Interstellar MHD Turbulence and Star Formation
  This chapter reviews the nature of turbulence in the Galactic interstellar
medium (ISM) and its connections to the star formation (SF) process. The ISM is
turbulent, magnetized, self-gravitating, and is subject to heating and cooling
processes that control its thermodynamic behavior. The turbulence in the warm
and hot ionized components of the ISM appears to be trans- or subsonic, and
thus to behave nearly incompressibly. However, the neutral warm and cold
components are highly compressible, as a consequence of both thermal
instability in the atomic gas and of moderately-to-strongly supersonic motions
in the roughly isothermal cold atomic and molecular components. Within this
context, we discuss: i) the production and statistical distribution of
turbulent density fluctuations in both isothermal and polytropic media; ii) the
nature of the clumps produced by thermal instability, noting that, contrary to
classical ideas, they in general accrete mass from their environment; iii) the
density-magnetic field correlation (or lack thereof) in turbulent density
fluctuations, as a consequence of the superposition of the different wave modes
in the turbulent flow; iv) the evolution of the mass-to-magnetic flux ratio
(MFR) in density fluctuations as they are built up by dynamic compressions; v)
the formation of cold, dense clouds aided by thermal instability; vi) the
expectation that star-forming molecular clouds are likely to be undergoing
global gravitational contraction, rather than being near equilibrium, and vii)
the regulation of the star formation rate (SFR) in such gravitationally
contracting clouds by stellar feedback which, rather than keeping the clouds
from collapsing, evaporates and diperses them while they collapse.
