Dynamically consistent analysis of Galactic WN4b stars

Abstract

Many Wolf-Rayet (WR) stars have optically thick winds that 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. Focussing on the regime of nitrogen-rich WN4-stars with strong emission lines, we employ 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 enables us to break parameter degeneracies, constrain the wind structure, and get a mass estimate. With this information, we can draw evolutionary implications and test current mass-loss descriptions for WR stars. 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 obtain new values for the stellar and wind parameters of the WN4b sample. We compare our results to previous studies employing grid models with a beta = 1 velocity structure and cross-check our derived parameters with stellar structure predictions from GENEC and FRANEC evolution tracks. For all six targets, we obtain a narrow range of stellar temperatures T~140 kK, in contrast to previous grid-model analyses. We confirm the existence of WRs with luminosities as low as log L/Lsol = 5.0 and M~5 Msol. All derived velocity fields include a plateau 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.

19 pages, 18 figures

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