A century ago, it was unclear whether the stars in the sky were clustered in groups, or widely spread in the universe. Without accurate stellar distances, it was impossible to obtain a reliable... Show moreA century ago, it was unclear whether the stars in the sky were clustered in groups, or widely spread in the universe. Without accurate stellar distances, it was impossible to obtain a reliable spatial stellar distribution to know which stars are part of our Galaxy and those that were beyond. Recently, the Gaia mission provided accurate position and velocity measurements for a billion stars in the Milky Way. However, these measurements are limited by dust that absorbs and scatter the optical light, particularly forward the Galactic plane. In contrast, radio waves are not affected, and therefore, can easily penetrate the Galactic plane providing complementary data. Radio campaigns are currently measuring the positions and velocities of bright stellar sources in the Galactic plane using VLBI. This thesis demonstrates how accurate are these astrometric measurements for young massive and evolved stars. These results are (1) compared with simulations of unobserved Galactic areas to determine the structural parameters of the Galaxy, and (2) cross-matched between optical and infrared surveys to characterize different stellar populations. Finally, a study of a particular binary system was carried out that demonstrates the stellar information can be obtained by having accurate astrometry at different frequencies and epochs. Show less
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
