  We report sensitive Spitzer IRS spectroscopy in the 10-20 micron 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 water, acetylene, or HCN. The difference indicates that
the inner planet formation region (within 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
(HI, [NeII], and [NeIII]) 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 HI 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
micron 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.
