The nitrogen spectra of Wolf-Rayet stars - A grid of models and its application to the Galactic WN sample

Adopting the "standard model" for Wolf-Rayet atmospheres, non-LTE radiation transfer calculations are performed which account for helium and nitrogen. Grids of theoretical models are presented for the whole relevant parameter range. The WN classification criteria are employed in order to identify the subtype domains, and inconsistencies are discussed. The (almost complete) sample of known Galactic WN stars is analyzed by comparing the observed spectra with the synthetic spectra of the grid models. This is the first time that nitrogen line analyses are performed for the whole WN sample, while previous comprehensive studies were restricted to helium models. The obtained parameters roughly confirm the results from the previous helium analyses, as far as late subtypes (WNL) and early subtypes with Strong lines (WNE-s) are concerned. For early subtypes with weak lines (WNE-w), however, the parameters are substantially revised. The hottest WN star, with a stellar (effective) temperature of 140 kK, is WR 2, which could not be analyzed previously from its helium lines due to the lack of He I. The other members of the WNE-w subgroup have stellar temperatures between 40 and 90 kK, thus populating the same temperature range as the strong-lined WNE-w, but with less dense winds. The luminosities are revised according to the new parameters. Moreover, reddening corrections are newly determined form comparing IUE data with the UV model fluxes. The average luminosity is now log L/L. = 5.5 for WNE stars (both, strong and weak lined), and log L/L. = 5.9 for WNL (not significantly revised). The empirical minimum WN luminosity is 10(5.0) L., reducing former incompatibilities with predictions from evolutionary calculations. The ratio between mechanical and radiative momentum flow is slightly affected by the revisions, but remains much higher than unity: 9, 9 and 29 for the WNL, WNE-w and WNE-s subclass, respectively.