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A massive white-dwarf merger product before final collapse


Gravitational-wave emission can lead to the coalescence of close pairs of compact objects orbiting each other1,2. In the case of neutron stars, such mergers may yield masses above the Tolman–Oppenheimer–Volkoff limit (2 to 2.7 solar masses)3, leading to the formation of black holes4. For white dwarfs, the mass of the merger product may exceed the Chandrasekhar limit, leading either to a thermonuclear explosion as a type Ia supernova5,6 or to a collapse forming a neutron star7,8. The latter case is expected to result in a hydrogen- and helium-free circumstellar nebula and a hot, luminous, rapidly rotating and highly magnetized central star with a lifetime of about 10,000 years9,10. Here we report observations of a hot star with a spectrum dominated by emission lines, which is located at the centre of a circular mid-infrared nebula. The widths of the emission lines imply that wind material leaves the star with an outflow velocity of 16,000 kilometres per second and that rapid stellar rotation and a strong magnetic field aid the wind acceleration. Given that hydrogen and helium are probably absent from the star and nebula, we conclude that both objects formed recently from the merger of two massive white dwarfs. Our stellar-atmosphere and wind models indicate a stellar surface temperature of about 200,000 kelvin and a luminosity of about 104.6 solar luminosities. The properties of the star and nebula agree with models of the post-merger evolution of super-Chandrasekhar-mass white dwarfs9, which predict a bright optical and high-energy transient upon collapse of the star11 within the next few thousand years. Our observations indicate that super-Chandrasekhar-mass white-dwarf mergers can avoid thermonuclear explosion as type Ia supernovae, and provide evidence of the generation of magnetic fields in stellar mergers.

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We thank T. Rauch and K. Werner for discussions and for providing atomic data through the Tübingen Model Atom Database in the framework of the German Virtual Observatory. V.V.G. acknowledges support from the Russian Science Foundation under grant 14-12-01096 and from the Russian Foundation for Basic Research (RFBR) under grant 19-02-00779. G.G. acknowledges financial support from Deutsche Forschunsgemeinschaft (DFG) under grant GR 1717/5-1. O.V.M. acknowledges support from RFBR under grant 16-02-00148 and from the Čzech Science Foundation under grant GA ČR 18-05665S. A.Y.K. acknowledges support from RFBR under grant 16-02-00148 and from the National Research Foundation (NRF) of South Africa. The TMAD tool (http://astro.uni-tuebingen.de/~TMAD) used in this study was constructed as part of the activities of the German Astrophysical Virtual Observatory. This research made use of the SIMBAD database, operated at CDS, Strasbourg, France.

Author information

V.V.G., G.G. and N.L. jointly analysed and interpreted observational data and wrote the manuscript. O.V.M. obtained and reduced the spectroscopic material. A.Y.K. provided ideas for the interpretation of the nebula. A.S.M. and O.I.S. obtained optical photometry data. All authors discussed the results and commented on the manuscript.

Competing interests

The authors declare no competing interests.

Correspondence to Vasilii V. Gvaramadze or Götz Gräfener.

Extended data figures and tables

  1. Extended Data Fig. 1 Neon features in our models.

    Comparison of models with neon surface mass fractions of 0.0 (red), 0.1 (green) and 0.5 (blue) with observations (black). The observed and calculated fluxes are divided by the same modelled continuum flux fc.

  2. Extended Data Fig. 2 Abundances.

    a, Comparison of the observed spectrum of J005311 (black line) with our best-fitting model (helium mass fraction Y = 0.0; blue) with models with increased helium abundance (Y = 0.1, green; Y = 0.2, red). b, Comparison of our best-fitting model (carbon mass fraction X(C) = 0.2; blue) with models with altered carbon abundance (X(C) = 0.1, green; X(C) = 0.3, red).

  3. Extended Data Fig. 3 Stellar temperature.

    a, Comparison of our best-fitting model (211,000 K; black) with models that are hotter (266,000 K; red) and cooler (178,000 K; blue) than the temperature range given in Table 1. b, Magnified profiles of the O vi 4,500 Å and C iv 4,660 Å lines.

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Fig. 1: New mid-infrared nebula in Cassiopeia.
Fig. 2: Spectral modelling of J005311.
Fig. 3: Position of J005311 in the Hertzsprung–Russell diagram.
Extended Data Fig. 1: Neon features in our models.
Extended Data Fig. 2: Abundances.
Extended Data Fig. 3: Stellar temperature.


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