Student: Jill Neeley, Graduate Student in Astrophysics, Iowa State University
Faculty Advisor: Massimo Marengo
Calibrating the extragalactic distance scale with RR Lyrae stars
My work focuses on the calibration of mid-infrared RR Lyrae (RRL) period-luminosity (PL) relations and their dependence on metallicity. These relations are the anchor of a Population II galactic and extragalactic distance scale, and are a crucial step towards providing a new, independent measurement of Hubble’s constant to 3% accuracy. I worked as part of the Carnegie RR Lyrae Program (CRRP), which has obtained over 900 hours of Spitzer Space Telescope observations of RRL in the field, bulge and Galactic globular clusters (GCs). For this project, I developed a method to extract high-precision photometry from crowded fields, by combining multiple catalogs at different wavelengths to minimize the effects of blending in the MIR. In Neeley et al. (2015), I obtained the first calibration of the zero point (using RRL with HST parallax) and slope (using RRL in the GC M4) of the PL relation at Spitzer wavelengths. The scatter in that relation is 0.05 mag, which is approaching the expected intrinsic scatter (~0.03 mag) and already less than half the scatter observed in Spitzer PL relations for classical Cepheids in the LMC. In parallel, I studied the dependence of iron abundance using theoretical period-luminosity-metallicity relations (Neeley et al. in prep.), finding up to 12% dispersion on the zero point over Galactic range of [Fe/H]. I am now analyzing additional GCs to empirically test for this metallicity effect and improve the statistics on the PL relation.