The widths of magnetised filaments in molecular clouds
  Filaments are an ubiquitous feature of molecular clouds, and appear to play a
critical role in assembling the material to form stars. The dominant filaments
are observed to have a rather narrow range of widths around $\sim 0.1$ pc, and
to be preferentially aligned perpendicularly to the direction of the local
magnetic field. We have previously argued that the observed filament widths can
be explained if filaments are formed by converging, mildly supersonic flows,
resulting from large-scale turbulent motions in the parent molecular cloud.
Here we demonstrate that the introduction of a magnetic field perpendicular to
the filament long axis does not greatly alter this conclusion, as long as the
mass-to-flux ratio is supercritical. The distribution of widths for
supercritical magnetised filaments formed via this mechanism is peaked at
slightly higher values, and is slightly broader, than for non-magnetised
filaments, but still reproduces the basic properties of the width distributions
derived from far-infrared observations of molecular clouds. In contrast,
subcritical filaments have width distributions with a fundamentally different
shape, and typically have much larger widths than those observed. Both
subcritical and supercritical filaments are consistent with the observed lack
of correlation between filament widths and filament surface densities.
