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 micron 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. EXES’s smaller beam size compared to most other detections allows us to focus on the hot core itself without confusion from surrounding sources. Previous observations at longer wavelengths detected colder components of these three molecules in emission, while the MIR observations are hotter and in absorption.

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 measurements at longer wavelengths. Several other recent observations towards star-forming regions also show similarly unexpectedly low isotope ratios. This points to the possibility that the isotope chemistry in these regions is not yet fully understood.

Our work demonstrates the importance of the MIR in accessing molecular transitions that originate in hotter material central to the hot core compared to more commonly studied transitions at longer wavelengths. SOFIA/EXES, together with complementary ground-based observations from TEXES, are currently the only instruments available that can deliver this science.