Keto, Eric, Rawlings, Jonathan, and Caselli, Paola. 2014. "Chemistry and radiative transfer of water in cold, dense clouds." Monthly Notices of the Royal Astronomical Society 440:2616-2624. https://doi.org/10.1093/mnras/stu426
The Herschel Space Observatory's recent detections of water vapour in the cold, dense cloud L1544 allow a direct comparison between observations and chemical models for oxygen species in conditions just before star formation. We explain a chemical model for gas-phase water, simplified for the limited number of reactions or processes that are active in extreme cold (2O (110-101) line. Despite discouragingly high optical depth caused by the large Einstein A coefficient, the subcritical excitation in the cold, rarefied H2 causes the line brightness to scale linearly with column density. Thus, the water line can provide information on the chemical and dynamical processes in the darkest region in the centre of a cold, dense cloud. The inverse P-Cygni profile of the observed water line generally indicates a contracting cloud. This profile is reproduced with a dynamical model of slow contraction from unstable quasi-static hydrodynamic equilibrium (an unstable Bonnor-Ebert sphere).
