Results from HOPS: A Multiwavelength Census of Orion Protostars
  Surveys with the Spitzer and Herschel space observatories are now enabling
the discovery and characterization of large samples of protostars in nearby
molecular clouds, providing the observational basis for a detailed
understanding of star formation in diverse environments. We are pursuing this
goal with the Herschel Orion Protostar Survey (HOPS), which targets 328
Spitzer-identified protostars in the Orion molecular clouds, the largest
star-forming region in the nearest 500 pc. The sample encompasses all phases of
protostellar evolution and a wide range of formation environments, from dense
clusters to relative isolation. With a grid of radiative transfer models, we
fit the 1-870 micron spectral energy distributions (SEDs) of the protostars to
estimate their envelope densities, cavity opening angles, inclinations, and
total luminosities. After correcting the bolometric luminosities and
temperatures of the sources for foreground extinction and inclination, we find
a spread of several orders of magnitude in luminosity at all evolutionary
states, a constant median luminosity over the more evolved stages, and a
possible deficit of high-inclination, rapidly infalling envelopes among the
Spitzer-identified sample. We have detected over 100 new sources in the
Herschel images; some of them may fill this deficit. We also report results
from modeling the pre- and post-outburst 1-870 micron SEDs of V2775 Ori (HOPS
223), a known FU Orionis outburster in the sample. It is the least luminous FU
Ori star with a protostellar envelope.
