RADIATIVE COOLING OF WARM MOLECULAR GAS

We consider the radiative cooling of warm (T greater-than-or-equal-to 100 K), fully molecular astrophysical gas by rotational and vibrationa l transitions of the molecules H2O, CO, and H-2. Using an escape proba bility method to solve for the molecular level populations, we have ob tained the cooling rate for each molecule as a function of temperature , density, and an optical depth parameter. A four-parameter expression proves useful in fitting the run of cooling rate with density for any fixed values of the temperature and optical depth parameter. We ident ify the various cooling mechanisms which are dominant in different reg ions of the astrophysically relevant parameter space. Given the assump tion that water is very abundant in warm regions of the interstellar m edium, H2O rotational transitions are found to dominate the cooling of warm interstellar gas over a wide portion of the parameter space cons idered. While chemical models for the interstellar medium make the str ong prediction that water will be produced copiously at temperatures a bove a few hundred degrees, our assumption of a high water abundance h as yet to be tested observationally. The Infrared Space Observatory an d the Submillimeter Wave Astronomy Satellite will prove ideal instrume nts for testing whether water is indeed an important coolant of inters tellar and circumstellar gas.