The First Mid-Infrared Detection of HNC in the Interstellar Medium:
  Probing the Extreme Environment Towards the Orion Hot Core
  We present the first mid-infrared (MIR) detections of HNC and H13CN in the
interstellar medium, and numerous, resolved HCN rovibrational transitions. Our
observations span 12.8 to 22.9 micron towards the hot core Orion IRc2, obtained
with the Echelon-Cross-Echelle Spectrograph aboard the Stratospheric
Observatory for Infrared Astronomy (SOFIA). Exceptional, ~5 km/s, resolution
distinguishes individual rovibrational transitions of the HNC and HCN P, Q, and
R branches; and the H13CN R branch. This allows direct measurement of the
species' excitation temperatures, column densities, and relative abundances.
HNC and H13CN exhibit a local standard rest velocity of -7 km/s that may be
associated with an outflow from nearby radio source I and an excitation
temperature of about 100 K. We resolve two velocity components for HCN, the
primary component also being at -7 km/s with temperature 165 K. The hottest
component, which had never before been observed, is at 1 km/s with temperature
309 K. This is the closest component to the hot core's centre measured to date.
The derived 12C/13C=13 is below expectation for Orion's Galactocentric
distance, but the derived HCN/HNC=72 is expected for this extreme environment.
Compared to previous sub-mm and mm observations, our SOFIA line survey of this
region shows that the resolved MIR molecular transitions are probing a distinct
physical component and isolating the chemistry closest to the hot core.
