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

TL;DR

This paper introduces a new method combining analytical, 1D, and 3D simulations to estimate the ages of H ii regions and OB associations in the Galaxy by analyzing their expansion in turbulent molecular clouds.

Contribution

It develops a novel approach to determine the ages of H ii regions using a grid of models that account for turbulence effects, validated on well-known regions.

Findings

Expansion of H ii regions is slowed by turbulence, reaching quasi-equilibrium.

The model ages agree with literature values for known regions.

Method can be applied to large surveys and extragalactic sources.

Abstract

This work aims at improving 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. Using analytical solutions, one-dimensional (1D), and three-dimensional (3D) simulations, we constrained the expansion of the ionized bubble depending on the turbulent 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 to date large sample of OB associations and was used on the H ii Region Discovery Survey (HRDS). 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 laws. The 3D turbulence is taken into account by 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 M16. We thus propose that this method can be used to give ages of young OB associations through the Galaxy such as the HRDS survey and also in nearby extra-galactic sources.