  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 24um, 60-100um,
and sub-millimeter continuum data toward a sample of 38 IRDCs. The 24um Spitzer
images reveal that while the IRDCs remain dark, many of the cores are
associated with bright 24um emission sources, which suggests that they contain
one or more embedded protostars. Combining the 24um, 60-100um, and
sub-millimeter 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 3-8um colors and the presence of 24um 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.
