Skip to main content

Thank you for visiting You are using a browser version with limited support for CSS. To obtain the best experience, we recommend you use a more up to date browser (or turn off compatibility mode in Internet Explorer). In the meantime, to ensure continued support, we are displaying the site without styles and JavaScript.

An ultra-massive white dwarf with a mixed hydrogen–carbon atmosphere as a likely merger remnant


White dwarfs are dense, cooling stellar embers consisting mostly of carbon and oxygen1, or oxygen and neon (with a few per cent carbon) at higher initial stellar masses2. These stellar cores are enveloped by a shell of helium, which in turn, is usually surrounded by a layer of hydrogen, generally prohibiting direct observation of the interior composition. However, carbon is observed at the surface of a sizeable fraction of white dwarfs3,4, sometimes with traces of oxygen, and is thought to be dredged up from the core by a deep helium convection zone5,6. In these objects, only traces of hydrogen are found7,8, as large masses of hydrogen are predicted to inhibit hydrogen–helium convective mixing within the envelope9. We report the identification of WD J055134.612+413531.09, an ultra-massive (1.14 solar masses (M)) white dwarf with a unique carbon–hydrogen mixed atmosphere (atomic ratio C∕H = 0.15). Our analysis of the envelope and interior indicates that the total hydrogen and helium mass fractions must be several orders of magnitude lower than predictions of single-star evolution10: less than 10−9.5 and 10−7.0, respectively. Due to the fast kinematics (129 ± 5 km s−1 relative to the local standard of rest), large mass and peculiar envelope composition, we argue that WD J0551+4135 is consistent with formation from the merger of two white dwarfs in a tight binary system11,12,13,14.

Access options

Rent or Buy article

Get time limited or full article access on ReadCube.


All prices are NET prices.

Fig. 1: The white dwarf sequence in the Gaia Hertzsprung–Russell diagram.
Fig. 2: Combined optical spectrum.
Fig. 3: Elemental mass fractions, Xi, against logarithmic mass depth.
Fig. 4: Lightcurves and Fourier transforms for WD J0551+4135 observations made with the TNT.

Data availability

The spectra of WD J0551+4135 are provided as Supplementary Data 1, the best-fitting model spectrum as Supplementary Data 2 and the lightcurves as Supplementary Data 3.

Code availability

The Koester model atmosphere and envelope codes, as well as the lpcode/lp-pul evolutionary/pulsation codes, are not made available. However, their associated references in the text can be consulted for further details.


  1. 1.

    Paczyński, B. Evolution of single stars. I. Stellar evolution from main sequence to white dwarf or carbon ignition. Acta Astron. 20, 47–58 (1970).

    ADS  Google Scholar 

  2. 2.

    Camisassa, M. E. et al. The evolution of ultra-massive white dwarfs. Astron. Astrophys. 625, A87 (2019).

    Google Scholar 

  3. 3.

    Hollands, M. A., Tremblay, P. E., Gänsicke, B. T., Gentile-Fusillo, N. P. & Toonen, S. The Gaia 20 pc white dwarf sample. Mon. Not. R. Astron. Soc. 480, 3942–3961 (2018).

    ADS  Google Scholar 

  4. 4.

    Kepler, S. O. et al. White dwarf and subdwarf stars in the Sloan Digital Sky Survey Data Release 14. Mon. Not. R. Astron. Soc. 486, 2169–2183 (2019).

    ADS  Google Scholar 

  5. 5.

    Koester, D., Weidemann, V. & Zeidler, E. M. Atmospheric parameters and carbon abundance of white dwarfs of spectral types C2 and DC. Astron. Astrophys. 116, 147–157 (1982).

    ADS  Google Scholar 

  6. 6.

    Pelletier, C., Fontaine, G., Wesemael, F., Michaud, G. & Wegner, G. Carbon pollution in helium-rich white dwarf atmospheres: time-dependent calculations of the dredge-up process. Astrophys. J. 307, 242–252 (1986).

    ADS  Google Scholar 

  7. 7.

    Coutu, S. et al. Analysis of helium-rich white dwarfs polluted by heavy elements in the Gaia era. Astrophys. J. 885, 74 (2019).

    ADS  Google Scholar 

  8. 8.

    Koester, D. & Kepler, S. O. Carbon-rich (DQ) white dwarfs in the Sloan Digital Sky Survey. Astron. Astrophys. 628, A102 (2019).

    ADS  Google Scholar 

  9. 9.

    Rolland, B., Bergeron, P. & Fontaine, G. On the spectral evolution of helium-atmosphere white dwarfs showing traces of hydrogen. Astrophys. J. 857, 56 (2018).

    ADS  Google Scholar 

  10. 10.

    Iben, J. I. & Renzini, A. Asymptotic giant branch evolution and beyond. Annu. Rev. Astron. Astrophys. 21, 271–342 (1983).

    ADS  Google Scholar 

  11. 11.

    Toonen, S., Nelemans, G. & Portegies Zwart, S. Supernova type Ia progenitors from merging double white dwarfs: using a new population synthesis model. Astron. Astrophys. 546, A70 (2012).

    ADS  Google Scholar 

  12. 12.

    Shen, K. J., Bildsten, L., Kasen, D. & Quataert, E. The long-term evolution of double white dwarf mergers. Astrophys. J. 748, 35 (2012).

    ADS  Google Scholar 

  13. 13.

    Cheng, S., Cummings, J. D. & Ménard, B. A cooling anomaly of high-mass white dwarfs. Astrophys. J. 886, 100 (2019).

    ADS  Google Scholar 

  14. 14.

    Gvaramadze, V. V. et al. A massive white-dwarf merger product before final collapse. Nature 569, 684–687 (2019).

    ADS  Google Scholar 

  15. 15.

    Gentile Fusillo, N. P. et al. A Gaia Data Release 2 catalogue of white dwarfs and a comparison with SDSS. Mon. Not. R. Astron. Soc. 482, 4570–4591 (2019).

    ADS  Google Scholar 

  16. 16.

    Gaia al. Gaia Data Release 2: summary of the contents and survey properties. Astron. Astrophys. 616, A1 (2018).

    Google Scholar 

  17. 17.

    Chandrasekhar, S. The highly collapsed configurations of a stellar mass (second paper). Mon. Not. R. Astron. Soc. 95, 207–225 (1935).

    ADS  MATH  Google Scholar 

  18. 18.

    Sion, E. M. et al. A proposed new white dwarf spectral classification system. Astrophys. J. 269, 253–257 (1983).

    ADS  Google Scholar 

  19. 19.

    Althaus, L. G., García-Berro, E., Isern, J., Córsico, A. H. & Miller Bertolami, M. M. New phase diagrams for dense carbon–oxygen mixtures and white dwarf evolution. Astron. Astrophys. 537, A33 (2012).

    ADS  Google Scholar 

  20. 20.

    Koester, D. Accretion and diffusion in white dwarfs: new diffusion timescales and applications to GD 362 and G 29-38. Astron. Astrophys. 498, 517–525 (2009).

    ADS  Google Scholar 

  21. 21.

    Cunningham, T., Tremblay, P.-E., Freytag, B., Ludwig, H.-G. & Koester, D. Convective overshoot and macroscopic diffusion in pure-hydrogen-atmosphere white dwarfs. Mon. Not. R. Astron. Soc. 488, 2503–2522 (2019).

    ADS  Google Scholar 

  22. 22.

    Cummings, J. D., Kalirai, J. S., Tremblay, P. E., Ramirez-Ruiz, E. & Choi, J. The white dwarf initial–final mass relation for progenitor stars from 0.85 to 7.5 M . Astrophys. J. 866, 21 (2018).

    ADS  Google Scholar 

  23. 23.

    Tremblay, P. E. et al. 3D model atmospheres for extremely low-mass white dwarfs. Astrophys. J. 809, 148 (2015).

    ADS  Google Scholar 

  24. 24.

    Curd, B. et al. Four new massive pulsating white dwarfs including an ultramassive DAV. Mon. Not. R. Astron. Soc. 468, 239–249 (2017).

    ADS  Google Scholar 

  25. 25.

    Dufour, P., Liebert, J., Fontaine, G. & Behara, N. White dwarf stars with carbon atmospheres. Nature 450, 522–524 (2007).

    ADS  Google Scholar 

  26. 26.

    Ferrario, L., Vennes, S., Wickramasinghe, D. T., Bailey, J. A. & Christian, D. J. EUVE J0317−855: a rapidly rotating, high-field magnetic white dwarf. Mon. Not. R. Astron. Soc. 292, 205–217 (1997).

    ADS  Google Scholar 

  27. 27.

    Córsico, A. H. & Althaus, L. G. Asteroseismic inferences on GW Virginis variable stars in the frame of new PG 1159 evolutionary models. Astron. Astrophys. 454, 863–881 (2006).

    ADS  Google Scholar 

  28. 28.

    De Gerónimo, F. C., Córsico, A. H., Althaus, L. G., Wachlin, F. C. & Camisassa, M. E. Pulsation properties of ultra-massive DA white dwarf stars with ONe cores. Astron. Astrophys. 621, A100 (2019).

    Google Scholar 

  29. 29.

    Córsico, A. H., Althaus, L. G., Miller Bertolami, M. M. & Kepler, S. O. Pulsating white dwarfs: new insights. Astron. Astrophys. Rev. 27, 7 (2019).

    ADS  Google Scholar 

  30. 30.

    Tanikawa, A. et al. Hydrodynamical evolution of merging carbon–oxygen white dwarfs: their pre-supernova structure and observational counterparts. Astrophys. J. 807, 40 (2015).

    ADS  Google Scholar 

  31. 31.

    Currie, M. J. et al. Starlink Software in 2013. Astron. Soc. Pac. Conf. 485, 391–394 (2014).

    ADS  Google Scholar 

  32. 32.

    Marsh, T. molly: 1D astronomical spectra analyzer. Astrophys. Source Code Libr. 1907.012 (2019).

  33. 33.

    Koester, D. White dwarf spectra and atmosphere models. Mem. Soc. Astron. Ital. 81, 921–931 (2010).

    ADS  Google Scholar 

  34. 34.

    Gaia Collaboration et al. Gaia Data Release 2: observational Hertzsprung–Russell diagrams. Astron. Astrophys. 616, A10 (2018).

  35. 35.

    Chambers, K. C. et al. The Pan-STARRS1 surveys. Preprint at (2016).

  36. 36.

    Morrissey, P. et al. The calibration and data products of GALEX. Astrophys. J. Suppl. 173, 682–697 (2007).

    ADS  Google Scholar 

  37. 37.

    Wall, R. E. et al. GALEX absolute calibration and extinction coefficients based on white dwarfs. Mon. Not. R. Astron. Soc. 489, 5046–5052 (2019).

    ADS  Google Scholar 

  38. 38.

    Tremblay, P.-E., Ludwig, H.-G., Steffen, M. & Freytag, B. Spectroscopic analysis of DA white dwarfs with 3D model atmospheres. Astron. Astrophys. 559, A104 (2013).

    Google Scholar 

  39. 39.

    Fontaine, G., Brassard, P. & Bergeron, P. The potential of white dwarf cosmochronology. Publ. Astron. Soc. Pac. 113, 409–435 (2001).

    ADS  Google Scholar 

  40. 40.

    Siess, L. Evolution of massive AGB stars. II. Model properties at non-solar metallicity and the fate of super-AGB stars. Astron. Astrophys. 476, 893–909 (2007).

    ADS  Google Scholar 

  41. 41.

    Siess, L. Evolution of massive AGB stars. III. The thermally pulsing super-AGB phase. Astron. Astrophys. 512, A10 (2010).

    ADS  Google Scholar 

  42. 42.

    Paquette, C., Pelletier, C., Fontaine, G. & Michaud, G. Diffusion coefficients for stellar plasmas. Astrophys. J. Suppl. 61, 177–195 (1986).

    ADS  Google Scholar 

  43. 43.

    Paquette, C., Pelletier, C., Fontaine, G. & Michaud, G. Diffusion in white dwarfs—new results and comparative study. Astrophys. J. Suppl. 61, 197–217 (1986).

    ADS  Google Scholar 

  44. 44.

    Iben, J. I. & MacDonald, J. The effects of diffusion due to gravity and due to composition gradients on the rate of hydrogen burning in a cooling degenerate dwarf. I. The case of a thick helium buffer layer. Astrophys. J. 296, 540–553 (1985).

    ADS  Google Scholar 

  45. 45.

    Spiegel, E. A. A generalization of the mixing-length theory of turbulent convection. Astrophys. J. 138, 216–225 (1963).

    ADS  MathSciNet  MATH  Google Scholar 

  46. 46.

    Zahn, J.-P. Convective penetration in stellar interiors. Astron. Astrophys. 252, 179–188 (1991).

    ADS  Google Scholar 

  47. 47.

    Tremblay, P. E. et al. Calibration of the mixing-length theory for convective white dwarf envelopes. Astrophys. J. 799, 142 (2015).

    ADS  Google Scholar 

  48. 48.

    Kupka, F., Zaussinger, F. & Montgomery, M. H. Mixing and overshooting in surface convection zones of DA white dwarfs: first results from ANTARES. Mon. Not. R. Astron. Soc. 474, 4660–4671 (2018).

    ADS  Google Scholar 

  49. 49.

    Cukanovaite, E. et al. Calibration of the mixing-length theory for structures of helium-dominated atmosphere white dwarfs. Mon. Not. R. Astron. Soc. 490, 1010–1025 (2019).

  50. 50.

    Dhillon, V. S. et al. ULTRASPEC: a high-speed imaging photometer on the 2.4-m Thai National Telescope. Mon. Not. R. Astron. Soc. 444, 4009–4021 (2014).

    ADS  Google Scholar 

  51. 51.

    Chote, P. et al. Puoko-nui: a flexible high-speed photometric system. Mon. Not. R. Astron. Soc. 440, 1490–1497 (2014).

    ADS  Google Scholar 

  52. 52.

    Tremblay, P.-E. et al. Core crystallization and pile-up in the cooling sequence of evolving white dwarfs. Nature 565, 202–205 (2019).

    ADS  Google Scholar 

Download references


M.A.H. acknowledges discussions on the nature of WD J0551+4135 with P. Bergeron and A. Karakas, and with M.-T. Belmonte on the quality of experimental atomic data. The research leading to these results has received funding from the European Research Council under the European Union’s Horizon 2020 research and innovation programme no. 677706 (WD3D). A.A. acknowledges support from the Faculty of Science, Naresuan University (grant no. R2562E029). V.S.D. and ULTRASPEC are funded by the STFC. This work presents results from the European Space Agency (ESA) space mission Gaia. Gaia data are being processed by the Gaia Data Processing and Analysis Consortium (DPAC). Funding for the DPAC is provided by national institutions, in particular the institutions participating in the Gaia MultiLateral Agreement (MLA). The Gaia mission website is and the Gaia archive website is The William Herschel Telescope is operated on the island of La Palma by the Isaac Newton Group of Telescopes in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias. This work is based on observations made with ULTRASPEC at the Thai National Observatory, which is operated by the National Astronomical Research Institute of Thailand (Public Organization).

Author information




M.A.H., P.-E.T. and B.T.G. led the project, including the interpretation of WD J0551+4135. M.E.C. calculated the interior CO/ONe-core models. D.K. calculated the envelope models and advised M.A.H. on atmospheric modelling. N.P.G.-F. acquired the initial Liverpool Telescope lightcurve. A.A., V.S.D. and T.R.M. acquired the TNT lightcurves. P.C. calibrated the Liverpool Telescope and TNT lightcurves and their amplitude spectra. A.H.C. calculated the pulsation properties of WD J0551+4135 from the CO/ONe interior models. M.J.H. and P.I. acquired the WHT spectroscopic data of WD J0551+4135. D.S. acquired and calibrated the Swift photometry of WD J0551+4135.

Corresponding author

Correspondence to M. A. Hollands.

Ethics declarations

Competing interests

The authors declare no competing interests.

Additional information

Peer review information Nature Astronomy thanks Francisco De Gerónimo and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Extended data

Extended Data Fig. 1 WHT spectroscopic observation log.

Observing log for WD J0551+4135 spectroscopy.

Extended Data Fig. 2 Astrompsheric/Stellar parameters for WD J0551+4135.

Results from our spectro-photometric fit. Error-ranges represent 1σ uncertainties.

Extended Data Fig. 3 WD J0551+4135 photometry.

Our model spectrum (grey) was fitted to Gaia, Pan-STARRS, and Swift photometry to determine the Teff and stellar radius. Fitting the Galex photometry instead of Swift, a cooler Teff of 12,400 K was found to be inconsistent with the optimal spectrum. The Galex magnitudes are therefore shown only to demonstrate the disagreement with the superior absolute calibration of Swift photometry.

Extended Data Fig. 4 WD J0551+4135 photometry.

Astrometry and photometry for WD J0551+4135. All astrometric data is from Gaia DR2, and thus at the J2015.5 epoch. Photometry is in units of magnitudes. Gaia magnitudes have been calculated in the AB system and include uncertainty in the Gaia zeropoints. Error-ranges represent 1σ uncertainties.

Extended Data Fig. 5 He/O Upper-limits.

Upper limits for He and O abundances. The solid red models correspond to the estimated 99th percentile upper limits, whereas the dotted curves indicate models with their respective elements at zero abundance.

Extended Data Fig. 6 Local white dwarf velocity distribution.

Our maximum likelihood fit (blue) to the v distribution of white dwarf with similar Gabs (grey) to WD J0551+4135. The LSR 3D velocity of WD J0551+4135 (red dashed) is beyond the 99th percentile of the corresponding 3D-distribution (orange).

Extended Data Fig. 7 TNT photometric observation log.

Observing log for TNT lightcurves of WD J0551+4135.

Supplementary information

Supplementary Data 1

Coadded WHT spectrum of WD J0551+4135. Columns are: air wavelength (Å), flux (mJy), and flux error (mJy).

Supplementary Data 2

Best-fitting model spectrum for WD J0551+4135. Columns are: vacuum wavelength (Å), and 4× Eddington flux (erg cm2 s–1 Å–1).

Supplementary Data 3

Lightcurve for WD J0551+4135. Columns are: BJD relative to 2019-01-24 00:00:00 utc (days), relative fluxes (mmi; milli-modulation intensity), and their uncertainties (mmi).

Rights and permissions

Reprints and Permissions

About this article

Verify currency and authenticity via CrossMark

Cite this article

Hollands, M.A., Tremblay, PE., Gänsicke, B.T. et al. An ultra-massive white dwarf with a mixed hydrogen–carbon atmosphere as a likely merger remnant. Nat Astron 4, 663–669 (2020).

Download citation

Further reading


Quick links

Nature Briefing

Sign up for the Nature Briefing newsletter — what matters in science, free to your inbox daily.

Get the most important science stories of the day, free in your inbox. Sign up for Nature Briefing