Precision RR Lyrae Distances for a Population II Route to the Distance Scale

Iowa State University, Ames IA

Investigators

Abstract

The investigator will analyze the change in brightness of a special class of variable stars, called RR Lyrae stars. These pulsating stars are very common in the old stellar clusters found in the Milky Way and other galaxies. These stars are important because they can be used as yardsticks to measure distances on scales of thousands to millions of light years. Distances are measured using a relation between their variability period (the time it takes these stars to cycle through their brightness variations), and their intrinsic luminosity. This "period-luminosity" relation makes them ideal "standard candles." The investigator will better calibrate measurements of distances across the universe, using RR Lyrae stars. Accurate distances are needed to better understand the expansion rate and the age of the cosmos. They also hope to achieve a better understanding of the physics of stars and of the stellar clusters where RR Lyrae stars are found. The Iowa State University Physics & Astronomy department engages the local community with numerous outreach activities. The student-run Planetarium shows, in particular, draw large crowds of all ages, and are offered monthly to the public, as well as to the local K-12 schools (over 40,000 students in Ames and nearby school districts). This project will refurbish telescope equipment previously located off-campus at the Fick observatory facility (now discontinued) making it available to the public as part of a new in-campus, wheelchair-accessible, observation site for outreach activity (e.g., star and Sun parties following planetarium shows for the general public and K-12 students). They will also prepare two new modules for planetarium shows based on the science of variable stars and the cosmological distance scale as a way to communicate to the local community an example of federally-funded research that is done at Iowa State University. Since Henrietta Leavitt discovered the Cepheid Period-Luminosity relation in 1908, pulsating stars have been an invaluable distance estimator. More than a century after this groundbreaking result, Cepheids are still the cornerstone of an ever more intricate cosmological distance ladder. Recent results from the Planck mission, however, reveal some tension between the value of cosmological parameters derived from the study of the cosmic background radiation with respect to "direct" measurements still crucially relying on Cepheids. With this project, the investigators aim to provide an alternative Population II stellar distance estimator based on the newly re-discovered period-luminosity relation of RR Lyrae variables and the availability of high precision parallaxes from the Gaia spacecraft. The investigators will (1) calibrate the Period-Luminosity-Metallicity (PLZ) and Period-Wesenheit-Metallicity (PWZ) relations for a large sample of Galactic RR Lyrae at optical and infrared wavelengths; (2) use the calibrated RR Lyrae PLZ and PWZ relations to precisely measure Galactic globular clusters' distances and calibrate globular clusters' distance scale estimators, enabling new fundamental tests of stellar evolution and stellar population models; and (3) validate the observational PLZ and PWZ relations against recently developed nonlinear, time-dependent, convective, hydrodynamic models of RR Lyrae stars calculated for a broad range of metal abundances. The investigators have demonstrated their ability to perform these measurements by completing the analysis of two of the target clusters. The investigators have all the data and models crucial to this project.