Radiation-magnetohydrodynamic simulations of HII regions and their
  associated PDRs in turbulent molecular clouds
  We present the results of radiation-magnetohydrodynamic simulations of the
expansion of HII regions and surrounding photodissociation regions in
turbulent, magnetised, molecular clouds on scales of up to 4 parsecs, including
the effects of ionising and non-ionising ultraviolet radiation and x rays from
young star clusters. We find that HII region expansion reduces the disordered
component of the B field, imposing a large-scale order on the field around its
border. The field in the neutral gas lies along the ionisation front, while the
field in the ionised gas tends to be perpendicular to this. The highest
pressure compressed neutral/molecular gas is driven towards approximate
equipartition between thermal/magnetic/turbulent energy densities, whereas
lower pressure neutral/molecular gas divides into quiescent, magnetically
dominated regions, and, on the other hand, turbulent, demagnetised regions. The
ionised gas shows approximate thermal/turbulent equipartition, but with
magnetic energy densities 1 to 3 orders of magnitude lower. A high velocity
dispersion (approx 8 km/s) is maintained in the ionised gas throughout our
simulations, despite the mean expansion velocity being significantly lower. The
B field does not significantly brake the HII region expansion on the length and
timescales accessible to our simulations, but it does tend to suppress the
small-scale fragmentation and radiation-driven implosion of neutral/molecular
gas that forms globules and pillars at the edge of the HII region. However, the
relative luminosity of ionising and non-ionising radiation has a much larger
influence than the presence or absence of the B field. When the radiation field
is relatively soft (as in the case of a lower mass cluster, with earliest
spectral type of B0.5), then fragmentation is less vigorous and a thick,
relatively smooth PDR forms. Movies available at
http://youtube.com/user/divBequals0
