Previous abstract Next abstract
Wolf-Rayet (W-R) stars are generally thought to be the descendants of massive and luminous OB stars which have lost most of their hydogen rich outer layers. During the W-R phase, massive stars experience an enormous mass-loss, of order $10^-^5$ M$_\odot$ $yr^-^1$, presumably driven by radiation pressure. A major theoretical problem has been to explain the apparently incredibly efficient conversion of radiative momentum to the gas in W-R stars, if it is assumed that each photon transfers its momentum via line scattering to the gas just once. Current estimates of the ratio of the momentum in the wind to the single scattering radiation pressure limit for the WN star WR 6 is 10.44 and the WC star WR 111 is 21.36 (Willis 1991, IAU Symp. 143, 265).
A possible solution to the momentum problem is multiple scattering in which a photon may transfer its momentum more than once (cf. Abbott \& Lucy 1985, ApJ, 288, 679). In a recent paper, Lucy \& Abbott (1993, ApJ, 405, 738) calculated a multitransfer model for EZ CMa = WR 6. They find that ionization stratification in the wind is a key factor for increasing the effectiveness of multiline transfer thus making possible mass-loss rates exeeding the single-scattering limit by the right amount.
The critical significance of ionization stratification to explain what drives the winds of W-R stars has motivated this research. Specifically, the two W-R stars on which we report have been used as case studies in the "standard model" of W-R atmospheres (cf. Hillier 1991, IAU Symp. 143, 59). In an accelerating outflow, one expects to find an inverse correlation between the ionization potential and the line width. We here combine high-resolution IUE spectra from the archival data base, optical spectra obtained at the Anglo-Australian Telescope, and infrared spectra gathered at UKIRT to measure the line widths in WR 6 and WR 111. This study includes a much larger number of transitions for each star than was previously used in any study of ionization stratification.
This work was supported by NASA grant NAG 5-2411.
Saturday program listing