Astrophys. J., 712, 274-286 (2010/March-3)
Spitzer spectroscopy of the transition object TW Hya.
NAJITA J.R., CARR J.S., STROM S.E., WATSON D.M., PASCUCCI I., HOLLENBACH D., GORTI U. and KELLER L.
Abstract (from CDS):
We report sensitive Spitzer IRS spectroscopy in the 10-20 µm region of TW Hya, 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 H2O, C2H2, or HCN. The difference suggests that the inner planet formation region (≲5 AU) of the gaseous disk has evolved physically and/or chemically away from the classical T Tauri norm. Nevertheless, TW Hya does show a rich spectrum of emission features of atoms (H I, [Ne II], and [Ne III]) and molecules (H2, OH, CO2, HCO+, and possibly CH3), 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 23,000 K 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 TW Hya disk.
circumstellar matter - planetary systems - protoplanetary disks - stars: individual: TW Hya - stars: pre-main sequence
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