Coronal and Chromospheric Structures in AR LAC: I. Data and Models

Abstract

Traditionally stellar sizes and shapes have been determined by analyzing the optical light curves of eclipsing binaries. Now it is possible to extend our knowledge to structures above the photosphere by making similar observations at X-ray, UV, and radio wavelengths. We report the first such observations, together with their geometrical and physical interpretation, which allow us to determine coronal and chromospheric properties of the stars in the RS CVn binary AR Lac. In this paper we discuss the properties of the system, Einstein observations which provide a reasonably good X-ray light curve, IUE data which enable us to tie the inferred coronal features to the surfaces, and radio observations which allow us to separate geometric and intrinsic variations by acting as a flare monitor. We also discuss the construction of simple coronal model which, when applied to the X-ray light curve, yields the sizes and shapes of the coronae in this system. In another paper (Paper II: Walter, Gibson, and Basri, 1982), the geometrical properties of the coronae are discussed and used to determine the physical conditions within.