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Broadband spectral surveys of protostars offer a rich view of the physical,chemical and dynamical structure and evolution of star-forming regions. TheHerschel Space Observatory opened up the terahertz regime to such surveys,giving access to the fundamental transitions of many hydrides and to thehigh-energy transitions of many other species. A comparative analysis of thechemical inventories and physical processes and properties of protostars ofvarious masses and evolutionary states is the goal of the Herschel CHEmicalSurveys of Star forming regions (CHESS) key program. This paper focusses on theintermediate-mass protostar, OMC-2 FIR 4. We obtained a spectrum of OMC-2 FIR 4in the 480 to 1902 GHz range with the HIFI spectrometer onboard Herschel andcarried out the reduction, line identification, and a broad analysis of theline profile components, excitation, and cooling. We detect 719 spectral linesfrom 40 species and isotopologs. The line flux is dominated by CO, H2O, andCH3OH. The line profiles are complex and vary with species and upper levelenergy, but clearly contain signatures from quiescent gas, a broad componentlikely due to an outflow, and a foreground cloud. We find abundant evidence forwarm, dense gas, as well as for an outflow in the field of view. Line fluxrepresents 2% of the 7 L_Sol luminosity detected with HIFI in the 480 to 1250GHz range. Of the total line flux, 60% is from CO, 13% from H2O and 9% fromCH3OH. A comparison with similar HIFI spectra of other sources is set toprovide much new insight into star formation regions, a case in point being adifference of two orders of magnitude in the relative contribution of sulphuroxides to the line cooling of Orion KL and OMC-2 FIR 4.

The Herschel/HIFI spectral survey of OMC-2 FIR 4 (CHESS): An overview of the 480 to 1902 GHz range