The Atomic-to-Molecular Transition in Galaxies. III. A New Method for Determining the Molecular Content of Primordial and Dusty Clouds

TL;DR

This paper introduces a new analytical method, based on Stromgren analysis, to determine the molecular content of clouds in galaxies, applicable to both primordial and dusty environments, with high accuracy and broad physical relevance.

Contribution

The authors develop a novel Stromgren-type analytical approach to calculate atomic and molecular hydrogen in irradiated clouds, avoiding dependence on photodissociation cross sections and extending applicability.

Findings

Achieves about 10% accuracy compared to PDR code results.

Provides simple analytic fits for observational and modeling use.

Applicable to a wide range of physical conditions in galactic environments.

Abstract

Understanding the molecular content of galaxies is a critical problem in star formation and galactic evolution. Here we present a new method, based on a Stromgren-type analysis, to calculate the amount of HI that surrounds a molecular cloud irradiated by an isotropic radiation field. We consider both planar and spherical clouds, and H_2 formation either in the gas phase or catalyzed by dust grains. Under the assumption that the transition from atomic to molecular gas is sharp, our method gives the solution without any reference to the photodissociation cross section. We test our results for the planar case against those of a PDR code, and find typical accuracies of about 10%. Our results are also consistent with the scaling relations found in Paper I of this series, but they apply to a wider range of physical conditions. We present simple, accurate analytic fits to our results that are suitable for comparison to observations and to implementation in numerical and semi-analytic models.