Star-forming cloud-dispersal timescales in TIGRESS-NCR simulations

Abstract

When stars form in molecular clouds, their radiation ionizes the surrounding gas, creating expanding HII regions. These regions, by disrupting the clouds in which stars are born, have a significant impact on the regulation of star formation. Their size can also be used by observers to estimate the ages of their progenitor star clusters. In this work, we investigate the evolution of HII regions in TIGRESS-NCR, state-of-the-art radiation-magnetohydrodynamic (RMHD) simulations of the multi-phase interstellar medium (ISM). We construct time series of the evolution of surface density and emission measure around the star clusters in the simulation. We average these surface density profiles to determine the size of the HII region as a function of star cluster age. We also develop a simple spherical model for the evolution of HII regions with correction factors to account for their non-radial expansion, the dust-absorption and escape of the ionizing photons, and other effects. By comparing this model to the HII region evolution determined from the simulation, we recommend particular values for these correction factors to achieve the best fit to the data. In doing so, we suggest a method by which observers can date star clusters based on the size of their surrounding HII regions.