The Power of SOFIA/FORCAST in Estimating Internal Luminosities of Low
  Mass Class 0/I Protostars
  With the Stratospheric Observatory for Infrared Astronomy (SOFIA) routinely
operating science flights, we demonstrate that observations with the Faint
Object infraRed CAmera for the SOFIA Telescope (FORCAST) can provide reliable
estimates of the internal luminosities, $L_{\rm int}$, of protostars. We have
developed a technique to estimate $L_{\rm int}$ using a pair of FORCAST
filters: one "short-wavelength" filter centered within 19.7-25.3 $\mu$m, and
one "long-wavelength" filter within 31.5-37.1 $\mu$m. These $L_{\rm int}$
estimates are reliable to within 30-40% for 67% of protostars and to within a
factor of 2.3-2.6 for 99% of protostars. The filter pair comprised of
F25.3$\mu$m and F37.1$\mu$m achieves the best sensitivity and most constrained
results. We evaluate several assumptions that could lead to systematic
uncertainties. The OH5 dust opacity matches observational constraints for
protostellar environments best, though not perfectly; we find that any improved
dust model will have a small impact of 5-10% on the $L_{\rm int}$ estimates.
For protostellar envelopes, the TSC84 model yields masses that are twice those
of the Ulrich model, but we conclude this mass difference does not
significantly impact results at the mid-infrared wavelengths probed by FORCAST.
Thus, FORCAST is a powerful instrument for luminosity studies targeting newly
discovered protostars or suspected protostars lacking detections longward of 24
$\mu$m. Furthermore, with its dynamic range and greater angular resolution,
FORCAST may be used to characterize protostars that were either saturated or
merged with other sources in previous surveys using the Spitzer Space Telescope
or Herschel Space Observatory.
