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Published September 20, 2025 | Version Published
Journal Article Open

A Magnetic White Dwarf Formed through a Binary Merger within 35 Million Yr

Creators

  • 1. ROR icon Sun Yat-sen University
  • 2. ROR icon Yunnan Observatories
  • 3. ROR icon University of Chinese Academy of Sciences
  • 4. ROR icon Anhui Science and Technology University
  • 5. ROR icon University of Valparaíso
  • 6. ROR icon Millennium Institute of Astrophysics
  • 7. ROR icon University of Hertfordshire
  • 8. Gemini Observatory/NSF's NOIRLab, 670 N. A'ohoku Place, HI 96720, USA
  • 9. ROR icon Nanjing University
  • 10. ROR icon Beijing Normal University
  • 11. ROR icon California Institute of Technology
  • 12. ROR icon University of Auckland
  • 13. ROR icon Chinese Academy of Sciences
  • 14. International Centre of Supernovae, Yunnan Key Laboratory, Kunming 650216, Yunnan, People's Republic of China

Abstract

White dwarfs (WDs) represent the final evolutionary stage of most stars, typically originating from progenitor stars with masses below approximately 8–10 M⊙. Formation through single-star evolution generally requires at least 25 Myr, with the youngest WDs often near the Chandrasekhar limit of 1.4 M⊙. In contrast, WDs formed via binary channels, such as mergers or mass transfer, can develop smaller masses in a shorter timescale and may exhibit unique characteristics, including strong surface magnetic fields and rapid rotation. Accurately determining the ages of these WDs is essential for understanding their formation. A valuable method involves studying WDs in star clusters, where member stars share the same age and chemical composition, allowing for precise constraints on the formation times and metallicities of the WDs’ progenitors. Here we report a WD found in the open cluster RSG 5, which is only 35 Myr old. The WD’s mass is lower than 1.05 M⊙, indicating it may not have formed through single-star evolution. The WD possesses an exceptionally strong surface magnetic field (≥200 MG), a short rotational period (∼6.5 minutes), and, most notably, a corotating half-ring of ionized circumstellar debris. This distinctive feature provides evidence for a binary merger origin, a scenario further substantiated by our stellar evolution models.

Copyright and License

© 2025. The Author(s). Published by the American Astronomical Society. Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.

Acknowledgement

H.Y., J.L., L.W., and C.L. acknowledge support from the Natural Science Foundation of China (NSFC) through grant 12233013. H.G. acknowledges the support from the Strategic Priority Research Program of the Chinese Academy of Sciences (grant No. XDB1160201), the NSFC (Nos. 12288102, 12090043, and 12173081), the National Key R&D Program of China (No. 2021YFA1600403), Yunnan Fundamental Research Projects (No. 202401BC070007), and the International Centre of Supernovae, Yunnan Key Laboratory (No. 202302AN360001). Z.G. is funded by ANID, Millennium Science Initiative, AIM23-001. Z.G. is supported by the China-Chile Joint Research Fund (CCJRF No. 2301) and Chinese Academy of Sciences South America Center for Astronomy (CASSACA) Key Research Project E52H540301. Z.L. was supported by the National Natural Science Foundation of China (Grant No. 12473034), the Yunnan Revitalization Talent Support Program—Young Talent Project, and the Yunnan Fundamental Research Project (Grant No. 202401AT070139). We thank Dr. Zach Vanderbosch from the California Institute of Technology for the observations of RSG 5-WD using the Keck telescope, and this paper utilizes the observational data he provided as the PI. We thank Ilaria Caiazzo from the Institute of Science and Technology Austria for her scientific discussions and constructive suggestions on this work.

Contributions

C.L. and H.G. are responsible as the corresponding authors for the entire work. C.L. conceived the study and designed the methodology. H.G. led the theoretical calculations and their physical interpretation. H.Y. is the main executor of this work; he first discovered the source of this study, wrote the main content of the article, and carried out all steps from data analysis to scientific calculations. J.L., R.Z., and L.W. contributed equally to this work; they jointly completed the main steps required, such as spectral analysis, magnetic field calculations, and stellar evolution calculations. J.E. provided scientific insights and suggestions for the theoretical explanation of this work. G.L. fitted the spectra of the white dwarf and determined its parameters. L.R. analyzed and interpreted the light curve of the white dwarf. C.F. observed RSG 5-WD using the Keck telescope. Z.C., C.C., B.M., and S.X. conducted observations of the white dwarf in this work (using the GMOS/North-Gemini) and offered comments and suggestions for the article’s writing. C.F., Y.S., Z.L., and X.Z. contributed scientific discussions for the section on binary star evolution in this work.

Facilities

Gaia - (DR3), Gemini-Gillett - (GMOS), Keck:I - KECK I Telescope (LRIS), PO:1.2m North America & Hawaii - (DR23).

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Additional details

Related works

Is new version of
Discussion Paper: arXiv:2509.01069 (arXiv)

Funding

National Natural Science Foundation of China
12233013
Chinese Academy of Sciences
XDB1160201
National Natural Science Foundation of China
12288102
National Natural Science Foundation of China
12090043
National Natural Science Foundation of China
12173081
Ministry of Science and Technology of the People's Republic of China
National Key R&D Program 2021YFA1600403
Yunnan Provincial Science and Technology Department
202401BC070007
Yunnan Provincial Science and Technology Department
Yunnan Key Laboratory 202302AN360001
Agencia Nacional de Investigación y Desarrollo
AIM23-001
Chinese Academy of Science South America Center for Astronomy
2301
Chinese Academy of Science South America Center for Astronomy
E52H540301
National Natural Science Foundation of China
12473034
Yunnan Provincial Science and Technology Department
Yunnan Fundamental Research Project 202401AT070139

Dates

Accepted
2025-08-22
Available
2025-09-15
Published

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Division of Physics, Mathematics and Astronomy (PMA)
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Published