Fri 1/14/00
Gene Capriotti
Michigan State

Relative Effects of Photoionization and Stellar Winds 
on the Structure and Dynamics of Gaseous Nebulae

In HII regions and in some planetary nebulae, embedded
stars transfer momentum to the nebulae not only by
radiative heating via photoionization but by impact
of their winds. When a fast stellar wind (v>1000 km/s)
is present, a hot (T>>10000K) bubble heated by the
wind expands into a warm (T~10000K) HII region heated and
mechanically driven by photoionization.
For simple, spherically symmetric, homogeneous model
nebulae, scaling laws are derived that allow comparison
of photoionization and wind effects for a wide range of
luminosities, wind strengths and nebular densities.
For luminosities and wind strengths characteristic of
zero-age main sequence O-type stars it appears that
winds play an insignificant role in establishing the
overall structure and dynamics of HII regions.