The assembly history of the Milky Way: insights from metal-poor stars and solar twins

The location of the Sun in the Milky Way gives us a great opportunity to study our Galaxy in detail. And later we can generalize obtained knowledge to further galaxies in the Universe. One of the approaches to exploring the history of the Milky Way is studying its stars. Stars provide us with information on their age, chemical composition and kinematics. With age, we can trace the timeline of the formation, its different stages, correlations and order of the events. Chemical composition shows us the composition of the environment in which the stars were formed and lived. Also, kinematics contains the dynamical history, how stars are moving in the Galaxy or even in the Universe, how and with what they interact and in tandem with chemistry it can provide the most probable location where the star was formed. In this work, I start with exploring very metal-poor stars. Due to their low metallicity, they represent the initial phases of the Milky Way formation. In Chapter 2 I derive precise ages with the isochrone fitting technique for single stars in the turnoff/sub-giant branch region of the color-magnitude diagram. To achieve high precision we have to carefully take into account all possible sources of uncertainties. An automatic algorithm was created for these purposes. As a result we get the mean age of the sample 13.8 ± 0.5 Gyr. Additionally, we found a group of very metal-poor stars ([Fe/H] < -2.0 dex) with relatively young ages (8 - 10 Gyr). In Chapter 3 I investigate the origin of very metal-poor stars. To do so I derive the kinematical properties of each star. Combining kinematics and chemistry we found that some stars have clear signatures to be part of the Halo population. However, the 8 oldest could have formed in the primordial Bulge because of their orbital and large alpha-abundance. The other group of the stars can be associated with Gaia-nceladus/Sausage and Thamnos 1, 2. Changing the target of the investigation to solar twins, we can study more recent phases of the Milky Way formation and particularly Thin disk. In Chapter 4 I perform a line-by-line differential spectroscopic analysis of our sample with respect to the Sun and enlarge it with the dataset analyzed by Casali et al. (2020). Obtained high-precision effective temperature, surface gravity and metallicity allow us to derive precise ages with an average uncertainty of 0.7 Gyr. Now, together with metallicity, we can study the age-metallicity relationship for the Thin disk. In Chapter 4 we did not detect any presence of separation into two populations for the Thin disk. Also, we demonstrate that for this kind of analysis, it is very important to take into account selection bias and statistical error together with all sources of uncertainties. Additionally, in Chapter 4 we derive the birth radius to study the chemical evolution of the Thin disk. As a result, we propose that to reach today’s chemical composition in the Thin disk the most probable scenario is that radial migration played an important role together with the possible contribution of Gaia-Enceladus/Sausage and Sagittarius. In the end, two completely different groups of stars show us the history of the Milky Way’s formation and evolution. Starting from the very beginning with very metal-poor stars we investigate that first to form was pristine Bulge. Then Halo and Disk were formed with the possible contribution from the accretion events. Then, from solar twins, we confirm that the Thin disk reached today’s chemical composition due to radial migration also with the possible contribution of accretion events. More important we do not see the bimodal distribution in the age-metallicity relation that suggests that the Thin disk was formed relatively smoothly.