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

At the onset of high-mass star formation, accreting protostars are deeplyembedded in massive cores made of gas and dust. Their spectral energydistribution is still dominated by the cold dust and rises steeply from near-tofar-infrared wavelengths. The young massive star-forming region IRDC18223-3 isa prototypical Infrared-Dark-Cloud with a compact mm continuum core that showsno protostellar emission below 8mum. However, based on outflow tracers, earlystar formation activity was previously inferred for this region. Here, wepresent recent Spitzer observations from the MIPSGAL survey that identify thecentral protostellar object for the first time at 24 and 70mum. Combining themid- to far-infrared data with previous mm continuum observations and the upperlimits below 8mum, one can infer physical properties of the central source. Atleast two components with constant gas mass M and dust temperature T arenecessary: one cold component (~15K and ~576M_sun) that contains most of themass and luminosity, and one warmer component (>=51K and >=0.01M_sun) toexplain the 24mum data. The integrated luminosity of ~177L_sun can be used toconstrain additional parameters of the embedded protostar from the turbulentcore accretion model for massive star formation. The data of IRDC18223-3 areconsistent with a massive gas core harboring a low-mass protostellar seed ofstill less than half a solar mass with high accretion rates of the order10^-4M_sun/yr. In the framework of this model, the embedded protostar isdestined to become a massive star at the end of its formation processes.