[Paper Review] The Galactic WN stars: Spectral analyses with line-blanketed model atmospheres versus stellar evolution models with and without rotation

W.‐R. Hamann, G. Gräfener+1 more|ArXiv.org|Aug 3, 2006

Stellar, planetary, and galactic studiesPhysics and Astronomy50 references190 citations

TL;DR

This study conducts a comprehensive spectral analysis of Galactic WN stars using advanced line-blanketed model atmospheres (PoWR) to derive precise stellar and wind parameters. Despite improved models and inclusion of rotation in evolutionary tracks, significant quantitative discrepancies remain between observed parameters and predictions, with no clear preference for rotating models, indicating that massive star evolution remains poorly understood despite recent theoretical advances.

ABSTRACT

CONTEXT: Very massive stars pass through the Wolf-Rayet (WR) stage before they finally explode. Details of their evolution have not yet been safely established, and their physics are not well understood. Their spectral analysis requires adequate model atmospheres, which have been developed step by step during the past decades and account in their recent version for line blanketing by the millions of lines from iron and iron-group elements. However, only very few WN stars have been re-analyzed by means of line-blanketed models yet. AIMS: The quantitative spectral analysis of a large sample of Galactic WN stars with the most advanced generation of model atmospheres should provide an empirical basis for various studies about the origin, evolution, and physics of the Wolf-Rayet stars and their powerful winds. METHODS: We analyze a large sample of Galactic WN stars by means of the Potsdam Wolf-Rayet (PoWR) model atmospheres, which account for iron line blanketing and clumping. The results are compared with a synthetic population, generated from the Geneva tracks for massive star evolution. RESULTS: We obtain a homogeneous set of stellar and atmospheric parameters for the Galactic WN stars, partly revising earlier results. CONCLUSIONS: Comparing the results of our spectral analyses of the Galactic WN stars with the predictions of the Geneva evolutionary calculations, we conclude that there is rough qualitative agreement. However, the quantitative discrepancies are still severe, and there is no preference for the tracks that account for the effects of rotation. It seems that the evolution of massive stars is still not satisfactorily understood.

Motivation & Objective

• To provide a homogeneous, empirical determination of stellar and atmospheric parameters for Galactic WN stars using state-of-the-art model atmospheres.
• To test whether modern stellar evolution models—including rotation—can reproduce the observed properties of WN stars.
• To assess the impact of iron-line blanketing and wind clumping on spectral fitting and parameter derivation.
• To evaluate the consistency between observed WN star distributions in the Hertzsprung-Russell diagram and synthetic populations generated from Geneva evolutionary tracks.
• To determine whether rotating models better explain the observed WN and WC star ratios and luminosity functions.

Proposed method

• Application of the Potsdam Wolf-Rayet (PoWR) model atmosphere code with full line blanketing from iron and iron-group elements.
• Use of pre-computed model grids covering key parameters: effective temperature, luminosity, mass-loss rate, and clumping factor.
• Spectral fitting of observed WN star spectra using non-LTE, spherically symmetric, stationary wind models with $eta$-law velocity law and $\beta = 1$.
• Incorporation of wind clumping via a volume-filling factor $f_{\rm V}$, with $D = f_{\rm V}^{-1} = 4$ assumed.
• Comparison of empirical parameters with synthetic populations generated from Geneva stellar evolution tracks, both with and without rotation.
• Population synthesis using Salpeter and steeper initial mass functions ($\beta = 1.35, 2.0$) to test model predictions against observed HRD distributions.

Experimental results

Research questions

• RQ1 Do line-blanketed model atmospheres provide a better fit to observed WN star spectra than previous models?
• RQ2 How well do current Geneva stellar evolution tracks—including rotation—reproduce the observed distribution of WN stars in the Hertzsprung-Russell diagram?
• RQ3 Is there a preference for rotating evolutionary models over non-rotating ones when compared to empirical data?
• RQ4 What is the predicted ratio of WC to WN stars in synthetic populations, and how does it compare to observations?
• RQ5 Can the observed luminosity function of WNL stars be reproduced by current evolutionary models?

Key findings

• The PoWR models with iron-line blanketing significantly improve spectral fits compared to earlier models, leading to revised stellar parameters for Galactic WN stars.
• Despite improved modeling, the observed luminosities of WNL stars exceed those predicted by both rotating and non-rotating evolutionary tracks, with no synthetic WNL star reaching $\log L/L_\odot > 6.2$.
• The observed WNE stars have larger photospheric radii than predicted by models, suggesting a slow-expanding, extended layer possibly driven by the 'hot iron bump' in opacity.
• The synthetic population with rotation produces a WNE-to-WNL ratio of 30:29, closely matching the observed 30:29, but fails to reproduce the temperature scatter of WNE stars.
• The predicted WC-to-WN ratio is lower in rotating models (17) than in non-rotating models (45 and 37), yet the observed ratio of ~0.9 is closer to the non-rotating case, though not definitively resolved.
• No clear preference for rotating models is found; both rotating and non-rotating tracks show severe quantitative discrepancies with observations, indicating that massive star evolution is still not satisfactorily understood.