A Massive Protostar Forming by Ordered Collapse of a Dense, Massive Core

We present 30 and 40 mu m 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 3.3 x 10(4) L-circle dot. 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 similar to 20-34 M-circle dot depending on the outflow cavity opening angle (35 degrees-50 degrees), and Sigma(cl) similar to 0.4-1 g cm(-2). After accounting for the foreground extinction and the flashlight effect, the true bolometric luminosity is similar to(0.7-2.2) x 10(5) L-circle dot. One of these models also has excellent agreement with the observed intensity profiles along the outflow axis at 10, 18, 31, and 37 mu m. 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.