Studies of Double and Multiple Stars Using Space Astrometry

This thesis introduces and summarizes four papers dealing with the study of double and multiple stars using space astrometry. The introduction gives a brief review of the role of binary observations in various fields of astronomy --- star formation, stellar masses and the mass-luminosity relation, initial mass function, detection of sub-stellar mass companions. The astrometric techniques are also outlined, specifically using the ESA space astrometry mission Hipparcos and the proposed GAIA mission. The first two papers describe the use of the Hipparcos Transit Data, distributed with the Hipparcos and Tycho Catalogues, which contain all observations of the recognized or suspected double and multiple stars in the observing program. Using these data, the technique of aperture synthesis imaging was demonstrated (Paper I). Details of the method and of another application of these data for model fitting were published (Paper II) in order to aid astronomers in using the Transit Data. As an example, it is shown how HIP 97237 (for which no astrometric data are given in the Hipparcos Catalogue) may be resolved as a binary by means of aperture synthesis imaging and its parameters determined by model fitting. In Paper III, the number of double star solutions in the Hipparcos Catalogue are analyzed. The observed statistics are compared with predictions from a model of binary distributions and the observation process. Limits are set on the distribution of semi-major axes, where the 1 to 10 AU range is most precisely determined from the frequency of acceleration solutions. For stars at distances of the order of 100 pc, this method fills the gap between spectroscopic and resolved (visual and speckle) binaries. Finally, in Paper IV, a model is presented of GAIA observations of stars within 100 pc. The aim is to study the detection of brown-dwarf companions to these stars by means of GAIA. It is found that essentially all brown-dwarf companions with periods less than 100 years will be detected, and that orbits and masses will be determined for those with periods less than 5-10 years.