Molecules in Massive Protostars from the Near to Mid Infrared: Unique Constraints from SOFIA/EXES

Massive stars form deep within cold molecular envelopes, where the energetic central object drives both a complex chemistry, due to heating of the gas and evaporation of ices, and complex kinematics such as jets, winds, and outflows.  The warm chemistry that takes place produces a variety of gas phase species, including CO, H2O, C2H2, and HCN, among others.  All of these molecules can be observed via vibrational and rotational transitions in the near to mid infrared.  While some of these species can be observed using ground based telescopes, EXES on SOFIA enables the observation of several transitions at wavelengths where the Earth's atmosphere is almost entirely opaque.  In particular, SOFIA/EXES has enabled the detection of ro-vibrational transitions of water arising from many low-energy states, including the ground rotational state.  I will present our study of water absorption in the massive protostar AFGL 2591, and discuss the unique information we learn from these observations.