2009ApJS..181..360C

Infrared Dark Clouds (IRDCs) contain compact cores which probably host the early stages of high-mass star formation. Many of these cores contain regions of extended, enhanced 4.5 µm emission, the so-called "green fuzzies," which indicate shocked gas. Many cores also contain 24 µm emission, presumably from heated dust which indicates embedded protostars. Because "green fuzzies" and 24 µm point sources both indicate star formation, we have developed an algorithm to identify star-forming cores within IRDCs by searching for the simultaneous presence of these two distinct indicators. We employ this algorithm on a sample of 190 cores found toward IRDCs, and classify the cores as "active" if they contain a green fuzzy coincident with an embedded 24 µm source, and as "quiescent" if they contain neither IR signature. We hypothesize that the "quiescent" cores represent the earliest "preprotostellar" (starless) core phase, before the development of a warm protostar, and that the "active" cores represent a later phase, after the development of a protostar. We test this idea by comparing the sizes, densities, and maser activity of the "active" and "quiescent" cores. We find that, on average, "active" cores have smaller sizes, higher densities, and more pronounced water and methanol maser activity than the "quiescent" cores. This is expected if the "quiescent" cores are in an earlier evolutionary state than the "active" cores. The masses of "active" cores suggest that they may be forming high-mass stars. The highest mass "quiescent" cores are excellent candidates for the elusive high-mass starless cores.