1998A&A...334..845S

We consider the relationships between the classification parameters of WN stars in the new 3-dimensional classification of Smith et al. (1996MNRAS.281..163S) and the corresponding and related parameters that define stellar atmosphere models. Specifically, we consider: FWHM of HeII4686 vs. v_∞; hydrogen content by direct inspection vs. hydrogen content by modelling and vs. colour (b-v)₀; ionisation subclass and M_v vs. effective temperature. From these data we argue that the WNb and only the WNb stars (i.e. stars with EW5411>40Å or FWHM4686>30Å) are entirely free of hydrogen. For the WNb stars, we consider the relationships of EW5411 and FWHM4686 to the derived temperature T_*; the mass loss rate; and the surface mass flux. It appears that, to first approximation, the stars are a one-parameter family and the spectral classification criteria are sufficient to give an indication of the intrinsic colour, absolute magnitude (not very accurately), effective temperature T_* and terminal velocity. Theoretical models suggest that the critical parameter defining most of the properties of a WNb star is its present mass. However, the behaviour of FWHM4686 suggests the presence of a second parameter that affects the mass loss rate and terminal velocity of the wind. We suggest that the second parameter may be either (or a combination of) the internal mean molecular weight or the rotation rate of the star. We further compare the relationships predicted by evolutionary models with those found for observed stars (using atmosphere models), highlighting the present difficulties in these comparisons.