Organic Molecules and Water in the Inner Disks of T Tauri Stars
  We report high signal-to-noise Spitzer IRS spectra of a sample of eleven
classical T Tauri stars. Molecular emission from rotational transitions of H2O
and OH and ro-vibrational bands of simple organic molecules (CO2, HCN, C2H2) is
common among the sources in the sample. The gas temperatures (200-800 K) and
emitting areas we derive are consistent with the emission originating in a warm
disk atmosphere in the inner planet formation region at radii < 2 AU. The H2O
emission appears to form under a limited range of excitation conditions, as
shown by the similarity in relative strengths of H2O features from star to star
and the narrow range in derived temperature and column density. Emission from
highly excited rotational levels of OH is present in all stars; the OH emission
flux increases with the stellar accretion rate, and the OH/H2O flux ratio shows
a relatively small scatter. We interpret these results as evidence for OH
production via FUV photo-dissociation of H2O in the disk surface layers. No
obvious explanation is found for the observed range in the relative emission
strengths of different organic molecules or in their strength with respect to
water. We put forward the possibility that these variations reflect a diversity
in organic abundances due to star-to-star differences in the C/O ratio of the
inner disk gas. Stars with the largest HCN/H2O flux ratios in our sample have
the largest disk masses. We speculate that such a trend could result if higher
mass disks are more efficient at planetesimal formation and sequestration of
water in the outer disk, leading to enhanced C/O ratios and abundances of
organic molecules in the inner disk. A comparison of our derived HCN to H2O
column density ratio to comets, hot cores, and outer T Tauri star disks
suggests that the inner disks are chemically active.
