Theory of cool loops and the dividing line

Abstract

Static models for coronal loops have been widely used to interpret observations of the coronae of cool stars. Although these models have been successful in explaining several features of the observations; they have been unsuccessful in accounting for two key features: (a) in dwarf stars they do not agree with the observed form of the differential emission measure at low temperatures, T < 10⁵ K; and (b) in certain giant stars they do not agree with the lack of emission at high temperatures, T > 10⁵ K (the so-called dividing line). It appears that in high gravity stars there is more cool material than the standard models of the transition region predict; whereas in low gravity stars there is less hot material than the loop models predict.

In this paper we describe a model for the structure of a coronal active region that can explain both the emission measure discrepancy and the dividing line. The key new idea underlying this model is that under certain conditions, the equations for a static coronal loop admit a class of cool solutions as well as the usual hot solutions. We describe the properties of the cool loop models and their dependence on stellar parameters such as magnetic field geometry and element abundances. We discuss in detail the conditions which permit a coronal flux tube to have a cool solution. It turns out that gravity plays a fundamental role in determining whether a cool state is possible. We discuss the implications of these results for the structure of stellar coronae.