Age, size, and position of H ii regions in the Galaxy

Expansion of ionized gas in turbulent molecular clouds

¹ Astrophysics Group, University of Exeter, EX4 4 QL Exeter, UK
e-mail: pascal.tremblin@cea.fr
² Maison de la Simulation, CEA-CNRS-INRIA-UPS-UVSQ, USR 3441, Centre d’étude de Saclay, 91191 Gif-Sur-Yvette, France
³ Nordita, KTH Royal Institute of Technology and Stockholm University, Roslagstullsbacken 23, 10691 Stockholm, Sweden
⁴ Department of Physics, West Virginia University, Morgantown WV 26506, USA
⁵ Laboratoire AIM Paris-Saclay (CEA/Irfu – Uni. Paris Diderot – CNRS/INSU), Centre d’études de Saclay, 91191 Gif-Sur-Yvette, France
⁶ Instituto de Ciencias Nucleares, Universidad Nacional Autónoma de México, Ap. 70-543, 04510 DF, Mexico
⁷ Aix-Marseille Université, CNRS, LAM (Laboratoire d’Astrophysique de Marseille) UMR 7326, 13388 Marseille, France
⁸ Univ. Bordeaux, LAB, UMR 5804, 33270 Floirac, France
⁹ CNRS, LAB, UMR 5804, 33270 Floirac, France

Received: 8 April 2014
Accepted: 2 June 2014

Abstract

Aims. This work aims to improve the current understanding of the interaction between H 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 H 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 H 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 H ii Region Discovery Survey (HRDS).

Results. Analytical solutions and numerical simulations showed that the expansion of H 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 H 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 M 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.