A Massive Protostar Forming by Ordered Collapse of a Dense, Massive Core
  We present 30 and 40 micron imaging of the massive protostar G35.20-0.74 with
SOFIA-FORCAST. The high surface density of the natal core around the protostar
leads to high extinction, even at these relatively long wavelengths, causing
the observed flux to be dominated by that emerging from the near-facing outflow
cavity. However, emission from the far-facing cavity is still clearly detected.
We combine these results with fluxes from the near-infrared to mm to construct
a spectral energy distribution (SED). For isotropic emission the bolometric
luminosity would be 3.3x10^4 Lsun. We perform radiative transfer modeling of a
protostar forming by ordered, symmetric collapse from a massive core bounded by
a clump with high mass surface density, Sigma_cl. To fit the SED requires
protostellar masses ~20-34 Msun depending on the outflow cavity opening angle
(35 - 50 degrees), and Sigma_cl ~ 0.4-1 g cm-2. After accounting for the
foreground extinction and the flashlight effect, the true bolometric luminosity
is ~ (0.7-2.2)x10^5 Lsun. One of these models also has excellent agreement with
the observed intensity profiles along the outflow axis at 10, 18, 31 and 37
microns. Overall our results support a model of massive star formation
involving the relatively ordered, symmetric collapse of a massive, dense core
and the launching bipolar outflows that clear low density cavities. Thus a
unified model may apply for the formation of both low and high mass stars.
