Caught in the Act: The Onset of Massive Star Formation

Combining mid-infrared data from the Spitzer Space Telescope with cold gas and dust emission observations from the Plateau de Bure Interferometer, we characterize the infrared dark cloud IRDC 18223-3 at high spatial resolution. The millimeter continuum data reveal a massive 184 M gas core with a projected size of 28,000 AU that has no associated protostellar mid-infrared counterpart. However, the detection of 4.5 m emission at the edge of the core indicates early outflow activity, which is supported by broad CO and CS spectral line-wing emission. Moreover, systematically increasing N2H+(10) line width toward the millimeter core center can be interpreted as additional evidence for early star formation. Furthermore, the N2H+(10) line emission reveals a less massive secondary core that could be in an evolutionary stage prior to any star formation activity.