viewed very close to edge-on. The short-wavelength peak is entirely due to photons scattered off the surface of the disk, while the long-wavelength peak is due to thermal emission from the disk itself. The Spitzer spectrum represents the first spectroscopic detection of scattered light out to 15 mm from a bona fide, isolated edge-on disk around a T Tauri star. The depth and the wavelength of the mid-infrared "valley" of the SED give direct constraints on the size distribution of large grains in the disk. Using a 2D continuum radiative transfer model, we find that a significant amount of 5-10 mm-sized grains is required in the surface layers of the disk at radii of 50-300 AU. The detection of relatively large grains in the upper layers implies that vertical mixing is effective, since grain growth models predict that the grains would otherwise settle deep in the disk on short timescales. Additionally, we tentatively detect the 9.66 mm S(3) line of H 2 and the 11.2 mm emission feature due to PAHs. Subject headings: circumstellar matter-infrared: ISM-planetary systems: protoplanetary disks-stars: individual (2MASS J162813702431391)-stars: pre-main-sequence