A Spitzer Survey of Mid-Infrared Molecular Emission from Protoplanetary
  Disks II: Correlations and LTE Models
  We present an analysis of Spitzer-IRS 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. We
note that the line detection efficiency is anti-correlated with the 13/30 um
SED 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
H-alpha 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_star. However, significant sample variation,
especially in molecular line ratios, remains. 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 10^{18} 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 ~500-1500 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.
