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

Abstract

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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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.

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Acknowledgements

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).

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.

Correspondence to M. A. Hollands.

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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).

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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

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