By Sarah Nickerson, Naseem Rangwala, and Joan Schmelz

Paper: The First Mid-infrared Detection of HNC in the Interstellar Medium: Probing the Extreme Environment toward the Orion Hot Core
Nickerson, et al., 2021/01, ApJ, 907, 51.

High-resolution molecular line surveys provide a chemical inventory for star forming regions — essential for establishing the relative importance of potential chemical networks, understanding organic chemistry associated with star formation, and providing constraints on the supply pathways of key organic molecules in Earth-like planet formation.

Previous high spectral resolution line surveys have been limited to radio, sub-mm, and far-infrared wavelengths, but mid-infrared (MIR) observations are the only way to study symmetric molecules like CH₄, C₂H₂, and C₂H₆ that have no dipole moment and thus cannot be observed through rotational transitions. The MIR astronomical missions such as the Infrared Space Observatory (ISO) and Spitzer had low to moderate resolving power. Therefore, they were only able to identify molecular bands and could not resolve their individual rovibrational transitions needed to identify specific molecules with certainty. Due to the lack of high spectral resolution data in the MIR, the model chemical networks require testing and refinement in this regime.

The first comprehensive high-resolution molecular-line survey targeting the longer MIR from 12.5–28.3 µm with SOFIA's Echelon-Cross- Echelle Spectrograph (EXES) instrument opens up a new, largely unexplored discovery space.

Researchers targeted the hot core IRc2 in Orion, the nearest and best-studied region of massive star formation. Here, some early results from this survey are described, including the first MIR detections of HNC and H₁₃CN in the target region.

HCN and HNC are astronomically ubiquitous. They are observed in our solar system, star-forming regions, and extragalactic sources. Though created in similar quantities at cool temperatures, the HNC molecule is less stable. The HCN/HNC abundance ratio is close to unity at the low temperatures in pre-stellar clouds and increases towards later stages in star-formation; thus this ratio can be used as a chemical clock.

Hot cores associated with high mass protostars are a rich source of chemistry in the interstellar medium (ISM) and known to harbour molecules such as HCN and HNC. They represent a key stage in stellar evolution as a young protostar heats its natal, icy mantles to unlock reservoirs of molecules.

Despite being the first hot molecular core discovered, IRc2 is atypical. Most hot cores envelop high mass protostars and are internally heated, while IRc2 is heated externally. An explosive event in the Orion BN/KL region 500 years ago possibly separated IRc2 from the protostar called Radio Source I. Radio Source I has no MIR component and is presently obscured by dust.

The derived ratio from the EXES spectra, HCN/HNC=72, offers insight into the origins of IRc2. A gas-grain chemical network was utilized to model the evolution of HCN/HNC in three phases: free fall collapse, warmup, and post warmup. The model reaches the derived HCN/HNC after 10⁶ years, suggesting that the hot core’s origin predates the explosive event. It is likely that Radio Source I was once embedded in IRc2 and heated it to reach HCN/HNC=72.

The derived 12C/13C=13 is lower than most measurements in the sub-mm/mm and radio, and over five times lower than expected for Orion’s galactocentric distance. This ratio is similar, however, to that derived from C₂H₂ measurements, also from EXES. If the HCN lines were optically thick and the HCN column density underestimated, this could explain this low ratio. However, the data do not show any of the expected signs of optically thick lines, suggesting that galactocentric distance may not be the only factor affecting the ratio.

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

All three molecules arise in cooler 100–200 K gas that is moving at a velocity similar to the outflow from the nearby high mass protostar Radio Source I, and are likely associated with it. There is also a second warmer HCN component at a separate velocity, the closest detection ever to the heart of the hot core itself. These results show that SOFIA allows us study the chemistry and physical conditions closest to the protostars without spatially resolving them.

The work presented here is part of a wider molecular survey of Orion IRc2 with EXES spanning 7.2 to 8 and 13.2 to 28.3 µm. The survey has detected a forest of molecular transitions in this wavelength region. Researchers are currently identifying and using them to build an inventory of molecules in the MIR to complement line surveys in other wavebands. Such comprehensive line surveys not only create a rich legacy archive, but also provide a valuable reference database for future James Webb Space Telescope observations.