The Herschel Orion Protostar Survey: Spectral Energy Distributions and
  Fits Using a Grid of Protostellar Models
  We present key results from the Herschel Orion Protostar Survey (HOPS):
spectral energy distributions (SEDs) and model fits of 330 young stellar
objects, predominantly protostars, in the Orion molecular clouds. This is the
largest sample of protostars studied in a single, nearby star-formation
complex. With near-infrared photometry from 2MASS, mid- and far-infrared data
from Spitzer and Herschel, and sub-millimeter photometry from APEX, our SEDs
cover 1.2-870 $\mu$m and sample the peak of the protostellar envelope emission
at ~100 $\mu$m. Using mid-IR spectral indices and bolometric temperatures, we
classify our sample into 92 Class 0 protostars, 125 Class I protostars, 102
flat-spectrum sources, and 11 Class II pre-main-sequence stars. We implement a
simple protostellar model (including a disk in an infalling envelope with
outflow cavities) to generate a grid of 30400 model SEDs and use it to
determine the best-fit model parameters for each protostar. We argue that
far-IR data are essential for accurate constraints on protostellar envelope
properties. We find that most protostars, and in particular the flat-spectrum
sources, are well-fit. The median envelope density and median inclination angle
decrease from Class 0 to Class I to flat-spectrum protostars, despite the broad
range in best-fit parameters in each of the three categories. We also discuss
degeneracies in our model parameters. Our results confirm that the different
protostellar classes generally correspond to an evolutionary sequence with a
decreasing envelope infall rate, but the inclination angle also plays a role in
the appearance, and thus interpretation, of the SEDs.
