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

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 μ m 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.