Are Infrared Dark Clouds Really Quiescent?
  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 1.1 mm for four infrared dark clouds (IRDCs, G28.34S, IRDC
18530, IRDC 18306, and IRDC 18308). We also present 1 mm/3 mm line surveys
using IRAM 30 m single-dish observations.
  Our results are: (1) At a spatial resolution of 10^4 AU, the 1.1 mm SMA
observations resolve each source into several fragments. The mass of each
fragment is on average >10 Msun, which exceeds the predicted thermal Jeans mass
of the whole clump by a factor of up to 30, indicating that thermal pressure
does not dominate the fragmentation process. Our measured velocity dispersions
in the 30 m 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 N2H+(1-0), C2H(1-0), HCN(1-0), and
H13CN(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 N2H+/NH2D 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 (~15 K), dense (>10^5 cm-3) cores and that
these IRDCs are not completely quiescent.
