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Alkali metals in white dwarf atmospheres as tracers of ancient planetary crusts

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Abstract

White dwarfs that accrete the debris of tidally disrupted asteroids1 provide the opportunity to measure the bulk composition of the building blocks, or fragments, of exoplanets2. This technique has established a diversity of compositions comparable to what is observed in the Solar System3, suggesting that the formation of rocky planets is a generic process4. The relative abundances of lithophile and siderophile elements within the planetary debris can be used to investigate whether exoplanets undergo differentiation5, yet the composition studies carried out so far lack unambiguous tracers of planetary crusts6. Here we report the detection of lithium in the atmospheres of four cool (<5,000 K) and old (cooling ages of 5–10 Gyr ago) metal-polluted white dwarfs, of which one also displays photospheric potassium. The relative abundances of these two elements with respect to sodium and calcium strongly suggest that all four white dwarfs have accreted fragments of planetary crusts. We detect an infrared excess in one of the systems, indicating that accretion from a circumstellar debris disk is ongoing. The main-sequence progenitor mass of this star was 4.8 ± 0.2 M, demonstrating that rocky, differentiated planets may form around short-lived B-type stars.

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Fig. 1: Optical spectra of the four white dwarfs with photospheric lithium.
Fig. 2: Number density abundance ratios of debris-accreting white dwarfs and Solar System benchmarks.
Fig. 3: An infrared excess at WD J2317+1830.

Data availability

The data that support the plots within this paper and other findings of this study are available from the ESO science archive facility, the GTC public archive, ING archive and SDSS database; or from the corresponding author upon reasonable request.

Code availability

The Koester model atmosphere and envelope codes are not publicly available, although details of their internal operation and input physics can be consulted from ref. 15.

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Acknowledgements

M.A.H. and P.-E.T. acknowledge useful discussions with H.-G. Ludwig and M. Steffen regarding neutral broadening of lithium lines. We also acknowledge J. McCleery for maintaining a database of 40 pc white dwarf spectra. M.A.H. and D.K. acknowledge atomic data from T. Leininger used for the Ca i unified profile. This research received funding from the European Research Council under the European Union’s Horizon 2020 research and innovation programme number 677706 (WD3D). B.T.G. was supported by the UK STFC under grant number ST/T000406/1 and a Leverhulme Research Fellowship. Funding for the Sloan Digital Sky Survey IV (www.sdss.org) was provided by the Alfred P. Sloan Foundation, the US Department of Energy Office of Science and the participating institutions. Based on observations collected at the European Organisation for Astronomical Research in the Southern Hemisphere under ESO programme 0102.C-0351. This article is based on observations (programme ITP08) made in the Observatorios de Canarias del IAC with the WHT operated on the island of La Palma by the Isaac Newton Group of Telescopes in the Observatorio del Roque de los Muchachos and with the Gran Telescopio Canarias (GTC), installed at the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias, on the island of La Palma.

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M.A.H. performed data reduction, analysis and interpretation and wrote the majority of the text. P.-E.T. and B.T.G. contributed to the data interpretation and writing of the article. D.K. developed the model atmosphere code used for the analysis. N.P.G.-F. contributed to the data reduction and analysis of photometric data.

Corresponding author

Correspondence to Mark A. Hollands.

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The authors declare no competing interests.

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Extended data

Extended Data Fig. 1 Astrometry and photometry for the four lithium-rich white dwarfs.

Pan-STARRS, SDSS and SkyMapper photometry are given in the AB-system, with the remainder in the Vega system. Positions are given in the J2015.5 epoch.

Extended Data Fig. 2 Atmospheric parameters for the four white dwarfs with photospheric lithium.

The abundances are in base 10 in terms of number ratio.

Extended Data Fig. 3 Best fitting models compared with the spectra and photometry of the four lithium-bearing white dwarfs.

In the right panel for WD J2317+1830, the disk model and white dwarf plus disk model are indicated by dotted and dashed curves, respectively. The spectrum of SDSS J1330+6435 has been smoothed with a Gaussian with a full width half maximum of 5 Å. Error bars correspond to 1σ uncertainties.

Extended Data Fig. 4 White dwarf envelope parameters for our sample.

The first row indicates the fractional convection zone mass. In subsequent rows, pairs correspond to the sinking timescale at the base of the convection zone in years, and (where abundances were determined) the elemental mass in the convection zone in g, i.e. (τZ/yr, mZ/g). Diffusion timescales are given for all elements commonly considered in white dwarf planetary abundance studies. The final row, ‘crust’, provides estimates for the total material within the white dwarf convection zones, assuming a continental crust composition, scaled from the Na masses.

Extended Data Fig. 5 SDSS spectra of three additional cool DZs with strong metal absorption features.

Lithium lines are not detected for any of these stars. Spectra have been smoothed by a Gaussian with a full width half maximum of 3 Å for clarity.

Supplementary information

Supplementary Information

Supplementary Figs. 1 and 2 and Tables 1 and 2.

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Hollands, M.A., Tremblay, PE., Gänsicke, B.T. et al. Alkali metals in white dwarf atmospheres as tracers of ancient planetary crusts. Nat Astron (2021). https://doi.org/10.1038/s41550-020-01296-7

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