Preview

PDF, English Download (18MB) | Terms of use

Abstract

The exact processes behind the formation and evolution of galaxies are interesting puzzles in modern astrophysics. Our Galaxy offers us the unique opportunity to be studied in detail, as we can obtain the 3D positions, 3D velocities and also the chemical information on a star-by-star basis. Different Galactic surveys have advanced in the effort of studying the Milky Way. The Gaia mission in particular provides the full 6D stellar position-velocity phase-space measurements for millions of its stars. By combining Gaia with chemical information from spectroscopic surveys, we can obtain a detailed physical picture of our Galaxy. In this thesis, we set out to investigate the stellar orbit distribution of the Milky Way, while also adding their chemical information ([Fe/H]) in a chemical tagging generalization approach. We first make use of the spectroscopic information from LAMOST, in combination with parallaxes and proper motions from Gaia. We develop a method to obtain improved spectrophotometric distances (with errors less than 6%) for 150 000 main sequence stars. With more precise distances at hand, we investigate the small-scale structure in the orbit distribution of the Galactic disc for ∼ 600 000 main sequence stars in LAMOST × Gaia. Most stars disperse from their birth sites and siblings, in orbit and orbital phase, becoming ‘field stars’. We explore and provide direct observational evidence for this process in the Milky Way disc, by quantifying the probability that orbit similarity among stars implies indistinguishable metallicity. We define the orbit similarity among pairs of stars through their distance in action-angle space ∆(J, θ) and their abundance similarity by ∆[Fe/H]. By grouping such star pairs into associations with a friend-of-friends algorithm linked by ∆(J,θ), we find that hundreds of mono-abundance groups –some clusters, some spread across the sky– are over an order-of-magnitude more abundant than expected for a smooth phase-space distribution, suggesting that we are witnessing the ‘dissolution’ of stellar birth associations into the field. We finally explore a significantly larger sample of 6.2 million stars with radial velocities in Gaia, which is not limited to main sequence stars. Although this sample does not have [Fe/H] information, we are able to recover the same major groups found in the previous sample in both action and angle space. Moreover, we are able to identify other known associations by simple inspection, opening up the possibility for this method to be applied to further characterize dissolving associations across the Galaxy.