Galactic Structure Revealed by Space Astrometry

Abstract: This work demonstrates how the structure of our Galaxy, the Milky Way, can be unveiled with the use of space astrometry. It consists of two parts, of which the first is a brief introduction to the fields of Galactic structure, covering the Galaxy's stellar constituents, how the Galaxy was formed and how it has evolved over time, both chemically and kinematically. Also the workings of space astrometry are presented, with emphasis on the European Space Agency's Hipparcos satellite, which delivered all astrometric measurements used in this thesis. The first part is concluded with brief summaries of the five papers which make up the main second part of the thesis, and with a short overview of future work. Paper I describes how the Hipparcos Catalogue is utilized to derive an improved model of the local Galactic structure. The availability of parallax data eliminates the basic indeterminacy of classical methods based on star counts, where the lack of distance information leads to severe degeneracy between different models. Paper II presents an accurate estimate of the density of matter in the solar neighbourhood. This is obtained by combining the observed spatial distribution of stars with their kinematics to deduce the total amount of matter needed to explain the observations. This dynamically determined mass density is 0.102+/-0.010 Solar masses/pc^3, which may be compared with an estimated 0.095 Solar masses/pc^3 in identified visible disk matter. This gives compelling evidence that there is no room for significant amounts of dark matter in the disk. In Paper III, the old and metal-rich moving group HR 1614 is investigated. The very existence of this and other moving groups has been questioned, but using Hipparcos parallaxes and proper motions in combination with radial velocities and metallicities, the existence and properties of this moving group are firml y established. This is further supported with numerical simulations of the dynamical evolution of stellar aggregates in the Galactic potential. Finally, Papers IV and V follow the evolution with time of two basic properties of the stars in the Galactic disk, namely their chemical content an d kinematics. Paper IV presents age and metallicity determinations for a large sample of stars observed by Hipparcos. The distribution of metallicity for the stars proves to be remarkably constant over the age of the disk, in sharp contrast to standard models of Galactic chemical evolution which usually assume a clear correlation between age and metallicity. Paper V concentrates on the kinematics of the disk, and how this has evolved over time. The increase in the stellar velocity dispersion over time proves to be rather modest for the Hipparcos stars, in accordance with models where the dynamical heating of the Galactic disk is dominated by scattering from molecular clouds and spiral structure.