Are infrared dark clouds really quiescent?

Context. The dense, cold regions where high-mass stars form are poorly characterized, 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. Aims: We investigate the initial conditions of HMSF by studying the dynamics and chemistry of HMSCs. Methods: We present continuum maps obtained from the Submillimeter Array (SMA) interferometry at 1.1 mm for four infrared dark clouds (IRDCs, G28.34 S, IRDC 18530, IRDC 18306, and IRDC 18308). For these clouds, we also present line surveys at 1 mm/3 mm obtained from IRAM 30 m single-dish observations. Results: (1) At an angular resolution of 2'' (~104 AU at an average distance of 4 kpc), the 1.1 mm SMA observations resolve each source into several fragments. The mass of each fragment is on average >10 M⊙, which exceeds the predicted thermal Jeans mass of the entire clump by a factor of up to 30, indicating that thermal pressure does not dominate the fragmentation process. Our measured velocity dispersions in the lines obtained by 30 m imply that non-thermal motion provides the extra support against gravity in the fragments. (2) Both non-detection of high-J transitions and the hyperfine multiplet fit of N2H+ (J = 1 → 0), C2H (N = 1 → 0), HCN(J = 1 → 0), and H13CN(J = 1 → 0) indicate that our sources are cold and young. However, the obvious detection of SiO and the asymmetric line profile of HCO+(J = 1 → 0) in G28.34 S 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 N2H+/NH2D and CO, our large-scale high-mass clumps exhibit similar chemical features to small-scale low-mass prestellar objects. Conclusions: This study of a small sample of IRDCs illustrates that thermal Jeans instability alone cannot explain the fragmentation of the clump into cold (T ~ 15 K), dense (>105 cm-3) cores and that these IRDCs are not completely quiescent