The relationship between the WR classification and stellar models II. The WN stars without hydrogen

We consider the relationships between the classification parameters of WN stars in the new 3-dimensional classification of Smith et al. (1996) and the corresponding and related parameters that define stellar atmosphere models. Specifically, we consider: FWHM of HeII 4686 vs. v∞; hydrogen content by direct inspection vs. hydrogen content by modelling and vs. colour (b-v)0; ionisation subclass and Mv vs. effective temperature. From these data we argue that the WN b and only the WN b stars (i.e. stars with EW 5411 > 40 Å or FWHM 4686 > 30 Å) are entirely free of hydrogen. For the WN b stars, we consider the relationships of EW 5411 and FWHM 4686 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 WN b star is its present mass. However, the behaviour of FWHM 4686 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.