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 ~ 78000 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 kms^{-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 beta power-law as well as a P \propto rho wind yield appropriate fit models. Adopting a continuous mass outflow we obtain a mass-loss rate of M_loss= (0.8 - 3.4)*10^{-11} M_{sun}yr^{-1} depending on the particular wind model.