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

Abstract

Abstract We present the first mid-infrared (MIR) detections of HNC and in the interstellar medium, and numerous resolved HCN rovibrational transitions. Our observations span 12.8–22.9 μm toward the hot core Orion IRc2, obtained with the Echelon-Cross-Echelle Spectrograph aboard the Stratospheric Observatory for Infrared Astronomy (SOFIA). In particular, ∼5 km s−1 resolution distinguishes individual rovibrational transitions of the HNC and HCN P, Q, and R branches; and the R branch. This allows direct measurement of the species’ excitation temperatures, column densities, and relative abundances. HNC and exhibit a local standard of rest velocity of −7 km s−1 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−1 with a temperature of 165 K. The hottest component, which had never before been observed, is at 1 km s−1 with a temperature of 309 K. This is the closest component to the hot core’s center measured to date. The derived is below expectation for Orion’s Galactocentric distance, but the derived HCN/HNC = 72 ± 7 is expected for this extreme environment. Compared to previous sub-millimeter and millimeter 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.