Age, size, and position of H ii ii regions in the Galaxy: Expansion of ionized gas in turbulent molecular clouds

Aims. This work aims to improve the current understanding of the interaction between Hii ii regions and turbulent molecular clouds. We propose a new method to determine the age of a large sample of OB associations by investigating the development of their associated Hii ii regions in the surrounding turbulent medium. Methods. Using analytical solutions, one-dimensional (1D), and three-dimensional (3D) simulations, we constrained the expansion of the ionized bubble depending on the turbulence level of the parent molecular cloud. A grid of 1D simulations was then computed in order to build isochrone curves for Hii ii regions in a pressure-size diagram. This grid of models allowed us to date a large sample of OB associations that we obtained from the Hii ii Region Discovery Survey (HRDS). Results. Analytical solutions and numerical simulations showed that the expansion of Hii ii regions is slowed down by the turbulence up to the point where the pressure of the ionized gas is in a quasi-equilibrium with the turbulent ram pressure. Based on this result, we built a grid of 1D models of the expansion of Hii ii regions in a profile based on Larson's laws. We take the 3D turbulence into account with an effective 1D temperature profile. The ages estimated by the isochrones of this grid agree well with literature values of well known regions such as Rosette, RCW 36, RCW 79, and Mii 16. We thus propose that this method can be used to find ages of young OB associations through the Galaxy and also in nearby extra-galactic sources. © 2014 ESO .