Toward Understanding the Origin of Turbulence in Molecular Clouds: Small
  Scale Structures as Units of Dynamical Multi-Phase Interstellar Medium
  In order to investigate the origin of the interstellar turbulence, detailed
observations in the CO J=1--0 and 3--2 lines have been carried out in an
interacting region of a molecular cloud with an HII region. As a result,
several 1,000 to 10,000 AU scale cloudlets with small velocity dispersion are
detected, whose systemic velocities have a relatively large scatter of a few
km/s. It is suggested that the cloud is composed of small-scale dense and cold
structures and their overlapping effect makes it appear to be a turbulent
entity as a whole. This picture strongly supports the two-phase model of
turbulent medium driven by thermal instability proposed previously. On the
surface of the present cloud, the turbulence is likely to be driven by thermal
instability following ionization shock compression and UV irradiation. Those
small scale structures with line width of ~ 0.6 km/s have a relatively high CO
line ratio of J=3--2 to 1--0, 1 < R(3-2/1-0) < 2. The large velocity gradient
analysis implies that the 0.6 km/s width component cloudlets have an average
density of 10^{3-4} cm^{-3}, which is relatively high at cloud edges, but their
masses are only < 0.05 M_{sun}.
