RR Lyraes RR Lyraes are variable stars, which means their apparent brightness as seen from Earth varies. Their intrinsic brightness is around 0.75 and their period is usually less than a day. They’re commonly found in globular clusters, though the stars involved in out research were field stars, and are used as standard candles which are astronomical objects with known luminosities to measure galactic distances. Period-Color and Amplitude-Color Relations in OGLE III RR Lyraes Joyce Wu 1, Dilyana Mihaylova 2, Shashi Kanbur 3, Chow-Choong Ngeow 4 (1) Ithaca College Physics Dept. (2) University of Rochester Physics and Astronomy Dept. (3) SUNY Oswego Physics Dept. (4) Graduate Institute of Astronomy, National Central University, Taiwan OGLE-III Survey The Optical Gravitation Lensing Experiment (OGLE-III) is a Polish astronomical survey primarily intended to detect gravitational microlensing events and transiting planets. Due to constant monitoring of the night sky, they were able to construct one of the largest catalogues of variable stars. This catalogue provided information of periods and V and I bands intensities for over 17,000 RR Lyraes in the Large Magellanic Cloud (LMC) and the Small Magellanic Cloud (SMC) that we used in our project. Fourier Decomposition and Light Curves We ran the OGLE-III data through a Fourier decomposition code which provided us theoretical light curves to fit to curves of our actual data. The figure above displays what a typical light curve from our data of RR Lyraes should look like. The Theory The theory begins with the Stefan Boltzmann Law which states that the luminosity is proportionate to the surface area of a sphere and the temperature. We looked at the ratio between the maximum and minimum, and assuming the radius is fairly similar at maximum and minimum light, we are left with an equation that states the amplitude should be directly related to the temperature at maximum and minimum temperature. Period-Color Relations As seen in the figures below and to the right, is that the period color for minimum is relatively flat, suggesting the amplitude of the star is reliant solely on the temperature of maximum light. Evidence of Evolved Stars in Data In the period-amplitude relations there is evidence in a curve parallel to the bulk of the data that may suggest there are evolved stars in the data moving off the horizontal branch that RR Lyraes occupy. Below are period amplitude relations in the SMC and LMC that are fitted with a curve of data from M3 RR Lyraes from a previous project. A paper by Smith et al. also theorizes the presence of evolved stars in the data. Amplitude-Color Relations Our results for the amplitude color relations do support the theory, as seen in the figures below. The slopes of the fit on the maximum light is.380 while the slope for the minimum light is.109 in the LMC and respectively,.372 and.129 in the minimum suggesting at the very least that the amplitude is much more reliant on the temperature at maximum. Acknowledgements The authors thank SUNY Oswego, National Central University and the Graduate Institute for Astronomy and the National Science Foundation's Office of International Science and Engineering's award