A molecular line study of the filamentary infrared dark cloud
  G304.74+01.32
  The aim of this study is to better understand the physical and chemical
properties of the filamentary IRDC G304.74+01.32. In particular, we aim to
investigate the kinematics and dynamical state of the cloud and clumps within
it, and the amount of CO depletion. All the submillimetre peak positions in the
cloud identified from our previous LABOCA 870-micron map were observed in
C17O(2-1) with APEX. Selected positions were also observed in the 13CO(2-1),
SiO(5-4), and CH3OH(5_k-4_k) transitions at ~1 mm wavelength. The C17O lines
were detected towards all target positions at similar radial velocities,
indicating that G304.74 is a coherent filamentary structure. CO does not appear
to be significantly depleted in the clumps. Two- to three methanol 5_k-4_k
lines near ~241.8 GHz were detected towards all selected target positions,
whereas SiO(5-4) was seen in only one of these positions. The 13CO(2-1) lines
show blue asymmetric profiles, indicating large-scale infall motions. The
clumps show trans- to supersonic non-thermal motions, and virial-parameter
analysis suggests that most of them are gravitationally bound. The external
pressure may also play a non-negligible role in the dynamics. This is
qualitatively consistent with our earlier suggestion that the filament was
formed by converging supersonic turbulent flows. The analysis suggests that the
fragmentation of the filament into clumps is caused by "sausage"-type
instability, in agreement with results from other IRDCs. The star-formation
activity in the cloud, such as outflows, is likely responsible in releasing CO
from the icy grain mantles back into the gas phase. Shocks related to outflows
may have also injected CH3OH, SiO, and DCN into the gas-phase in SMM 3.
