Mapping the Structure of the Milky Way with Mira Stars

Studying the structure of the Milky Way is an intricate task due to the position of the solar system almost in the plane of the Galactic disk and the need to explore the Galaxy from within. Additionally, these studies are hindered by the strongly variable and light-of-sight dependent interstellar extinction, which is highest in the plane of the Galactic disk and towards the Galactic center. Despite these diÖculties, many attempts have been made for hundreds of years to map the Milky Way using various indicators. One such indicator that is well suited for studying the three-dimensional structure of the Galaxy is a class of pulsating stars of diÄerent types. The main aim of this thesis was to study the three-dimensional structure of the Milky Way using long-period, high-amplitude, intermediate-age pulsating stars, also known as Mira-type variables. This goal was achieved through a series of analyses and the combination of ground-based optical observations from the Optical Gravitational Lensing Experiment (OGLE) sky survey databases and infrared observations collected by the VISTA near-infrared Y JKs survey of the Magellanic System (VMC), with the infrared data collected by the Spitzer and Wide-field Infrared Survey Explorer (WISE) space telescopes. The thesis consists of four scientific articles published in a peer-revied journal which are logically connected and lead to a solution to the problem at hand. The first part of the thesis is based on the Mira-type variables discovered in the Large Magellanic Cloud (LMC) and focuses on the analysis of the Mi ras variability over a wide range of wavelengths, covering 0.1-40 µm. In this study, detailed Mira variability templates were created and the amplitude ratio and phase lag between the OGLE I-band and other bands were determined. Synthetic period–luminosity relations (PLRs) were also presented separately for oxygen-rich (O-rich) and carbon-rich (C-rich) Miras in 42 bands of already operating and future sky surveys. In the second part of the thesis, the Mira variability templates were used to precisely measure the mean magnitudes in the WISE and Spitzer bands. They, in turn, were used to fit and determine the PLRs separately for O-rich and C-rich Miras, using both linear and quadratic models, which were later calibrated at the distance to the LMC. These relations allow for measuring distances to Mira type variables with an accuracy of 5% for O-rich Miras and 12% for C-rich ones. The third part of the thesis was focused on discovering as many Mira-type variables in the Milky Way as possible using photometric data from the OGLE sky survey. This led to the discovery of 65,981 Miras, of which more than two thirds were previously unknown. This allowed for the publication of the largest collection of Milky Way Mira-type variables in the history of astronomy. The final part of the thesis was devoted to the analysis of the three-dimensio nal structure of the Milky Way based on the discovered Mira variables. A com plex 44-parameter Galactic bulge model was fitted to the Miras distribution, taking into account distance uncertainties by implementing the Bayesian hierar chical inference method. The distance to the Galactic Center and the inclination of the major axis of the bulge to the line of sight were measured. Additionally, the existence of the controversial X-shaped structure in the Mira-type variables was confirmed.