Previous
abstract 
Next
abstract
Wolf-Rayet (W-R) stars are believed to be the descendants of massive OB stars experiencing very high mass loss, of order $10^{-5}$ M$_\odot$ yr$^{-1}$. Although it is generally assumed that the stellar winds of W-R stars are driven by radiation pressure, single-line scattering has been found insufficient to account for the observed wind momentum. This discrepancy is known as the ``momentum problem". Lucy \& Abbott (1993, ApJ, 405, 738) recently presented a model for the WN star WR 6 which solves the momentum problem for this star by including multiple-line scattering.
In the model by Lucy \& Abbott, ionization stratification in the wind is the key factor in increasing the momentum deposition of the stellar radiation. {}From an observational point of view, ionization stratification in W-R winds can be investigated by measuring the widths of Doppler broadened emission lines displaying a range of ionization potentials. This is the method adopted here.
In this paper, we show the results of our study of a small sample of W-R stars of different subtypes along the WN sequence. We combine IUE UV spectra, optical spectra from the literature, and new IR spectra, to discuss the ionization stratification in the sample stars. This work is intended to build towards a more complete set of measurements which will allow us to investigate whether there is any systematic behavior of the ionization stratification that relates to the size of the momentum problem, as one might expect on the basis of Lucy \& Abbott's model.
We acknowledge support by NASA grant NAG5-2411.
