This thesis explores the chemistry of interstellar and circumstellar molecules during star formation and death. From the perspective of chemical physics, the most important outcome of this thesis... Show moreThis thesis explores the chemistry of interstellar and circumstellar molecules during star formation and death. From the perspective of chemical physics, the most important outcome of this thesis lies in that the rates for two important reactions are determined accurately for the first time: N2 photodissociation and reaction rates (both state-to-state and thermal) of OH with H. In particular, accurate N2 photodissociation rate and shielding functions were calculated. The results are very useful in many astrophysical fields. In addition, a new method for accurately calculating molecular shielding functions in full 3D radiation field was proposed, and was employed to give new predictions for molecule distributions in a C-rich AGB star, IRC +10216, and an O-rich AGB star, IK Tau, based on the latest progress from both observations and simulations. By far, these results are the most accurate ones, and can be directly compared to the future observations. Show less
The CS J = 2-1 emission line at 98 GHz has been searched for in 10 diffuse molecular clouds. CS column densities are derived by performing statistical equilibrium calculations for the rotational... Show moreThe CS J = 2-1 emission line at 98 GHz has been searched for in 10 diffuse molecular clouds. CS column densities are derived by performing statistical equilibrium calculations for the rotational population distribution which includes collisional excitation by electrons as well as by neutral species. Because CS has a large dipole moment, the electron collisions completely dominate the CS excitation in diffuse clouds, where the electron fraction is high. This result suggests that, in general, molecules with large dipole moments may be relatively more detectable by millimeter emission in diffuse clouds than in dense clouds. A detailed model of the gas-phase sulfur chemistry in diffuse clouds is developed to interpret the observations. Show less
Recent work on the vacuum UV absorption spectrum of CO to the description of the photodissociation of interstellar CO and its principal isotopic varieties is discussed. The effects of line... Show moreRecent work on the vacuum UV absorption spectrum of CO to the description of the photodissociation of interstellar CO and its principal isotopic varieties is discussed. The effects of line broadening, self-shielding, shielding by H and H2, and isotope-selective shielding are examined as functions of depth into interstellar clouds. The photodissociation rates of the isotopic species are larger than that of (C-12)O inside the clouds by up to one to two orders of magnitude. A simple approximation to the attenuation by line absorption is given in tabular form. Computed abundances of CO and related species C and C+ are presented for a variety of interstellar clouds ranging from diffuse clouds to dense photodissociation regions. Several series of models of translucent clouds are presented which illustrate how the CO abundance increases rapidly with total cloud thickness. The variations of the isotopic abundances with depth and their sensitivity to temperature and total cloud thickness are explored in detail. Show less
