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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. Despitetheir importance, very little is known about their formation process and only afew studies have been devoted to characterize the first phases in the evolutionof intermediate mass protostars. Here we consider in great detail the case ofthe brightest and closest known young IM protostar: FIR4 in the OMC2 componentof the Orion molecular cloud complex. Methods. We analyzed the availablecontinuum emission (maps and SED) through one-dimensional dust radiativetransfer calculations. We ran large grids of models to find the envelope modelthat best fits the data. The derived dust density and temperature profiles havebeen then used to compute the gas temperature profile, equating gas cooling andheating terms across the envelope. Last, we computed the water line spectrumfor various possible values of water abundance. Results. The luminosity of FIR4has been reevaluated to 1000 Lo, making FIR4 definitively an Intermediate Massprotostar. The envelope surrounding FIR4 has a relatively shallow density powerlaw index, ~ 0.6. The most surprising result is that the gas and dust arethermally decoupled in the interior of the envelope, where the dust icessublimate at 100 K. This has important consequences in the interpretation ofthe line data. We provide the predictions for the water spectrum, and discussin detail the lines which will be observed by the Herschel Space Observatory .

Physical structure and water line spectrum predictions of the Intermediate Mass protostar OMC2-FIR4