The 5.5-8 microns spectral region is rich in vibrational and rovibrational transitions of organic molecules and their isotopologues, including pre-biotic molecules. Spectral signatures from water (in particular the ground state v2 vibrational band), formaldehyde, methane, ammonia, CH3, HCN, and more complex organics can be emitted from warm and hot regions around young stellar objects, as well as the inner regions (<a few AU) of protoplanetary disks. High resolution mid-IR spectra are hence an important tool to understand the mechanics of dust grain evaporation in stellar environments, and eventually to retrace chemical evolution during planet formation.

While the 5.5-8 microns region is inaccessible from the ground, it is observable with the EXES instrument aboard SOFIA. Thanks to its high spectral resolution modes up to R~90000, much higher than what Spitzer IRS could offer, profiles of blended and individual transitions can be analyzed to estimate molecular abundances and excitation temperatures. In addition, measurements of the gas velocity through Doppler-shifts can help to identify the source region for each molecule.

Over the past several years, EXES observations have contributed to a rich inventory of mid-IR spectra from YSOs and protoplanetary disks. The available public database includes sources such as: GV Tau's disk (project 05_0097, Carr et al., in prep), and massive protostars AFGL 2136, AFGL 2591 (projects 04_0120/05_0041 - Barr et al. 2020, 2018, Indriolo et al. 2020), Orion IRc2 (project 05_0043/06_0061, Nickerson et al. in prep) and high-mass YSOs NGC 7538 IRS 1 and IRS 9 (project 75_0024).All calibrated data is available from the IRSA SOFIA Archive.

Gaussian fits for selected HCN lines normalized to their baselines

Gaussian fits for selected HCN lines normalized to their baselines. The HCN fluxes have been corrected for atmospheric absorption. HCN is best fit by a double Gaussian showing two velocity components. The vertical dotted line indicates the systematic, ambient cloud velocity 9 km s−1 (Zapata et al. 2012)

Nickerson et al. 2020
EXES
Absorption lines
chemical abundance
dust
protoplanetary disk
star formation
stellar evolution and models
Young stars and protostellar objects