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Discovery of a transitional type of evolved massive star with a hard ionizing flux

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Abstract

Wolf–Rayet (WR) stars are the evolved descendants of the most massive stars and show emission-line-dominated spectra formed in their powerful winds. Marking the final stage before core collapse, the standard picture of WR stars has been that they evolve through three well-defined spectral subtypes known as WN, WC and WO. Here we present a detailed analysis of five objects that defy this scheme, demonstrating that WR stars can also evolve directly from the WN stage to the WO stage (WN/WO). Our study reveals that this direct transition is connected to low metallicity and weaker winds. The WN/WO stars and their immediate WN precursors are hot and emit a high flux of photons capable of fully ionizing helium. The existence of these stages unveils that high-mass stars that manage to shed off their outer hydrogen layers in a low-metallicity environment can spend a considerable fraction of their lifetime in a stage that is difficult to detect in integrated stellar populations, but at the same time yields a hard ionizing flux. The identification of the WN-to-WO evolution path for massive stars has significant implications for understanding the chemical enrichment and ionizing feedback in star-forming galaxies, in particular at earlier cosmic times.

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Fig. 1: Spectral appearance of WN/WO stars compared with known WN- and WO-type objects.
Fig. 2: HRD with the positions of WR stars and in comparison with the calculated GENEC tracks.
Fig. 3: Spectral change from WN/WC to WN/WO with temperature.
Fig. 4: Illustration of the ionizing fluxes for different WR types.

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Data availability

All observational material employed in the current study is available publicly from the sources listed in ‘Observational data’ in Methods. The resulting synthetic spectra from the atmosphere models that support the findings of this study are plotted in the Extended Data figures. The raw model data are available upon request from the corresponding author.

Code availability

The standard branch of the PoWR atmosphere model code is described in refs. 16,17 and available at https://github.com/powr-code. The customized version by the corresponding author is described in refs. 1,18 and available on request. A brief conceptual overview is further given in ‘Stellar atmosphere modelling’ in Methods with the employed set of atomic data summarized in Extended Data Table 1.

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Acknowledgements

This research is based on observations made with the NASA/ESA Hubble Space Telescope obtained from the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. These observations are associated with programmes 15822 (principal investigator A.A.C.S.) and 17426 (principal investigator A.A.C.S.). This paper benefited from discussions at the International Space Science Institute (ISSI) in Bern through ISSI International Team project 512 (Multiwavelength View on Massive Stars in the Era of Multimessenger Astronomy, principal investigator L.M.O.). R.R.L. and J.J. are members of the International Max Planck Research School for Astronomy and Cosmic Physics at the University of Heidelberg (IMPRS-HD). A.A.C.S., R.R.L. and V.R. acknowledge support by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) in the form of an Emmy Noether Research Group – Project-ID 445674056 (SA4064/1-1, principal investigator A.A.C.S.). A.A.C.S. and V.R. further acknowledge support from the Deutsches Zentrum für Luft und Raumfahrt (DLR) grant grants 50 OR 2503 (principal investigator A.A.C.S.) and 50 OR 2306 (principal investigators V.R. and A.A.C.S.) as well as from the Federal Ministry of Education and Research (BMBF) and the Baden-Württemberg Ministry of Science as part of the Excellence Strategy of the German Federal and State Governments. J.J. acknowledges funding from the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) Project-ID 496854903 (SA4064/2-1, principal investigator A.A.C.S.). E.R.H. and J.S.V. are supported by STFC funding under grant number ST/V000233/1. L.M.O. acknowledges the funding provided by the DFG grant 443790621. D.P. acknowledges financial support by the Deutsches Zentrum für Luft und Raumfahrt (DLR) grant FKZ 50OR2005. R.H. acknowledges support from the World Premier International Research Centre Initiative (WPI Initiative), MEXT, Japan, the European Union’s Horizon 2020 research and innovation programme (ChETEC-INFRA, Grant No. 101008324) and the IReNA AccelNet Network of Networks (National Science Foundation, Grant No. OISE-1927130). This article is based upon work from the ChETEC COST Action (CA16117). I.M. acknowledges support from the Australian Research Council (ARC) Centre of Excellence for Gravitational Wave Discovery (OzGrav), through project number CE230100016. This project was co-funded by the European Union (Project 101183150 - OCEANS).

Author information

Authors and Affiliations

  1. Zentrum für Astronomie der Universität Heidelberg, Astronomisches Rechen-Institut, Heidelberg, Germany

    Andreas A. C. Sander, Roel R. Lefever, Joris Josiek & Varsha Ramachandran

  2. Interdisziplinäres Zentrum für Wissenschaftliches Rechnen, Universität Heidelberg, Heidelberg, Germany

    Andreas A. C. Sander

  3. Armagh Observatory and Planetarium, Armagh, UK

    Erin R. Higgins & Jorick S. Vink

  4. Astrophysics Research Centre, School of Mathematics and Physics, Queen’s University Belfast, Belfast, UK

    Erin R. Higgins

  5. Astrophysics Group, Keele University, Staffordshire, UK

    Raphael Hirschi

  6. Kavli IPMU (WPI), University of Tokyo, Kashiwa, Japan

    Raphael Hirschi

  7. Institut für Physik & Astronomie, Universität Potsdam, Potsdam, Germany

    Lidia M. Oskinova, Daniel Pauli, Max Pritzkuleit, Wolf-Rainer Hamann & Helge Todt

  8. Institute of Astronomy, KU Leuven, Leuven, Belgium

    Daniel Pauli

  9. Department of Astronomy, University of Wisconsin, Madison, WI, USA

    John S. Gallagher III

  10. Department of Physics and Astronomy, Macalester College, St. Paul, MN, USA

    John S. Gallagher III

  11. School of Physics and Astronomy, Monash University, Clayton, Victoria, Australia

    Ilya Mandel

  12. ARC Center of Excellence for Gravitational-wave Discovery (OzGrav), Melbourne, Victoria, Australia

    Ilya Mandel

  13. The School of Physics and Astronomy, Tel Aviv University, Tel Aviv, Israel

    Tomer Shenar

Authors
  1. Andreas A. C. Sander
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  2. Roel R. Lefever
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  3. Joris Josiek
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  4. Erin R. Higgins
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  5. Raphael Hirschi
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  6. Lidia M. Oskinova
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  7. Daniel Pauli
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  8. Max Pritzkuleit
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  9. John S. Gallagher III
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  10. Wolf-Rainer Hamann
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  11. Ilya Mandel
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  12. Varsha Ramachandran
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  13. Tomer Shenar
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  14. Helge Todt
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  15. Jorick S. Vink
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Contributions

A.A.C.S. developed the hypothesis, developed the hydrodynamically consistent atmosphere PoWR code branch used in this work, calculated the final atmosphere models and wrote most of the paper. R.R.L. calculated the hydrodynamically consistent atmosphere models, performed the spectral re-classification and contributed to the paper. J.J. calculated the presented and related GENEC evolution models and contributed to the interpretation of the objects. E.R.H. provided WR star structure models and contributed to the discussion about their evolutionary nature. R.H. contributed to the paper, in particular with respect to the evolutionary interpretation and the discussion of the abundances. L.M.O. contributed the X-ray investigation of the studied targets and commented on the paper. D.P. calculated MESA evolution models, contributed to the discussion on the evolutionary status and commented on the paper. M.P. calculated early atmosphere models for one of the WN/WO targets. J.S.G. contributed to the paper and the discussion regarding the wider context of the targets. W.-R.H. developed the core and large parts of the PoWR atmosphere code and contributed to the discussion. I.M., V.R. and T.S. contributed to the discussion regarding the nature and impact of the studied objects. T.S. further contributed the script to select the best-matching BPASS models. H.T. collected a considerable part of the underlying atomic data to the atmosphere models, contributed to the PoWR atmosphere code, and to the atmosphere analysis and discussion. J.S.V. contributed to the discussion on the evolutionary status and the hydrodynamically consistent atmosphere models.

Corresponding author

Correspondence to Andreas A. C. Sander.

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Extended data

Extended Data Fig. 1 Spectral visualization of potential WN+WO binaries or chance alignments.

Upper panel (a): Comparison of the normalized optical spectrum of M33WR 206 with those of the WN2-star WR 2 and the WO-star WR 142. To check whether the unique appearance of M33WR 206 could also be the product of a WN and WO star, we show two weighted combinations of the latter two stars with the WN2-star providing either 80 % (dash-dotted line) or 95 % (dotted line) of the flux to the optical spectrum. In either case, unobserved features appear with the most prominent ones being highlighted in the figure. Lower panel (b): Similar comparison, but now using the WN4 star WR 6 as the WN component, illustrating that for cooler WN subtypes even more additional features would appear.

Extended Data Fig. 2 Visualization of the quantitative spectral analysis results of M33WR 206.

Observed spectral and photometric data (blue) for M33WR 206 over-plotted with the best-fitting atmosphere model (red). The uppermost panel shows the spectral energy distribution while the lower panels compare the normalized observed and model spectrum the UV and optical regime. The presence (or absence) of major spectral lines is indicated by black vertical dashes annotated with the corresponding ion.

Extended Data Fig. 3 Visualization of the quantitative spectral analysis results of M31WR 99-1.

Observed spectral and photometric data (blue and green) for M31WR 99-1 over-plotted with the best-fitting atmosphere model (red). The uppermost panel shows the spectral energy distribution while the lower panels compare the normalized observed and model spectrum the UV and optical regime. For the UV (secondmost upper panel), no observation is available. The third panel contains also a zoom-in around the N V 4933/44 line compared to models with different nitrogen abundance (indicated by different colors and line styles). The presence (or absence) of major spectral lines is indicated by black vertical dashes annotated with the corresponding ion.

Extended Data Fig. 4 Visualization of the quantitative spectral analysis results of BAT99 5.

Observed spectral and photometric data (blue and green) for BAT99 5 over-plotted with the best-fitting atmosphere model (red). The uppermost panel shows the spectral energy distribution while the lower panels compare the normalized observed and model spectrum the UV and optical regime. The UV panel (secondmost upper panel) contains an inlet showing a zoom-in around the C IV 1550 wind line compared to models with different carbon abundance (indicated by different colors and line styles). The presence (or absence) of major spectral lines is indicated by black vertical dashes annotated with the corresponding ion.

Extended Data Fig. 5 Comparison in the Hertzsprung Russell Diagram (HRD) with single-star evolution models.

The positions of the discovered WN/WO stars (yellow) and the three other WN2 stars (blue) are compared to GENEC models with an LMC-like metallicity (left panel, Eggenberger et al. 2021 as well as earlier tracks from Meynet et al. (1994) with enhanced mass loss often employed in Starburst99 population synthesis models (right panel). Error bars for the observed WR stars are derived from acceptable spectral reproduction within a sample of at least 100 star-specific models.

Extended Data Fig. 6 Comparison with BPASS evolution models.

HRDs with the best-matching BPASS v2.2.1 (Eldridge et al. 2017, Stanway & Eldridge 2018 single (grey) and binary (light purple) tracks for each of the analyzed WN/WO (yellow) and WN2 stars (blue). For binary solution, the secondary track is indicated by a dashed path. The combined luminosity of primary and secondary is indicated by a semi-transparent filled circle. Error bars for the observed WR stars are derived from acceptable spectral reproduction within a sample of at least 100 star-specific models.

Extended Data Fig. 7 Advanced evolution of the custom GENEC models calculated with our adapted wind mass-loss scheme.

Effective temperature (without wind correction, upper panel), mass (middle panel) and mass-loss rate (lower panel) as a function of age, focusing on the time beyond central hydrogen burning. Similar to the HRD plot in Fig. 4, the surface composition is indicated by the track color and dots mark steps of 50 kyr. For comparison, the derived the effective temperatures, masses, and mass-loss rates of the three WN2 stars and M33WR 206 are indicated as dashed light blue horizontal lines.

Extended Data Fig. 8 HRD with custom MESA models demonstrating the impact of enhanced mixing about the He burning core.

The derived positions of our sample stars compared to tracks from custom MESA calculations without (left) and with (right) overshooting during the core-He burning. The models also use an increased mass loss in the cool star regime to remove a substantial part of the envelope which otherwise employ the standard ‘Dutch’ prescription. The surface composition is indicated by the track color and dots mark steps of 30 kyr.

Extended Data Table 1 Number of levels and line transitions used in the PoWRHD models
Full size table
Extended Data Table 2 Derived stellar parameters for WR 46
Full size table

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Sander, A.A.C., Lefever, R.R., Josiek, J. et al. Discovery of a transitional type of evolved massive star with a hard ionizing flux. Nat Astron (2025). https://doi.org/10.1038/s41550-025-02719-z

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