Changes in the Light Curves of Short-Period W Ursae Majoris Binaries: Program Summary

We are observing and analyzing changes in the light curves of a few W Ursae Majoris binaries. This paper summarizes the objectives of our program and the rationale behind our choice of stars and observational strategy. It also describes, briefly, our approach to the seasonal optimization of our reductions, and our two primary analytic approaches (phase-bin and Wilson-Devinney). 1. Program objectives The W UMa stars, overcontact binaries of late spectral type, are fascinating laboratories for understanding stellar structure and evolution. They typically consist of two stars of very different masses in physical contact, with mass ratios higher than ten-to-one in some cases. Despite nearly forty years of research on these stars, we still do not understand the details of their internal structure. New three-dimensional modeling codes are being developed that will enable researchers to make progress in understanding both the internal structure and evolution of these stars. Developing and testing these models fully will require observations of a magnitude heretofore unseen. The primary science objective of our program is to characterize the nature and time scales of changes—even very subtle changes—in the shapes of the light curves of several magnetically-active stars. Rather than providing a few disjointed “snapshots” of such systems, it is our intent to provide “movies” of their behavior over yearly, monthly, weekly, and even daily time scales. Such observations should provide powerful feedback to theoretical hydrodynamic models of the behavior and evolution of magnetically active stars. Genet et al., JAAVSO Volume 34, 2005 55 The secondary science objective of our program is to search for transits of large close-in planets, i.e. “hot Jupiters,” across those binaries in our program that have high orbital inclinations. If hot Jupiters are orbiting W UMa binaries, there are several reasons why their transits have not yet been detected. Although essentially all known W UMa systems are eclipsing binaries, their orbital inclinations are, in the main, nowhere near 90 degrees. Thus one would only expect to observe transits on a small subset of W UMa binaries and, even then, only for close-in orbits. To further compound observational difficulties, hot Jupiter transits of W UMa binaries would produce quasi-periodic rather than truly periodic transits since the binaries are orbiting around each other as the planet moves across our line-of-sight (Doyle et al. 2000). Also, the ever-changing star spots and other surface photometric phenomena would mask subtle transit signatures. Thus it is not surprising such transits have not yet been discovered, although their evidence may already exist unrecognized in some high-precision observations. Our planned multi-year observations of the same binaries led us to adopt, as a tertiary science objective, the search for “cold Jupiters” (Jupiter-mass planets in Jupiter-distance orbits), brown dwarfs, or other third bodies via the light-traveltime effect on eclipse times-of-minima (Deeg et al. 2000). Since Jupiter shifts our own solar system’s barycenter by five seconds peak-to-peak over the course of six years, one might be able to detect a similar shift in an eclipsing binary’s barycenter caused by a third body if the precision of the September–December (2004) seasonal eclipse timing was 1 second or better (3 sigma). Intermittent mass loss, drifting star spots, and other transient phenomena may mask subtle third-body effects, although separation may be possible (Kalimeris et al. 2002). Supporting our three science objectives are two technical objectives. The first is fine tuning our reduction process for each of our major sets of observations. While such optimization would not be worthwhile under ordinary circumstances, our large data sets and the full automation of our reduction and analysis processes allow us to parametrically explore and optimize such reduction decisions as ensemble star inclusion, weighting strategies, etc. Our second technical objective is to develop our phase-bin analysis process for detecting and evaluating small changes in light-curve shapes, including those that could be caused by the transits of hot Jupiters. Our phase-bin technique has been developed specifically for the analysis of a sizeable number of complete orbit-inone-night light curves.