  We present a new SOFIA/FORCAST mid-IR survey of luminous protostars and
crowded star-forming environments in Cygnus X, the nearest million-solar mass
molecular cloud complex. We derive bolometric luminosities for over 1000
sources in the region with these new data in combination with extant Spitzer
and UKIDSS photometry, with 63 new luminous protostar candidates identified by
way of the high quality SOFIA/FORCAST data. By including FORCAST data, we
construct protostellar luminosity functions (PLFs) with improved completeness
at the high luminosity end. The PLFs are well described by a power law function
with an index of ~-0.5. Based on the Herschel temperature and column density
measurements, we find no obvious dependence of the PLFs on the local gas
temperature, but PLFs in regions of high stellar density or gas column density
exhibit some excess at higher luminosities. Through the comparison between our
observed PLFs and existing accretion models, both the turbulent core (TC) and
the competitive accretion (CA) models are consistent with our results, while
the isothermal sphere (IS) model is disfavored. The implications of these
results on the star formation process are discussed.
