A Theoretical Investigation into the Properties of RR Lyraes at Maximum and Minimum Light G. Feiden, S. M. Kanbur (Physics Department, SUNY Oswego), R.Szabó, R. Buchler (Physics Department, University of Florida), C. C. Ngeow (Astronomy Department, UIUC) & Z. Kolláth (Konkoly Observatory, Hungary). 209 th AAS Meeting, January 2007, Seattle, WA Abstract: We construct a large grid of full amplitude hydrodynamic models of RRab stars. We confirm earlier findings that, at minimum light, the period-color relation is tighter and flatter than at maximum light, the reason being the interaction of the hydrogen ionization front and photosphere at low densities. We propose using the properties of RRab stars with periods greater than log(P) = -0.2 to estimate reddening. Introduction: RR Lyraes are intrinsically variable stars which vary in brightness with regular periods of the order of hours. Knowledge of their absolute brightness leads to information both about their distance and their age: both are vitally important to know in order to discriminate between different theories about the origin, structure and fate of our Universe. A major problem in estimating their absolute magnitudes for globular clusters is reddening. One method for estimating reddening is to use the period-color (PC) properties of RRab stars at minimum light. Previous observational and theoretical work (Kanbur & Fernando 2005, MNRAS 359:L15; and references therein) has suggested that at minimum light, RRab stars follow a much flatter PC relation. Here we construct a large grid of models with updated physics to investigate the physics behind the properties of these stars at minimum light. A simple application of the Stefan-Boltzmann law suggests that if the PC relation is flatter at minimum light, then the amplitude-color (AC) relation should be steeper at maximum light than at minimum light. Analysis & Preliminary Results: We integrate the time dependent equations of momentum, mass and energy conservation, together with a Saha equation of state, radiative transfer in the diffusion approximation and a numerical recipe to compute non-local time dependent turbulent convection. The system is completed by an initial model in hydrostatic equilibrium and well defined boundary conditions at the center and surface. Each model is specified by a mass, luminosity, effective temperature and composition, with 0.3Msun < M < 0.9Msun, 10Lsun < L < 100Lsun, 5500K < T < 7000K and X=0.7, Z= After full amplitude pulsation was reached, the theoretical variations of the photospheric temperature were converted to V-I colors using Kurucz model atmospheres. Figures 1 and 2 displays the PC and AC relations (at maximum and minimum light), respectively. Figure 3 presents the “distance” in mass distribution between the photosphere and HIF for the models. We clearly see the flatter and tighter relation at minimum light and a distinct slope at maximum light. Also the PC relation at minimum light gets tighter for log (P) > This property can be used to estimate the reddening for RRab variables. Acknowledgement: GF notes the support of SUNY Oswego. SK would like to acknowledge the HST Grant HST-AR RSz acknowledges support of the Hungarian Eötvös fellowship. Figure 1: Plots of the theoretical PC relations at maximum (top) and minimum (bottom) light. The larger and smaller dispersion at maximum and minimum light is evident, as is the flatter slope at minimum light. Figure 2: Amplitude-Color relation at minimum (top panel) and minimum (bottom panel) light for the theoretical models. Figure 3: The distance from the base of the hydrogen ionization front to the photosphere or optical depth 2/3.