A Spitzer Survey of Mid-infrared Molecular Emission from Protoplanetary Disks. II. Correlations and Local Thermal Equilibrium Models

We present an analysis of Spitzer Infrared Spectrograph observations of H₂O, OH, HCN, C₂H₂, and CO₂ emission, 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, H₂O 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 H₂O emission show that line strength is primarily related to the best-fit emitting area, and this accounts for most source-to-source variation in H₂O emitted flux. Best-fit temperatures and column densities cover only a small range of parameter space, near ~10¹⁸ cm^-2 and 450 K for all sources, suggesting a high abundance of H₂O 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.