THE STRUCTURE OF WOLF-RAYET WINDS .1. OBSERVATION OF IONIZATION STRATIFICATION IN WR-6 AND WR-111

Although it is widely believed that the winds of Wolf-Rayet stars are driven by radiation pressure, a major problem has been to explain theo retically the observation of very efficient conversion of radiative mo mentum to the gas. Known as the ''momentum problem,'' this has led to alternative models in which the radiation force is only one of the for ces acting on a W-R wind-others could be due to stellar rotation and a magnetic field, or result from stellar pulsations. In a recent paper, Lucy & Abbott (1993) showed that radiation alone could drive the wind of WR 6 if strong ionization stratification in the wind causes suffic ient line overlap so as to increase the momentum deposition via multip le scattering to the observationally required amount. Ionization strat ification in an accelerated outflow should be observable as an inverse correlation of ionization potential with line width, and this has bee n noticed in a few stars in the literature. We here present multiwavel ength data sets which demonstrate observationally that the ionization in the winds of the two prototype stars of the WN and WC sequences, WR 6 (WN 5) and WR 111 (WC 5), decreases outward. We discuss how the dat a compare with the ''standard model'' for these two stars.