SIMBAD references

A detailed theoretical model for the thermal balance, chemistry, and radiative transfer within centrally heated, quiescent molecular cloud cores (Doty & Neufeld) is used to predict the properties of the 183 GHz 3₁₃-2₂₀ rotational masing transition of para-H₂O in Orion. The model suggests weak masing for this transition, with maximum negative optical depths at line center in the range 0.12←τ_max<0.34. The ensuing maser gain is a nonmonotonic function of the water abundance, peaking at intermediate values. The model results are compared with the observations of Cernicharo et al. for this transition. Fits to their data suggest there are two components: a ``narrow'' (Δv=2.47 km.s<sup>–1</sup>) feature consistent with previous studies of relatively quiescent gas and a ``broad'' (Δv=10.5 km.s^–1) feature consistent with the well-known shocks in Orion. Both chemical and uniform abundance models are compared with these data. The gas-phase chemical models are inconsistent with the observed constraints on the peak temperature and total source luminosity - confirming that the role of shock and/or grain mantle chemistry is important. If uniform abundances are assumed, the water abundance is conservatively constrained to the range 3x10^–6<x(H₂O)<3x10^–5.