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.
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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 https://www.cosmos.esa.int/gaia and the Gaia archive website is https://archives.esac.esa.int/gaia. 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).
The authors declare no competing interests.
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.
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Observing log for WD J0551+4135 spectroscopy.
Results from our spectro-photometric fit. Error-ranges represent 1σ uncertainties.
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.
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.
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.
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).
Observing log for TNT lightcurves of WD J0551+4135.
Coadded WHT spectrum of WD J0551+4135. Columns are: air wavelength (Å), flux (mJy), and flux error (mJy).
Best-fitting model spectrum for WD J0551+4135. Columns are: vacuum wavelength (Å), and 4× Eddington flux (erg cm–2 s–1 Å–1).
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).
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Hollands, M.A., Tremblay, P., Gänsicke, B.T. et al. An ultra-massive white dwarf with a mixed hydrogen–carbon atmosphere as a likely merger remnant. Nat Astron (2020). https://doi.org/10.1038/s41550-020-1028-0