THE CHROMOSPHERIC STRUCTURE OF THE COOL GIANT STAR G-HERCULIS

Non-LTE calculations of semiempirical chromospheric models are present ed for 30 g Her (M6 III). This star is one of the coolest (T-eff = 325 0 K) SRb (semiregular) variable stars and has a mass perhaps as great as 4 M(.). Chromospheric features we have observed in its spectrum inc lude Mg II h and k; C II] UV0.01, which is sensitive to electron densi ty; Mg I lambda 2852; Ca II H, K, and IRT; Ca I lambda 4227 and lambda 6573; Al II] UV1; and H alpha. We pay special attention to fitting th e C II intersystem lines and the Mg II resonance lines but use all the other features as constraints to some extent. The equations of radiat ive transfer and statistical equilibrium are solved self-consistently for H I, H-, H-2, He I, C I, C II, Na I, Mg I, Mg II, Al I, Al II, Ca I, and Ca II with the equivalent two-level technique. To simplify thes e calculations, a one-dimensional hydrostatic, plane-parallel atmosphe re is assumed. We investigate 10 separate ''classical'' chromospheric models, differing most importantly in total mass column density above the temperature minimum. Synthetic spectra from these models fit some but not all of the observations. These comparisons are discussed in de tail. However, we find that no single-component classical model in hyd rostatic equilibrium is able to reproduce both the Mg II line profiles and the relative strengths of the C II] lines. In all these models, c hromospheric emission features are formed relatively close to the star (less than or similar to 0.05R double dagger). The circumstellar envi ronment has a thick, cool component overlying the Mg II emission regio n, which is relatively static and very turbulent. Finally, we find tha t thermalization in the Mg II h and k lines in the coolest giant stars is controlled by continuum absorption from Ca I 4 rho(3 rho 0) 3PO bo und-free opacity and not collisional de-excitation as is the case for warmer K giants.