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Identified as extinction features against the bright Galactic mid-infraredbackground, infrared dark clouds (IRDCs) are thought to harbor the veryearliest stages of star and cluster formation. In order to better characterizethe properties of their embedded cores, we have obtained new 24um, 60-100um,and sub-millimeter continuum data toward a sample of 38 IRDCs. The 24um Spitzerimages reveal that while the IRDCs remain dark, many of the cores areassociated with bright 24um emission sources, which suggests that they containone or more embedded protostars. Combining the 24um, 60-100um, andsub-millimeter continuum data, we have constructed broadband spectral energydistributions (SEDs) for 157 of the cores within these IRDCs and, using simplegray-body fits to the SEDs, have estimated their dust temperatures,emissivities, opacities, bolometric luminosities, masses and densities. Basedon their Spitzer/IRAC 3-8um colors and the presence of 24um point sourceemission, we have separated cores that harbor active, high-mass star formationfrom cores that are quiescent. The active `protostellar' cores typically havewarmer dust temperatures and higher bolometric luminosities than the morequiescent, perhaps `pre-protostellar', cores. Because the mass distributions ofthe populations are similar, however, we speculate that the active andquiescent cores may represent different evolutionary stages of the sameunderlying population of cores. Although we cannot rule out low-massstar-formation in the quiescent cores, the most massive of them are excellentcandidates for the `high-mass starless core' phase, the very earliest in theformation of a high-mass star.

The early stages of star formation in Infrared Dark Clouds: characterizing the core dust properties