Building a High-Resolution Mid-Infrared Spectral Line Database for Hot Molecular Cores

Hot molecular cores associated with high mass protostars are a rich source of chemistry in the ISM, connecting star formation to planetary systems such as our own. They represent a key stage in stellar evolution as a young protostar heats its natal, icy mantle to unlock reservoirs of molecules.

Molecular line surveys are commonly conducted at longer wavelengths, in the far-infrared to the sub-mm/mm. The mid-infrared (MIR), however, provides the only access to rovibrational transitions and molecules with no permanent dipole moment, and probes hot core material closest to embedded protostars. Nonetheless, the MIR has historically been underutilized in analogous astrochemical studies for reasons including atmospheric interference and low available spectral resolution from space-based missions.

We will present ongoing work on building a database of molecular transitions towards multiple hot cores in the MIR. The primary data used are from the EXES spectrograph aboard the Stratospheric Observatory for Infrared Astronomy (SOFIA). This is the only instrument that accesses the entire MIR with the spectral resolution to distinguish individual molecular transitions and identify molecules with certainty. We will present results on various molecular species (H₂O, CH₄, SO₂, etc.) from Orion IRc2 survey that covers 7.2 to 25 microns with EXES. We have established about 350 unique features and have identified nine species with two isotopes. We will also present preliminary comparisons of IRc2 with other hot cores (AFGL 2591, AFGL 2136, NGC 7538 IRS 1, Mon R2, and W3) using data from the SOFIA archive.

Though the James Webb Space Telescope (JWST) will be more sensitive than SOFIA, its low spectral resolution could lead to confusion in identifying the contribution of gaseous molecular species. Our database will greatly enhance the inventory of bright, resolved line features of hot cores in the MIR, providing an invaluable reference for planning future JWST observations and inform astrochemical models.