  We present results of numerical simulations carried out with a 2D radiation
hydrodynamics code in order to study the impact of massive stars on their
surrounding interstellar medium. This first paper deals with the evolution of
the circumstellar gas around an isolated 60 Msun star. The interaction of the
photoionized HII region with the stellar wind bubble forms a variety of
interesting structures like shells, clouds, fingers, and spokes. These results
demonstrate that complex structures found in HII regions are not necessarily
relics from the time before the gas became ionized but may result from
dynamical processes during the course of the HII region evolution. We have also
analyzed the transfer and deposit of the stellar wind and radiation energy into
the circumstellar medium until the star explodes as a supernova. Although the
total mechanical wind energy supplied by the star is negligible compared to the
accumulated energy of the Lyman continuum photons, the kinetic energy imparted
to the circumstellar gas over the star's lifetime is 4 times higher than for a
comparable windless simulation. Furthermore, the thermal energy of warm
photoionized gas is lower by some 55%. Our results document the necessity to
consider both ionizing radiation and stellar winds for an appropriate
description of the interaction of OB stars with their circumstellar
environment.
