WC stars - Hot stars with cold winds

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Abstract
The atmospheric structure of WC stars is studied. Detailed models reveal that the ionization of the envelope systematically decreases farther out into the wind. C(4+) and He(2+) dominate close to the star while C(2+) and He(+) dominate the outer regions. Ionizations from excited states play a key role in determining the envelope ionization structure. A 'Stromgren sphere' ionization front occurs in the stellar wind due to the proximity of the C(2+) ionization edge to that of He(+). The temperature structures at densities less than 10 to the 11th can be derived from a simple 'nebular analysis'. The line and continuum fluxes of one model are compared with those observed for the WC5 star HD 165763, and the formation of individual lines is discussed in detail. The correlation of line width with ionization and excitation potential is theoretically discussed. The terminal velocity and velocity law are examined, and the C/He and O/C abundance estimates and their implications for the evolution of WC stars are considered.

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