Abstract
Astrometry aims at producing a threedimensional 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 mid2014.
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 Tycho2 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 TychoGaia 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 longbaseline proper motion results.
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 Tycho2 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 TychoGaia 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 longbaseline proper motion results.
Original language  English 

Qualification  Doctor 
Awarding Institution  
Supervisors/Advisors 

Award date  2015 Nov 28 
Publisher  
ISBN (Print)  9789176235461 
ISBN (electronic)  9789176235478 
Publication status  Published  2015 
Bibliographical note
Defence detailsDate: 20151128
Time: 14:00
Place: Lundmarksalen, Astronomy building, Sölvegatan 27
External reviewer(s)
Name: BailerJones, Coryn
Title: [unknown]
Affiliation: Max Planck Institute for Astronomy, Heidelberg

Subject classification (UKÄ)
 Astronomy, Astrophysics and Cosmology
Free keywords
 proper motions
 parallaxes
 positions
 map of the Milky Way
 joint solution
 Gaia
 detection of exoplanets
 astrometry
 catalogue combination
 TychoGaia Astrometric Solution
 Hundred Thousand Proper Motion project
 Fysicumarkivet A:2015:Michalik