Context :

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 characterize the first phases in the evolution of intermediate mass 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 best fits the data .
The derived dust density and temperature profiles have been then used to compute the gas temperature profile , equating gas cooling and heating terms across the envelope .
Last , we computed the water line spectrum for various possible values of water abundance .

Results : The luminosity of FIR4 has been reevaluated to 1000 L _ { \odot } , making FIR4 definitively an Intermediate Mass protostar .
The envelope surrounding FIR4 has a relatively shallow density power law index , \sim 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 which will be observed by the Herschel Space Observatory .

Conclusions :