  At least 25 per cent of massive stars are ejected from their parent cluster,
becoming runaways or exiles, travelling with often-supersonic space velocities
through the interstellar medium (ISM). Their overpressurised H II regions
impart kinetic energy and momentum to the ISM, compress and/or evaporate dense
clouds, and can constrain properties of both the star and the ISM. Here we
present one-, two-, and (the first) three-dimensional simulations of the H II
region around a massive star moving supersonically through a uniform,
magnetised ISM, with properties appropriate for the nearby O star Zeta Oph. The
H II region leaves an expanding overdense shell behind the star and, inside
this, an underdense wake that should be filled with hot gas from the shocked
stellar wind. The gas column density in the shell is strongly influenced by the
ISM magnetic field strength and orientation. H-alpha emission maps show the H
II region remains roughly circular, although the star is displaced somewhat
from the centre of emission. For our model parameters, the kinetic energy
feedback from the H II region is comparable to the mechanical luminosity of the
stellar wind, and the momentum feedback rate is >100X larger than that from the
wind and about 10X larger than the total momentum input rate available from
radiation pressure. Compared to the star's eventual supernova explosion, the
kinetic energy feedback from the H II region over the star's main sequence
lifetime is >100X less, but the momentum feedback is up to 4X larger. H II
region dynamics are found to have only a small effect on the ISM conditions
that a bow shock close to the star would encounter.
