Stellar Ages for Galactic Archaeology: Methods and Applications

Galactic Archaeology is the study of the formation and evolution of the Milky Way through the properties of stars and stellar populations. The combination of stellar ages, chemical compositions, positions, and velocities can reveal the history of star formation and chemical enrichment in different regions of the Galaxy. A large number of stellar spectroscopic, photometric, and astrometric surveys are currently providing the data necessary to determine the properties of millions of stars spanning almost the entire distance from the Galactic centre to its outskirts. These data do not give stellar ages directly, but ages can be estimated by comparing the surface properties of the stars with models of stellar evolution.

This thesis covers the work presented in five papers concerning stellar ages for studies of Galactic Archaeology. In Paper I, we investigate the ages of the Gaia benchmark stars, a sample of 33 nearby stars used as a validation sample in large surveys. By combining new age estimates from model fitting with age estimates found in the literature, we define benchmark ages for 16 of the stars. In Paper III we present a new method for estimating the sample age-metallicity distribution (SAMD) of a population of stars. This method combines the full age-metallicity probability density functions from model fitting of individual stars and is found to yield more precise age distributions than the distribution of individual age estimates.

The remaining papers are devoted to the study of the formation history of stellar populations in the Milky Way disc and halo. In Paper II we study the properties of two stellar populations in the halo and find that they formed at the same time more than 10 Gyr ago, but at significantly different metallicities. This is consistent with the prevailing interpretation that the most metal-poor population has been accreted from a dwarf galaxy and that the other population is made up of kinematically hot Milky Way disc stars. In Paper IV we revisit the age distribution of the accreted component and find it to be consistent with a single population of stars with ages in the range 10 to 12 Gyr. We also investigate the properties of a population of stars on retrograde orbits claimed to be accreted from another distinct dwarf galaxy. This population seems to be of a mixed origin as indicated, for example, by a multimodal age distribution. Finally, in Paper V we calculate the SAMD for a sample of almost 200 000 stars in the Milky Way disc. The SAMD shows two local minima, at ages of approximately 5 and 10 Gyr, indicating that the disc stars formed in three main phases. First along a single chemical evolution track in the inner disc until around 10 Gyr ago, then across the disc with outwardly decreasing metallicity, and finally in a burst of star formation 2 to 4 Gyr ago.