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We present key results from the Herschel Orion Protostar Survey (HOPS):spectral energy distributions (SEDs) and model fits of 330 young stellarobjects, predominantly protostars, in the Orion molecular clouds. This is thelargest sample of protostars studied in a single, nearby star-formationcomplex. With near-infrared photometry from 2MASS, mid- and far-infrared datafrom Spitzer and Herschel, and sub-millimeter photometry from APEX, our SEDscover 1.2-870 $\mu$m and sample the peak of the protostellar envelope emissionat ~100 $\mu$m. Using mid-IR spectral indices and bolometric temperatures, weclassify our sample into 92 Class 0 protostars, 125 Class I protostars, 102flat-spectrum sources, and 11 Class II pre-main-sequence stars. We implement asimple protostellar model (including a disk in an infalling envelope withoutflow cavities) to generate a grid of 30400 model SEDs and use it todetermine the best-fit model parameters for each protostar. We argue thatfar-IR data are essential for accurate constraints on protostellar envelopeproperties. We find that most protostars, and in particular the flat-spectrumsources, are well-fit. The median envelope density and median inclination angledecrease from Class 0 to Class I to flat-spectrum protostars, despite the broadrange in best-fit parameters in each of the three categories. We also discussdegeneracies in our model parameters. Our results confirm that the differentprotostellar classes generally correspond to an evolutionary sequence with adecreasing envelope infall rate, but the inclination angle also plays a role inthe appearance, and thus interpretation, of the SEDs.

The Herschel Orion Protostar Survey: Spectral Energy Distributions and Fits Using a Grid of Protostellar Models