Photochemical Heating of Dense Molecular Gas

TL;DR

This paper investigates how photochemical reactions contribute significantly to heating dense molecular gas, especially through chemical reactions initiated by photodissociation and photoionization products, sometimes surpassing direct heating effects.

Contribution

It provides a detailed analysis of chemical heating mechanisms in dense molecular gas, highlighting their importance relative to direct heating processes.

Findings

Chemical heating can be as large or larger than direct heating.

Total photochemical heating approaches 10 eV per photodissociation in dense gas.

Chemical reactions significantly influence the thermal balance of molecular clouds.

Abstract

Photochemical heating is analyzed with emphasis on the heating generated by chemical reactions initiated by the products of photodissociation and photoionization. The immediate products are slowed down by collisions with the ambient gas and heat the gas. In addition to this direct process, heating is also produced by the subsequent chemical reactions initiated by these products. Some of this chemical heating comes from the kinetic energy of the reaction products and the rest from collisional de-excitation of the product atoms and molecules. In considering dense gas dominated by molecular hydrogen, we find that the chemical heating is sometimes as large if not much larger than the direct heating. In very dense gas the total photochemical heating approaches 10 eV per photodissociation (or photoionization), competitive with other ways of heating molecular gas.