The Early Stages of Star Formation in Infrared Dark Clouds: Characterizing the Core Dust Properties

Identified as extinction features against the bright Galactic mid-infrared background, infrared dark clouds (IRDCs) are thought to harbor the very earliest stages of star and cluster formation. In order to better characterize the properties of their embedded cores, we have obtained new 24 μm, 60-100 μm, and submillimeter continuum data toward a sample of 38 IRDCs. The 24 μm Spitzer images reveal that while the IRDCs remain dark, many of the cores are associated with bright 24 μm emission sources, which suggests that they contain one or more embedded protostars. Combining the 24 μm, 60-100 μm, and submillimeter continuum data, we have constructed broadband spectral energy distributions (SEDs) for 157 of the cores within these IRDCs and, using simple graybody fits to the SEDs, have estimated their dust temperatures, emissivities, opacities, bolometric luminosities, masses, and densities. Based on their Spitzer/Infrared Array Camera 3-8 μm colors and the presence of 24 μm point-source emission, we have separated cores that harbor active, high-mass star formation from cores that are quiescent. The active "protostellar" cores typically have warmer dust temperatures and higher bolometric luminosities than the more quiescent, perhaps "pre-protostellar," cores. Because the mass distributions of the populations are similar, however, we speculate that the active and quiescent cores may represent different evolutionary stages of the same underlying population of cores. Although we cannot rule out low-mass star formation in the quiescent cores, the most massive of them are excellent candidates for the "high-mass starless core" phase, the very earliest in the formation of a high-mass star.