RR Lyrae as distance indicators and stellar population tracers – Marina Rejkuba

Distance is one of the key parameters necessary for the study of physical properties of individual stellar systems. Through the cosmic distance ladder, it contributes to our understanding of the cosmological model and the expansion rate of the Universe. The disagreement between the Hubble constant (H0) measurement via the traditional population I Cepheid-Supernovae distance ladder and the Planck inference from the observations of the early Universe has sparked a flurry of studies of the possible systematics in distance measurements, as well as theoretical ideas focusing on new physics that could explain the discrepant H0 values.
Population II distance indicators, such as RR Lyrae and Tip of the Red Giant Branch (TRGB) stars are found in all types of galaxies and are an interesting alternative for the local distance ladder towards Supernovae host galaxies. In particular, the RR Lyrae stars can be used to robustly test the accuracy of other local distance indicators within the Local Group. Besides using RR Lyrae for accurate and precise distance measurements, these low mass and old variable stars can also be used as tracers of stellar populations, thus providing insights to the early star formation and galaxy assembly processes.
In this presentation I will describe a couple of recent studies of RR Lyrae stars in the Milky Way Globular Clusters and nearby galaxies used to derive the most accurate distance calibration as well as to probe the structure and build-up of the Milky Way bulge.