The Protostar in the Massive Infrared Dark Cloud IRDC 18223-3

At the onset of high-mass star formation, accreting protostars are deeply embedded in massive cores made of gas and dust. Their spectral energy distribution is still dominated by the cold dust and rises steeply from near- to far-infrared wavelengths. The young massive star-forming region IRDC 18223-3 is a prototypical infrared dark cloud with a compact millimeter continuum core that shows no protostellar emission below 8 μm. However, based on outflow tracers, early star formation activity was previously inferred for this region. Here we present recent Spitzer observations from the MIPSGAL survey that identify the central protostellar object for the first time at 24 and 70 μm. Combining the mid- to far-infrared data with previous millimeter continuum observations and the upper limits below 8 μm, one can infer the physical properties of the central source. At least two components with constant gas mass M and dust temperature T are necessary: one cold component (~15 K and ~576 M☉) that contains most of the mass and luminosity, and one warmer component (≥51 K and ≥0.01 M☉) to explain the 24 μm data. The integrated luminosity of ~177 L☉ can be used to constrain additional parameters of the embedded protostar from the turbulent core accretion model for massive star formation. The data of IRDC 18223-3 are consistent with a massive gas core harboring a low-mass protostellar seed of still less than half a solar mass with high accretion rates of the order 10-4 M☉ yr-1. In the framework of this model, the embedded protostar is destined to become a massive star at the end of its formation processes.