The First Mid-Infrared Detection of HNC in the Interstellar Medium: Probing the Extreme Environment Towards the Orion Hot Core

TL;DR

This study reports the first mid-infrared detection of HNC in the interstellar medium, providing detailed measurements of excitation temperatures, column densities, and velocities in the Orion Hot Core, revealing new insights into its extreme environment.

Contribution

It presents the first MIR detections of HNC and H13CN in the interstellar medium, with high-resolution observations that resolve rovibrational transitions and measure physical conditions near the Orion Hot Core.

Findings

Detected HNC and H13CN in MIR for the first time in the ISM.

Resolved multiple velocity components with distinct temperatures.

Found a lower 12C/13C ratio and high HCN/HNC ratio consistent with extreme environment.

Abstract

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.