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Published April 1, 2007 | Published
Journal Article Open

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


We present deep Spitzer IRS low-resolution (λ/Δλ ~ 100) 5-35 μm spectroscopy of the edge-on disk the "Flying Saucer" (2MASS J16281370-2431391) 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 μm 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 μm-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 μm S(3) line of H_2 and the 11.2 μm emission feature due to PAHs.

Additional Information

© 2007 American Astronomical Society. Received 2006 December 11; accepted 2007 February 15; published 2007 February 27. We are grateful for a constructive review by an anonymous referee. K. M. P. is supported by NASA through Hubble Fellowship grant 01201.01 awarded by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., for NASA, under contract NAS 5-26555. Support for this work, part of the Spitzer Space Telescope Legacy Science Program, was provided by NASA through contracts 1224608 and 1230779 issued by the Jet Propulsion Laboratory, California Institute of Technology, under NASA contract 1407. E. F. v. D. acknowledges an NWO Spinoza prize.

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