Studies of the chemistry and physics of starless and prestellar cores

Concentrations of dense and cold gas embedded in gigantic molecular clouds, the so-called starless or prestellar cores, represent the initial stages of star formation in the interstellar medium. The dominating gas component in the cores is molecular hydrogen, but a variety of other elements such as helium, oxygen, carbon and nitrogen are also present in appreciable amounts. A notable addition to this list is deuterium which can, at the low temperatures prevalent in the cores, be treated as another element. In fact, it turns out that deuterium is increasingly important toward the very centers of the cores where the density is highest. In addition to the gas, interstellar dust is also present in the cores with a total mass of about one hundreth of the total gas mass. Chemical interaction between the gas and the dust and the relatively high gas density allow for rich chemical evolution to take place during the lifetime of a core. The chemistry affects the gas temperature because the gas cools mainly by molecular line radiation, and the gas temperature in turn determines the thermal pressure which is mainly responsible for stabilizing the core against gravitational collapse. Consequently, the chemical composition of a core influences its physical evolution. Observing the molecular line radiation emitted by the various chemical species yields information on, e.g., the kinematics and the temperature of the gas. We can also deduce molecular column densities based on observations. However, the observationally measured column densities are averages over different lines of sight and hence possible spatial variations in the number densities of the various species, and consequently in the gas temperature, can be difficult to detect. This is particularly important toward the centers of the cores where molecules are expected to freeze onto the surfaces of dust grains and hence to disappear from the gas phase. For this reason, numerical models of the chemistry are needed to correctly interpret the various observations. Indeed, studying the chemical and physical properties of starless and prestellar cores theoretically, using numerical methods, is the focus of this thesis which consists of five original journal articles. In two of the articles, we discuss the chemistry of light deuterated ions in the context of the so-called complete depletion model, in which all elements heavier than helium are frozen onto the surfaces of dust grains. In the latter of these two articles, we expand upon the complete depletion model by explicitly taking into account the chemical evolution of heavier species preceding freeze-out, and our analysis yields the first theoretical prediction of the eventual depletion of the singly-deuterated hydrogen molecule, HD. One of the articles focuses solely on the physics of starless and prestellar cores by studying their stability against gravitational collapse. For the physical core model