Effects of Strong Magnetic Fields on Photoionised Clouds
  Simulations are presented of the photoionisation of three dense gas clouds
threaded by magnetic fields, showing the dynamical effects of different initial
magnetic field orientations and strengths. For moderate magnetic field
strengths the initial radiation-driven implosion phase is not strongly affected
by the field geometry, and the photoevaporation flows are also similar. Over
longer timescales, the simulation with an initial field parallel to the
radiation propagation direction (parallel field) remains basically
axisymmetric, whereas in the simulation with a perpendicular initial field the
pillar of neutral gas fragments in a direction aligned with the magnetic field.
For stronger initial magnetic fields, the dynamics in all gas phases are
affected at all evolutionary times. In a simulation with a strong initially
perpendicular field, photoevaporated gas forms filaments of dense ionised gas
as it flows away from the ionisation front along field lines. These filaments
are potentially a useful diagnostic of magnetic field strengths in H II regions
because they are very bright in recombination line emission. In the strong
parallel field simulation the ionised gas is constrained to flow back towards
the radiation source, shielding the dense clouds and weakening the ionisation
front, eventually transforming it to a recombination front.
