# Tycho-Gaia and beyond: Combining data for precision astrometry

Sammanfattning: Astrometry aims at producing a three-dimensional map of positions and motions of stars and other celestial bodies in a consistent coordinate system covering the whole sky. This is best done from space, which provides a way of scanning the entire sky by a single instrument in a thermally stable environment. Space astrometry was pioneered by the Hipparcos satellite (1989–1993), and the successor mission Gaia began nominal operations in mid-2014. Determining a good astrometric solution for a star, i.e., all five astrometric parameters for its position, parallax and proper motion, requires a certain minimum stretch of observational data. The conditions for a good solution might not be met in the early phases of a space mission, for stars at the detection limit, or for transient objects such as supernovae. If the available observations are too few or do not span a sufficiently long time interval, additional constraints could be added to reduce the degrees of freedom of the mathematical problem. An example is the assumption that parallax and proper motion are exactly zero. Alternatively one can add prior information to lift the parameter degeneracy, at the cost of losing independence to external data. This doctoral thesis discusses the incorporation of prior information in an astrometric solution of Gaia data, with the aim to improve our understanding of these data early in the mission. Prior information is taken from the Hipparcos and Tycho-2 catalogues as well as a Galactic model. The influence of a prior on the astrometric solution is discussed in detail and the feasibility of joint solutions is demonstrated through simulations of various combination scenarios. One major result of the research work presented is the development and demonstration, through simulations, of a Tycho-Gaia Astrometric Solution (TGAS). Applied to real Gaia data it would allow us to obtain a full astrometric solution one year earlier than originally foreseen, with the additional benefit of long-baseline proper motion results.

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