EDP Sciences
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Volume 379, Number 3, December I 2001
Page(s) 925 - 935
Section Stellar atmospheres
DOI http://dx.doi.org/10.1051/0004-6361:20011117
A&A 379, 925-935 (2001)
DOI: 10.1051/0004-6361:20011117

The warm circumstellar envelope and wind of the G9 IIb star HR 6902

T. Kirsch, R. Baade and D. Reimers

Hamburger Sternwarte, Universität Hamburg, Gojenbergsweg 112, 21029 Hamburg, Germany

(Received 29 August 2000 / Accepted 31 July 2001)

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 SiIV and CIV absorption up to a distance of 3.3 giant radii ($R_{\rm g}$). Line ratio diagnostics yields an electron temperature of ${\sim} 78 000$ 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 ( ${< } 3 {R_{\rm g}}$) of the bright giant is consistent with a hydrostatic density distribution. The derived line broadening velocity of ${\sim} 70 {\rm km s^{-1}}$ 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 $\beta$ 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 $\dot{M}=0.8 {-} 3.4 \times 10^{-11} M_{\odot}$ yr-1 depending on the particular wind model. The emission lines observed during total eclipse are attributed mostly to resonance scattering of B star photons 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 circumstellar shell. As an illustration we present theoretical SiIV and CIV emission profiles showing model-dependent line shifts and asymmetries.

Key words: binaries: eclipsing -- binaries: spectroscopic -- mass loss -- stars: individual: HR 6902 -- ultraviolet: stars

Offprint request: R. Baade, rbaade@hs.uni-hamburg.de

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