2001ApJ...550..346A

It is known that most water in the interstellar medium is formed in the OH+H₂ reaction. It is also known that the newly formed water molecules from this reaction are born in highly excited vibrational and rotational states. Even at densities of 10⁹ cm^–3 in the interstellar medium these states relax in a fast IR radiation cascade to the ground state(s) of water emitting IR, far-infrared (FIR), and microwave light. Because this effect has not been included in the astrophysical literature before, we analyze here the intensity of this spontaneous emission at 6 µm, in the FIR and the microwave region. We calculate stationary rotational state distributions using a quantitative model that includes the nascent formation, the subsequent spontaneous emission, photolysis, and collisional redistribution. The radiation cascade yields FIR radiation and an alternative pump mechanism for water masers. For regions of massive water formation it is shown that the stationary rotational population is much less affected by collisions than by the nascent water formation. It is speculated that nascent reactive water might be responsible for the near-IR spectra at 6 µm, for the pure rotational transitions in the FIR radiation, and for the H₂O maser radiation.