2018ApJ...856....9R

We present high spectral resolution observations from 12.96 to 13.33 microns toward Orion IRc2 using the mid-infrared spectrograph, Echelon-Cross-Echelle Spectrograph (EXES), at Stratospheric Observatory for Infrared Astronomy (SOFIA). These observations probe the physical and chemical conditions of the Orion hot core, which is sampled by a bright, compact, mid-infrared background continuum source in the region, IRc2. All 10 of the rovibrational C₂H₂ transitions expected in our spectral coverage are detected with high signal-to-noise ratios (S/Ns), yielding continuous coverage of the R-branch lines from J = 9-8 to J = 18-17, including both ortho and para species. Eight of these rovibrational transitions are newly reported detections. The isotopologue, ¹³CCH₂, is clearly detected with a high S/N. This enabled a direct measurement of the ¹²C/¹³C isotopic ratio for the Orion hot core of 14 ± 1 and an estimated maximum value of 21. We also detected several HCN rovibrational lines. The ortho and para C₂H₂ ladders are clearly separate, and tracing two different temperatures, 226 K and 164 K, respectively, with a non-equilibrium ortho to para ratio (OPR) of 1.7 ± 0.1. Additionally, the ortho and para V_LSR values differ by about 1.8 ± 0.2 km s^–1, while the mean line widths differ by 0.7 ± 0.2 km s^–1, suggesting that these species are not uniformly mixed along the line of sight to IRc2. We propose that the abnormally low C₂H₂ OPR could be a remnant from an earlier, colder phase, before the density enhancement (now the hot core) was impacted by shocks generated from an explosive event 500 years ago.