RR Lyrae in the Cosmic Distance Ladder

I create near-infrared (J, H, and K filters) lightcurves with a minimum of 20 evenly spaced epochs for over 50 RR Lyrae. In addition, I have analyzed high-resolution spectra from the Southern African Large Telescope (SALT) for many of the same stars. All of these RR Lyrae have parallaxes measured by Gaia with precision greater than 10%. This has allowed me to construct the largest homogeneous set of both near-infrared lightcurves and high-resolution spectra. I am able to constrain the period-luminosity-metallicity (P-L_{J, H, K}-[Fe/H]) relationships to high precision. The near-infrared period-luminosity-metallicity relation is ideal due to lessened intrinsic luminosity spread (Bono et al. 2000), but also due to the lessened effect of interstellar reddening. These relations can be used as a first rung of the Cosmic Distance Ladder to measure a new value of H₀ independent of the previously used Cepheid variables. In addition, only a few RR Lyrae have had high-resolution spectroscopy analysis metallicity determinations. With the addition of dozens of well-determined metallicities for these RR Lyrae, we also better calibrate the commonly Delta-S method of determining an RR Lyrae's metallicity from its lightcurve.