High Spectral Resolution SOFIA/EXES Observations of C2H2 towards
  Orion-IRc2
  We present high-spectral resolution observations from 12.96 - 13.33 microns
towards Orion IRc2 using the mid-infrared spectrograph, EXES, on 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 ten of the rovibrational C2H2 transitions expected in
our spectral coverage, are detected with high S/N, 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 high
signal-to-noise. 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, 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. 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 yrs ago.
