Dynamically consistent analysis of Galactic WN4b stars

Context . Many Wolf-Rayet (WR) stars have optically thick winds that essentially cloak the hydrostatic layers of the underlying star. In these cases, traditional spectral analysis methods are plagued by degeneracies that make it difficult to constrain parameters such as the stellar radius and the deeper density and velocity structure of the atmosphere. Aims . Focussing on the regime of nitrogen-rich WN4 stars with strong emission lines, we employed hydrodynamically consistent modelling using the POWR HD code branch to perform a next-generation spectral analysis. The inherent coupling of the stellar and wind parameters enabled us to break parameter degeneracies, constrain the wind structure, and get a mass estimate. With this information, we were able to draw evolutionary implications and test current mass-loss descriptions for WR stars. Methods . We selected a sample of six Galactic WN4b stars. Applying updated parallaxes from Gaia DR3 and calculating POWR HD models that sufficiently resemble most of their spectral appearance, we obtained new values for the stellar and wind parameters of the WN4b sample. We compared our results to previous studies employing grid models with a prescribed β = 1 velocity structure and cross-checked our derived parameters with stellar structure predictions from GENEC and FRANEC evolution tracks. Results . For all six targets, we obtain a narrow range of stellar temperatures T * ~ 140 kK, in sharp contrast to previous grid-model analyses. We confirm the existence of WRs with luminosities as low as log L/L ⊙ = 5.0 and M * ≈ 5 M ⊙ . All derived velocity fields include a plateau-like feature at ~85% of the terminal velocity. Both the distance updates and the switch to dynamically consistent atmospheres lead to substantial parameter adjustments compared to earlier grid-based studies. A comparison of the derived mass-loss rates favours a different description for the WN4b sample than for WN2 stars analysed with the same methodology. Conclusions . WN4b winds are launched by the hot iron opacity bump, placing these hydrogen-free stars near or slightly hotter than the He zero age main sequence. Similar to a recent analysis of WN2 stars, we have thus solved the WR radius problem for the WN4b stars, but this conclusion cannot be extrapolated to regimes strongly affected by radiatively driven turbulence. Evolutionary models struggle to reproduce the empirical parameter combinations. The observed stars typically require lower mass loss in the current WR stage than predicted, but require further prior stripping in order to arrive at the observed stage.