If the microwave region can be observed, then the rotational spectrum can be used to calculate the temperature of the materials.
Does the rotational spectrum as a function of T vary with concentration (consider rotational-to vibrational, rotational-to-translational inelastic collisions as well...?)
The rotational modes of a molecule are much more sensitive to temperature than the vibrational modes. They both have temperature dependence, but you get a greater distribution of rotational states than you do with vibrational states over the same temperature range. By knowing what the materials are and the pressure range, the rotational spectra can be matched with those recorded under known conditions or computer modeled. Even though it is a gas disk around a star, the pressures are likely more like a vacuum than something like stmospheric pressure. Under these conditions, intermolecular collisions would not contribure significantly to the spectrum. It's not a direct function of temperature, like temperature is a function of the wavelength maximum of black body radiation. Astonomers and microwave spectroscopists do a lot of collaboration on thing like this. The organic chemicals in space are highly unsaturated hydrocarbons that could never exist in the atmospheric conditions on Earth (e.g. molecules like H-C=C=C=C=C-H) and their existence is known only through careful, temperature-sensitive modeling.