Constraints on the outer atmospheric structure of late-type giant stars with IUE : methods and application to Arcturus (alpha Boo K2III).

Abstract

Methods for empirically analyzing the emission lines formed in the outer atmospheres of late-type low-gravity stars are described and applied to the K2III star Alpha Boo. The methods are based on electron-density measurements, emission-measure analysis, and determination of the optical depths of emission lines. The reduction and measurement of IUE data of Alpha Boo is described. The important atomic and radiative processes which determine the emission-line fluxes of the 'diagnostic' emission lines in late-type giant stars are identified, and the effects of the scattering of radiation in optically thick lines are examined using results from probabilistic radiative transfer theory, and models for line excitation are obtained. Empirical constraints on the outer atmospheric structure of Alpha Boo are derived using the observed line fluxes and widths and the previously described models.