High-Resolution Mid-Infrared Molecular Line Survey of the Orion Hot Core

Molecular line surveys provide a chemical inventory for star forming regions and are essential for studying their chemistry, kinematics and physical conditions. Previous high spectral resolution surveys have been limited to radio, sub-mm and FIR wavelengths. Mid-infrared missions such as ISO and Spitzer had low to moderate resolving power that were only able to link broad features with particular molecular bands and could not resolve the individual rovibrational transitions needed to identify specific molecules with certainty. Mid-infrared observations are the only way to study symmetric molecules that have no dipole moment and thus cannot be detected in the submillimeter line surveys from ALMA. We present results from an on-going high resolution (R ~ 60,000) line survey of the Orion hot core between 12.5 - 28.3 microns, using the EXES instrument on the SOFIA airborne observatory. SOFIA's higher-resolution and smaller beam compared to ISO allows us to spatially and spectrally isolate the emission towards the hot core. This survey will provide the best infrared measurements (to date) of molecular column densities and physical conditions - providing strong constraints on the current chemical network models for star forming regions. Specifically, we will present: (a) resolved rovibrational transitions of C2H2, detected in both the Ortho and Para states, transitions from its 13CCH2 isotopologue and HCN. We find that the Ortho and Para C2H2 clearly trace different temperatures with the C2H2 Ortho to Para ratio (OPR) of 1.7 ± 0.1, which is far from its equilibrium value of 3. Additionally, the ortho and para VLSR values differ by about 1.8 ± 0.2 km/s, while, the mean line widths differ by 0.7 ± 0.2 km/s, suggesting that these species are not uniformly mixed along the line of sight to IRc2. These results could not be measured from previous, more limited observations of C2H2. (b) resolved line profiles for some of the atomic species that provide new information and constraints on the physical and chemical process associated with the hot core. (c) predictions for detecting new gas phase molecules with this survey.