STAR FORMATION ACTIVITY OF CORES WITHIN INFRARED DARK CLOUDS

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 mu m emission, the so-called green fuzzies, which indicate shocked gas. Many cores also contain 24 mu m emission, presumably from heated dust which indicates embedded protostars. Because green fuzzies and 24 mu m 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 mu m 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.