AAS 197, January 2001
Session 42. Dust and Theory of ISM
Display, Tuesday, January 9, 2001, 9:30am-7:00pm, Exhibit Hall

## [42.12] Relative Effects of Ionizing Radiation and Winds from O-type Stars on the Structure and Dynamics of HII Regions

E.R. Capriotti, J.F. Kozminski (Michigan State)

In HII regions (and some planetary nebulae), the structure and dynamics are affected not only by radiative heating via photoionization but also by winds from the embedded O-type stars. When a fast wind (v \geq 108 cm s-1) turns on, a hot (T \gg 104 K) bubble heated by the wind expands into the warm (T ~q 104 K) surrounding nebula which is heated and mechanically driven by photoionization. The bubble and a shell of swept-up, i.e. shocked, nebular gas evolve under the influence of mechanical effects and radiative cooling.

Simple scaling laws are derived for spherically symmetric, homogeneous model nebulae that allow comparison of wind effects and photoionization effects for wide ranges of nebular densities, stellar luminosities, and wind strengths. Lifetimes, energies, volumes, and masses of wind-affected material are compared to similar quantities for nebular material heated and driven by photoionization.

For the simple models considered in this study, winds with strengths expected for zero-age O-type stars are of little influence in the ultimate overall structure and dynamics of radiation-bounded HII regions, excepting for the best case for wind" models featuring nebulae of very high density (\rho \gtrsim 10-19 g cm-3, N \gtrsim 105-106 cm-3). Consequently, even though heating by photoionization seems to dominate the evolution of HI regions and the formation of HII regions surrounding O-type stars, dense molecular clouds may be more strongly influenced by winds.

Winds can also be more influential in the overall structure and dynamics of gas-bounded HII regions of moderate density if the system is sufficiently optically thin to ionizing radiation.