A Universal Relation for Inelastic Molecular Collisional Rates in Support of NASA FIR/SUBMM Observations of Molecular Gas

The understanding of stellar and planetary system formation has accelerated in the past decade due to an explosion of observations of young stellar objects (YSOs), including protoplanetary disks (PPDs), in the far infrared (FIR) by the Spitzer and Herschel space observatories. With the recent call for Cycle 2 observations on SOFIA and the anticipated launch of JWST, YSO observations from the near infrared to the FIR will grow in number, in improved resolution, and in higher sensitivity, critically testing the ability of spectral models to extract reliable scientific content from such observations. Here we propose a joint computational effort in quantum chemistry, atomic/molecular scattering, and spectral modeling to predict non-LTE emission from YSOs, focusing on extremely complex systems from small species of high electronic angular momentum to relatively high-dimensional molecules. In particular, we propose to compute first-principles electronic potential energy surfaces (PES) for Fe, TiO, and C2H2 interacting with He, H, and H2. These PESs will then be used in fully-quantal scattering calculations to predict inelastic fine-structure, rotational, and rovibrational excitation rate coefficients, as appropriate. This collisional data, for which no explicit measurements or calculations are currently available, will be used to compute non-LTE spectra with the Cloudy spectral simulation package. In additional, a full TiO chemical network, including new TiO photodissociation cross sections, will be constructed and used, along with the TiO non-LTE emission, to predict the TiO opacity in PPDs. Results from this proposed work will aid in the interpretation of available and future YSO spectra obtained with ISO, Spitzer, Herschel, SOFIA, and JWST, as well as enhance the scientific return from such observations. The resulting molecular data will be distributed to the astrophysical modeling community through various databases (e.g., LAMDA and VAMDC).