2011ApJ...731..130S

We present an analysis of Spitzer Infrared Spectrograph observations of H₂O, OH, HCN, C₂H₂, and CO₂emission, and Keck-NIRSPEC observations of CO emission, from a diverse sample of T Tauri and Herbig Ae/Be circumstellar disks. We find that detections and strengths of most mid-IR molecular emission features are correlated with each other, suggesting a common origin and similar excitation conditions for this mid-infrared line forest. Aside from the remarkable differences in molecular line strengths between T Tauri, Herbig Ae/Be, and transitional disks discussed in Pontoppidan et al., we note that the line detection efficiency is anti-correlated with the 13/30 µm spectral slope, which is a measure of the degree of grain settling in the disk atmosphere. We also note a correlation between detection efficiency and Hα equivalent width, and tentatively with accretion rate, suggesting that accretional heating contributes to line excitation. If detected, H₂ O line fluxes are correlated with the mid-IR continuum flux, and other co-varying system parameters, such as L_{sstarf}. However, significant sample variation, especially in molecular line ratios, remains, and its origin has yet to be explained. Local thermal equilibrium (LTE) models of the H₂ O emission show that line strength is primarily related to the best-fit emitting area, and this accounts for most source-to-source variation in H₂ O emitted flux. Best-fit temperatures and column densities cover only a small range of parameter space, near ∼10¹⁸/cm2 and 450 K for all sources, suggesting a high abundance of H₂ O in many planet-forming regions. Other molecules have a range of excitation temperatures from ∼500to1500 K, also consistent with an origin in planet-forming regions. We find molecular ratios relative to water of ∼10^–3 for all molecules, with the exception of CO, for which n(CO)/n(H₂O) ∼ 1. However, LTE fitting caveats and differences in the way thermo-chemical modeling results are reported make comparisons with such models difficult, and highlight the need for additional observations coupled with the use of line-generating radiative transfer codes.