The warm circumstellar envelope and wind of the G 9 IIb star HR 6902

ASTRONOMY AND ASTROPHYSICS The warm circumstellar envelope and wind of the G9 IIb star HR 6902 Abstract. IUE observations of the eclipsing binary system HR 6902 obtained at various epochs spread over four years indicate the presence of warm circumstellar material enveloping the G9 IIb primary. The spectra show Si iv and C iv absorption up to a distance of 3.3 giant radii (R g). Line ratio diagnostics yields an electron temperature of ∼ 78 000 K which appears to be constant over the observed height range. Applying a least square fit absorption line analysis we derive column densities as a function of height. We find that the inner envelope (< 3 R g) of the bright giant is consistent with a hydrostatic density distribution. The derived line broadening velocity of ∼ 70 km s −1 is sufficient to provide turbulent pressure support for the required scale height. However, an improved agreement with observations over the whole height regime including the emission line region is obtained with an outflow model. We demonstrate that the common β power-law as well as a P ∝ ρ wind yield appropriate fit models. Adopting a continuous mass outflow we obtain a mass-loss rate of ˙ M = 0.8 − 3.4 × 10 −11 M ⊙ yr −1 depending on the particular wind model. The emission lines observed during total eclipse are attributed mostly to resonance scattering of B star pho-tons in the extended envelope of the giant. By means of a multi-dimensional line formation study we show that the global envelope properties are consistent with the wind models derived from the absorption line analysis. We argue that future high resolution UV spectroscopy will resolve the large-scale velocity structure of the circumstel-lar shell. As an illustration we present theoretical Si iv and C iv emission profiles showing model-dependent line shifts and asymmetries.