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Description
The dense, cold regions where high-mass stars form are poorly characterised, yet they represent an ideal opportunity to learn more about the initial conditions of high-mass star formation (HMSF), since high-mass starless cores (HMSCs) lack the violent feedback seen at later evolutionary stages. We present continuum maps obtained from the Submillimeter Array (SMA) interferometry at 1.1mm for four infrared dark clouds (IRDCs, G28.34S, IRDC 18530, IRDC 18306, and IRDC 18308). We also present 1mm/3mm line surveys using IRAM 30m single-dish observations. Our results are: (1) At a spatial resolution of 10^4^AU, the 1.1mm SMA observations resolve each source into several fragments. The mass of each fragment is on average >10M_{sun}_, which exceeds the predicted thermal Jeans mass of the whole clump by a factor of up to 60, indicating that thermal pressure does not dominate the fragmentation process. Our measured velocity dispersions in the 30m lines imply that non-thermal motions provides the extra support against gravity in the fragments. (2) Both non-detection of high-J transitions and the hyperfine multiplet fit of N_2_H^+^(1-0), C_2_H(1-0), HCN(1-0), and H^13^CN(1-0) indicate that our sources are cold and young. However, obvious detection of SiO and the asymmetric line profile of HCO^+^(1-0) in G28.34S indicate a potential protostellar object and probable infall motion. (3) With a large number of N-bearing species, the existence of carbon rings and molecular ions, and the anti-correlated spatial distributions between N_2_H^+^/NH_2_D and CO, our large-scale high-mass clumps exhibit similar chemical features as small-scale low-mass prestellar objects. This study of a small sample of IRDCs illustrates that thermal Jeans instability alone cannot explain the fragmentation of the clump into cold (~15K), dense (>10^5^cm^-3^) cores and that these IRDCs are not completely quiescent.
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