AGB stars in the early universe I: Towards a portable chemical and thermal evolution model
Stars in the asymptotic giant branch (AGB) are in the penultimate stage of their lives as low- mass dwarfs. During this stage, they produce in their cool, bloated envelopes vast quantities of dust composed from metals forged deep in their interiors from nuclear reactions. They expel these dust grains in shells blown out from the star by strong winds powered by radiation pressure. In order to better understand the role of asymptotic giants as sources of metal and dust enrichment in the early, metal-poor universe, we create a portable chemical and thermal network considering physics on all the major primordial species including H; H+; H−; H2; H+2 ; He; He+; He++; D; D+; HD; e− as well as the most important metal coolants C, Si, O and their singly ionized state C+, Si+, O+. We involve in our code all of the important heating and cooling processes including atomic lines, molecular lines, continuum radiation and absorption, metal fine structure lines, and collisional heating and cooling including that between dust grains and gas particles. We test our reaction network by numerically solving chemical and thermal rate equations in a one-zone freefall model. Our results show that the presence of dust and metals in a cloud creates at lower densities a significant excess of molecular hydrogen, an important coolant, compared with dust and metal-free clouds.