Bipolar H{\sc ii} regions produced by cloud/cloud collisions
  We suggest that Bipolar H{\sc ii} Regions may be the aftermath of collisions
between clouds. Such a collision will produce a shock-compressed layer, and a
star cluster can then condense out of the dense gas near the centre of the
layer. If the clouds are sufficiently massive, the star cluster is likely to
contain at least one massive star, which emits ionising radiation, and excites
an H{\sc ii} region, which then expands, sweeping up the surrounding neutral
gas. Once most of the matter in the clouds has accreted onto the layer,
expansion of the H{\sc ii} Region meets little resistance in directions
perpendicular to the mid-plane of the layer, and so it expands rapidly to
produce two lobes of ionised gas, one on each side of the layer. Conversely, in
directions parallel to the mid-plane of the layer, expansion of the H{\sc ii}
Region stalls due to the ram-pressure of the gas that continues to fall towards
the star cluster from the outer parts of the layer; a ring of dense neutral gas
builds up around the waist of the Bipolar H{\sc ii} Region, and may spawn a
second generation of star formation. In this paper we present a dimensionless
model for the flow of ionised gas in a Bipolar H{\sc ii} Region created
according to the above scenario, and predict the characteristics of the
resulting freefree continuum and recombination-line emission. This
dimensionless model can be scaled to the physical parameters of any particular
system. Our intention is that these predictions will be useful in testing the
scenario outlined above, and thereby providing indirect support for the role of
cloud/cloud collisions in triggering star formation.
