The First Mid-Infrared Detections of HNC and H¹³CN in the Interstellar Medium

We present the first mid-infrared (MIR) detections of HNC and H¹³CN in the interstellar medium, and numerous HCN transitions. Our observations span 12.8 to 22.9 µm towards the hot core Orion IRc2, obtained with the Echelon-Cross-Echelle Spectrograph aboard the Stratospheric Observatory for Infrared Astronomy (SOFIA/EXES). 5 km/s resolution distinguishes individual rovibrational transitions of the three molecules, allowing direct measurement of their excitation temperatures, column densities, and relative abundances. HNC and H¹³CN share temperatures of 100 K with a local standard of rest velocity of -7 km/s. HCN shows two velocity components at -7 km/s at 165 K, and 1 km/s at 309 K.

The -7 km/s velocity component measured for all three molecules is similar to an outflow from the nearby high mass protostar Radio Source I, and are likely associated with it. The 1 km/s component is the hottest measured HCN to date towards IRc2 and closest to the hot core’s centre. We utilize a gas-grain chemical network to model the HCN/HNC evolution, which reaches our derived HCN/HNC=72 after 10⁶ years. This is much older than the region’s explosive event 500 years ago, suggesting that the hot core’s origins predate this event. Our derived ¹²C/¹³C=13 is lower than expected for the region.