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 gray-body fits to the SEDs, have estimated their dust temperatures, emissivities, opacities, bolometric luminosities, masses and densities. Based on their Spitzer/IRAC 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 -2temperatures 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.Subject headings: dust, extinction -stars: formation -infrared: stars -submillimeter which achieved a 1σ sensitivity of ∼ 30, 70, and 150 mJy at 60, 75, and 90 µm respectively. For the remaining 7 bright cores (those with 1.2 mm fluxes between 1-2 Jy), 4 repeats of a 3 sec exposure were combined. All spectra were obtained in the pointed observation mode. At these wavelengths, Spitzer has an angular resolution of ∼13-24 (9.8 pixels).Because of the contamination by the second order diffracted light and an inoperative detector module, the wavelength coverage of the spectra was restricted to 65-97 µm. The spectra were acquired in two epochs: 2006 October and 2007 May. Data from 2006 were processed using the S14.4.0 pipeline version, while 2007 data were reduced using version S16.1.0. The pointed SED-mode observation provides a set of six pairs of data frames between the target position ('on') and nearby sky position ('off'). For all analysis, we use the pipeline produced post-basic calibrated data (post-BCDs), which deliver mosaic images for the 'on' and 'off' spectra.We set the scan mirror to chop between the 'on' and 'off' positions with a chop throw of 1 to 3 . The 'off' position was selected for each individual core to be nearby and free from 1.2 mm continuum emission. However, because the chop distance and visibility of the cores were limited, many of the...
