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

Abstract

Empirical constraints on the outer atmospheric structure of α Boo are derived using spectra obtained with the International Ultraviolet Explorer (IUE) satellite at high resolution. The methods can be applied to giant stars later than ∼K0 which lie on the ‘wind’ side of the ‘corona/wind division’ in the H–R diagram. The methods are based on the interpretation of emission-line fluxes and profiles, combining emission measures, column density measurements from opacity sensitive line ratios and electron density measurements from C II] line ratios. The constraints so-derived apply only to the emission regions where most of the flux is created and by themselves say nothing about cooler material and regions with small emission measures. This is the most complete examination of a ’non-coronal’ star using the available diagnostic emission lines accessible to IUE. The present analysis reveals the importance of radiative processes in determining the observed spectra, resulting from the low particle densities and relatively strong emission lines (especially H Lyman-α) in red giants. A consistent set of constraints is obtained only when such effects are taken into account. Examination of the C II] intersystem line fluxes shows that the atmospheres are not geometrically extended in the sense suggested by previous workers. This has important consequences for models of the outer atmosphere and winds of such stars. The low-temperature constraints (⁠|$T_e\leqslant8000 \enspace \text K$|⁠) agree remarkably well with the overall structure of the hydrostatic model chromosphere of Ayres & Linsky which is based only on Ca IIH and K and Mg IIh and k emission lines. The emission region may be inhomogeneous with line fluxes being generated in regions of higher than average density. Above ∼8000 K where the Ayres & Linsky model is expected to be unreliable, the emission measures of C IIλ1335. H Lyα and Si III]λ1892 are considerably larger than those in the model. The maximum temperature of the emitting material above 104 K may be as low as 1.5×104 K on the basis of the Si III]λ1892 emission measure.