The detection of protostellar condensations in infrared dark cloud cores

Infrared dark clouds ( IRDCs) are a distinct class of interstellar molecular cloud identified as dark extinction features against the bright mid-infrared Galactic background. Here we present high angular resolution millimeter continuum images obtained with the IRAM Plateau de Bure Interferometer toward four high-mass ( 200-1800 M-circle dot) IRDC cores that show evidence for active high-mass star formation ( M > 8 M-circle dot). We detect twelve bright (> 7 mJy), compact ( less than or similar to 2, less than or similar to 0.024 pc) condensations toward these cores. Two of the cores ( G024.60+00.08 MM1 and G024.60+00.08 MM2) are resolved into multiple protostellar condensations, while one core ( G022.35+00.41 MM1) shows two condensations. The remaining core ( G024.33+00.11 MM1) contains a single, compact protostellar condensation with a very rich molecular spectrum, indicating that this is a hot molecular core associated with an early stage in the formation of a high-mass protostar. The derived gas masses for these condensations suggest that each core is forming at least one high-mass protostar ( M-gas > 8 M-circle dot), and three cores are also forming lower mass protostars ( M-gas similar to 2-5 M-circle dot). A comparison of the ratios of the gas masses ( M-G) to the Jeans masses ( M-J) for IRDCs, cores, and condensations, provides broad support for the idea of hierarchical fragmentation. The close proximity of multiple protostars of disparate mass indicates that these IRDCs are in the earliest evolutionary states in the formation of stellar clusters.