FROM ULTRACOMPACT TO EXTENDED H-II REGIONS

The dynamical evolution of H II, regions and wind-driven bubbles in de nse clouds is studied. In particular, we address two different issues: (1) the conditions under which ultracompact H II) regions can reach p ressure equilibrium with their surrounding medium (and thereby stall t heir expansion) and (2) the appearance of a powerful dynamic instabili ty in expanding H II regions. At pressure equilibrium, the ionized reg ions become static, and as long as the ionization sources and the ambi ent gas densities remain about constant, the resulting UCHII regions a re stable and long-lived. The equilibrium sizes and densities, R(S,eq) similar to 3 x 10(-2)F(48)(1/3)T(HII,4)(2/3)P(7)(-2/3) pc and n(i,eq) similar to 4 10(4)P(7)T(HII,4)(-1) cm(-3) (where F-48 is the photoion izing in units of 10(-7) dyne cm(-2), and T-HII,T-4 is the ion tempera ture in units of 10(4) K), are similar to those actually observed in U CHII regions. Similarly, ultracompact wind-driven bubbles can reach pr essure equilibrium, and the resulting final sizes are similar to those of UCHII's. The same is true for a combined ultracompact structure co nsisting of an interior wind-driven cavity and an external HII region. For nonmoving stars in a constant-density medium, the lifetimes for a ll types of ultracompact objects only depend on the stellar lifetimes. For cases with a density gradient, depending on the core size and slo pe of the density distribution, some regions never reach the static eq uilibrium condition. A powerful dynamic instability appears when cooli ng is included in the neutral gas swept up by an HII region or a combi ned wind-II n region structure. This instability was first studied by Giuliani and is associated with the thin-shell instability described b y Vishniac. The internal ionization front exacerbates the growth of th e thin-shell instability, creating a rapid shell fragmentation, and ou r numerical simulations confirm the linear analysis of Giuliani. The f ragments tend to merge as the evolution proceeds, creating dense and m ore massive clumps, and are slowly eroded by ionization fronts. Thus t he resulting structures have a variety of shapes, sizes, densities, an d lifetimes. Intriguing features such as ''elephant trunks'' and comet ary-like globules can easily be explained as a result of this instabil ity.