Terminal velocities for W-R stars can be obtained from ultraviolet P--Cygni absorption profiles (Prinja et al. 1990) or from infrared HeI profiles (Howarth & Schmutz 1992). Recent studies have shown that there exists a good correlation between terminal velocity and stellar temperature (see Table 1) although no significant differences are found between Galactic and LMC stars (Crowther & Smith 1996b).
Figure 1: Comparison of wind performance numbers (see text) for Galactic (open)
and LMC (filled-in) WNL stars and LBVs versus surface hydrogen
abundance (by mass) adapted from Crowther & Smith (1996b).
The single scattering limit is shown (dotted-line),
while a clear distinction in wind density is apparent between
hydrogen-rich and hydrogen-poor WNL stars. Lower wind velocities and
mass-loss rates for LMC stars, anticipated from radiation driven
wind theory are not observed
Observationally, mass-loss rates of W-R stars can be determined
from radio emission fluxes, assuming spherical symmetry and homogeneity,
provided stellar wind velocities, chemical abundances and the outer
wind ionization balance are known. However, the difficulty in obtaining
radio observations of most W-R stars, coupled with uncertain chemistries
and ionization meant that progress was slow prior to the Standard Model.
With few exceptions, log (
/
yr
)
-5 to -4,
with no obvious relation to
spectral type, binarity or metallicity. The theoretically expected
relation between mass-loss and luminosity has been tentatively
confirmed (Hamann et al. 1995), at least for those without hydrogen.
Winds of O--stars have mass-loss rates that are
reasonably explained by radiatively driven wind theory
(Puls et al. 1996).
The wind performance number 
/(L/c) indicates
the efficiency at which radiation momentum is imparted to the wind,
and is equal to unity in the `single scattering limit' (i.e. scattering
of a photon in a single line). In contrast, W-R stars often have
performance numbers substantially greater than unity
as shown in Fig. 1 for Galactic and LMC WNL stars, causing severe
difficulties for radiatively driven wind theory, unless multiple scattering
is achieved (see Springmann 1994). Hillier (1996)
discusses various options which could solve this problem, while other
driving mechanisms have also been proposed (e.g. pulsational instabilities,
Glatzel et al. 1993, Langer et al. 1994). Although current radiation
driven wind theory predicts lower wind velocities and mass-loss rates
at lower metallicity, these are not observed for LMC stars,
while there is also a significant correlation between wind performance
number and helium enrichment (Fig. 1).