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Session 17 - Molecular Clouds and Star Formation.
Oral session, Monday, June 10
Humanities 2650,

[17.08] From Ultracompact to Extended HII Regions

J. Franco, G. Garc\'\ia-Segura (Instituto de Astronomia-UNAM)

The dynamical evolution of HII regions and wind-driven bubbles in dense clouds is studied. In particular, we address two different issues: 1) the conditions in which ultracompact HII (UCHII) regions can reach pressure equilibrium with their surrounding medium (and thereby stall their expansion), and 2) the appeareance of a powerful dynamic instability in expanding HII regions. At pressure equilibrium the ionized regions become static and, as long as the ionization sources and the ambient gas densities remain about constant, the resulting UCHII regions are stable and long lived. The equilibrium sizes and densities, R_S,eq \sim 3 \times 10^-2 F_48^1/3 T_HII,4^2/3 P_7^-2/3 pc and n_i,eq\sim 4 \times 10^4 P_7 T^-1_HII,4 cm^-3 (where F_48 is the photoionizing flux in units of 10^48 s^-1, P_7 is the pressure in units of 10^-7 dyn cm^-2, and T_HII,4 is the ion temperature in units of 10^4 K), are similar to those actually observed in UCHII regions. Similarly, ultracompact wind-driven bubbles can reach pressure equilibrium and the resulting final sizes are similar to those of UCHIIs. The same is true for a combined ultracompact structure consisting of an interior wind-driven cavity and an external HII region. For non-moving stars in a constant density medium, the lifetimes for all types of ultracompact objects only depend on the stellar lifetimes. For cases with a density gradient, depending on the core size and slope of the density distribution, some regions never reach the static equilibrium condition.

A powerful dynamic instability appears when cooling is included in the neutral gas swept up by an HII region, or a combined wind-HII region structure. This instability was first studied by Giuliani (1979), and is associated with the thin-shell instability described by Vishniac (1983). The internal ionization front exacerbates the growth of the thin-shell instability, creating a rapid shell fragmentation, and our numerical simulations confirm the linear analysis of Giuliani. The fragments tend to merge as the evolution proceeds, creating dense and more massive clumps, and are slowly eroded by ionization fronts. Thus, the resulting structures have a variety of shapes, sizes, densities, and lifetimes. Intriguing features such as ``elephant trunks'' and cometary-like globules can be easily explained as a result of this instability.

Program listing for Monday