Passage of a Gamma-Ray Burst Through a Molecular Cloud: Cloud Ionization Structure

TL;DR

This paper models gamma-ray burst radiation passing through a dense molecular cloud, analyzing complex ionization and destruction processes affecting gas, dust, and molecules, with implications for understanding interstellar medium responses.

Contribution

It introduces a comprehensive simulation model that incorporates multiple ionization and destruction mechanisms of gas and dust in molecular clouds exposed to gamma-ray bursts, highlighting the role of metal ions and helium.

Findings

Metal ion column densities vary with ionization state.

Helium ionization significantly influences X-ray absorption.

Metallicity affects the dominance of helium in ionization processes.

Abstract

The model is constructed of the propagation of gamma-ray burst radiation through a dense molecular cloud. The main processes of the interaction of the radiation with the interstellar gas are taken into account in the simulations: the ionization of H and He atoms, the ionization of metal ions and the emission of Auger electrons, the photoionization and the photodissociation of H$_2$ molecules, the absorption of ultraviolet radiation via Lyman and Werner band transitions of H$_2$, the thermal sublimation of dust grains. The ionization of metal ions by X-ray radiation determines the gas ionization fraction in the cloud region, where the gas is predominantly neutral. The ionization of inner electron shells of ions is accompanied by the emission of Auger electrons, giving rise to metal ions in a high ionization state. In particular, the column densities of Mg, Si, Fe in the ionization states I$-$IV are much lower than the column densities of these ions in the ionization state V and higher. The photoionization of metal ions by ultraviolet radiation takes place at distances smaller than the dust-destruction radius and only for neutral atoms with an ionization threshold below 13.6 eV. The simulations have confirmed the previously made suggestion that the ionization of He atoms plays an important role in the absorption of radiation in the X-ray wavelength range. For a low metallicity, $\rm [M/H] \leq -1$, the role of He atoms is dominant.