Spitzer spectroscopy of the transition object TW Hya

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We report sensitive Spitzer IRS spectroscopy in the 10-20 μm region of TWHya, a nearby T Tauri star. The unusual spectral energy distribution of the source, that of a "transition object," indicates that the circumstellar disk in the system has experienced significant evolution, possibly as a result of planet formation. The spectrum we measure is strikingly different from that of other classical T Tauri stars reported in the literature, displaying no strong emission features of H O, C H , or HCN. The difference suggests that the inner planet formation region (≲5AU) of the gaseous disk has evolved physically and/or chemically away from the classical T Tauri norm. Nevertheless, TWHya does show a rich spectrum of emission features of atoms (H I, [Ne II], and [Ne III]) and molecules (H , OH, CO , HCO , and possibly CH ), some of which are also detected in classical T Tauri spectra. The properties of the neon emission are consistent with an origin for the emission in a disk irradiated by X-rays (with a possible role for additional irradiation by stellar EUV). The OH emission we detect, which also likely originates in the disk, is hot, arising from energy levels up to 23,000K above ground, and may be produced by the UV photodissociation of water. The H I emission is surprisingly strong, with relative strengths that are consistent with case B recombination. While the absence of strong molecular emission in the 10-20 μm region may indicate that the inner region of the gaseous disk has been partly cleared by an orbiting giant planet, chemical and/or excitation effects may be responsible instead. We discuss these issues and how our results bear on our understanding of the evolutionary state of the TWHya disk. © 2010 The American Astronomical Society. 2 2 2 2 2 3 +

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Astrophysical Journal

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