Relative effects of ionizing radiation and winds from O-type stars on the structure and dynamics of HII regions

The structure and dynamics of H II regions (and some planetary nebulae) are affected not only by radiative heating via photoionization but also by winds from the embedded O-type stars. When a fast wind (v greater than or equal to 10(8) cm s(-1)) turns on, a hot (T >> 10(4) K) bubble heated by the wind expands into the warm (T similar or equal to 10(4) 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. Previous calculations of wind effects make no direct comparison of the dynamical effects of the heating through photoionization. In addition, most previous calculations deal with particular combinations of wind strength and interstellar gas density. Here, we derive simple scaling laws for spherically symmetric, homogeneous model nebulae with uniform density and make direct comparison of wind effects and photoionization effects for much wider ranges of nebular densities, stellar luminosities, and wind strengths than have been considered in previous works. 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, photoionization dominates as winds with strengths expected for zero-age O-type stars are of little influence in the ultimate overall structure and dynamics of radiation-bounded H II regions, except for the "best case for wind" models featuring nebulae of very high density (rho greater than or similar to 10(-19) g cm(-3), N greater than or similar to 10(5)-10(6) cm(-3)). Consequently, even though heating by photoionization seems to dominate the evolution of H I regions and the formation of H ii 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 H ii regions of moderate density if the system is sufficiently optically thin to ionizing radiation.