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
In this thesis we study the earliest stages of high-mass star formation. Class II methanol masers are only associated with massive star formation and are a unique probe of these environments.... Show moreIn this thesis we study the earliest stages of high-mass star formation. Class II methanol masers are only associated with massive star formation and are a unique probe of these environments. Through observations we have studied where and when the methanol maser emission occur in relation to the protostar. We have found that for a fair fraction of the sources the methanol masers appear on size scales of ca. 1000 AU, in the equatorial region of the massive protostar. It appears that infall, rather than rotation, is the dominant motion. We propose that the maser emission occur close to or in a shock interface, possibly related to the accretion flow of the more extended gas in the protostellar envelope onto an accretion disk. The morphology and kinematics of the thermal methanol gas support the hypothesis that the maser region is also the region where the methanol molecules are released from the icy mantles of the du st grains. We have also estimated the temperature and column density of the methanol gas in the outflows and find evidence for radiative excitation of the methanol gas at the location of the maser emission. Show less
