Physical structure and water line spectrum predictions of the intermediate mass protostar OMC2-FIR4

Aims. Intermediate mass (IM) stars are an important component of our Galaxy, as they significantly contribute to the interstellar FUV field and, consequently, play an important role in the energy balance of the ISM. Despite their importance, very little is known about their formation process and only a few studies have been devoted to characterizing the first phases in the evolution of IM protostars. Here we consider in great detail the case of the brightest and closest known young IM protostar: FIR4 in the OMC2 component of the Orion molecular cloud complex. Methods. We analyzed the available continuum emission (maps and SED) through one-dimensional dust radiative transfer calculations. We ran large grids of models to find the envelope model that reproduces the data best. The derived dust density and temperature profiles were then used to compute the gas temperature profile, equating gas cooling and heating terms across the envelope. Finally, we computed the water line spectrum for various possible values of water abundance. Results. The luminosity of FIR4 has been reevaluated to 1000 L(circle dot), making FIR4 definitively an IM protostar. The envelope surrounding FIR4 has a relatively shallow density power-law index, similar to 0.6. The most surprising result is that the gas and dust are thermally decoupled in the interior of the envelope, where the dust ices sublimate at 100 K. This has important consequences in the interpretation of the line data. We provide the predictions for the water spectrum, and discuss in detail the lines that will be observed by the Herschel Space Observatory.