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

We present deep Spitzer-IRS low-resolution (lambda/Delta lambda ~ 100) 5-35micron spectroscopy of the edge-on disk ``the Flying Saucer'' (2MASSJ16281370-2431391) in the Ophiuchus molecular cloud. The spectral energydistribution exhibits the characteristic two-peak shape predicted for acircumstellar disk viewed very close to edge-on. The short-wavelength peak isentirely due to photons scattered off the surface of the disk, while thelong-wavelength peak is due to thermal emission from the disk itself. TheSpitzer spectrum represents the first spectroscopic detection of scatteredlight out to 15 micron from a bona-fide, isolated edge-on disk around a T Tauristar. The depth and the wavelength of the mid-infrared "valley" of the SED givedirect constraints on the size distribution of large grains in the disk. Usinga 2D continuum radiative transfer model, we find that a significant amount of5-10 micron-sized grains is required in the surface layers of the disk at radiiof 50-300 AU. The detection of relatively large grains in the upper layersimplies that vertical mixing is effective, since grain growth models predictthe grains would otherwise settle deep in the disk on short time scales.Additionally, we tentatively detect the 9.66 micron S(3) line of H2 and the11.2 micron emission feature due to PAHs.Comment: Accepted for publication in ApJ Letter