Core crystallization and pile-up in the cooling sequence of evolving white dwarfs

Abstract

White dwarfs are stellar embers depleted of nuclear energy sources that cool over billions of years1. These stars, which are supported by electron degeneracy pressure, reach densities of 107 grams per cubic centimetre in their cores2. It has been predicted that a first-order phase transition occurs during white-dwarf cooling, leading to the crystallization of the non-degenerate carbon and oxygen ions in the core, which releases a considerable amount of latent heat and delays the cooling process by about one billion years3. However, no direct observational evidence of this effect has been reported so far. Here we report the presence of a pile-up in the cooling sequence of evolving white dwarfs within 100 parsecs of the Sun, determined using photometry and parallax data from the Gaia satellite4. Using modelling, we infer that this pile-up arises from the release of latent heat as the cores of the white dwarfs crystallize. In addition to the release of latent heat, we find strong evidence that cooling is further slowed by the liberation of gravitational energy from element sedimentation in the crystallizing cores5,6,7. Our results describe the energy released by crystallization in strongly coupled Coulomb plasmas8,9, and the measured cooling delays could help to improve the accuracy of methods used to determine the age of stellar populations from white dwarfs10.

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Fig. 1: Effects of crystallization on the cooling of white dwarfs.
Fig. 2: Observational Gaia colour–magnitude Hertzsprung–Russell diagram for white dwarfs within 100 pc of the Sun.
Fig. 3: Observational Gaia Hertzsprung–Russell diagram for white dwarfs with SDSS spectra.
Fig. 4: Luminosity function for massive white dwarfs within 100 pc of the Sun.

Data availability

The Gaia DR2 catalogue of white dwarfs used in this study is available from the University of Warwick astronomy catalogues repository, https://warwick.ac.uk/fac/sci/physics/research/astro/research/catalogues/gaia_dr2_white_dwarf_candidates_v2.csv. All modelling was performed with our extensive white-dwarf evolution code. We have opted not to make this multi-purpose code available, but the cooling sequences calculated for this work are available on request.

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Acknowledgements

This research received funding from the European Research Council under the European Union’s Horizon 2020 research and innovation programme number 677706 (WD3D) and under the European Union’s Seventh Framework Programme (FP/2007- 2013)/ERC Grant Agreement number 320964 (WDTracer). This work made use of data from the European Space Agency (ESA) mission Gaia (https://www.cosmos.esa.int/gaia), processed by the Gaia Data Processing and Analysis Consortium (DPAC, https://www.cosmos.esa.int/web/gaia/dpac/consortium). Funding for the DPAC was provided by national institutions, in particular the institutions participating in the Gaia Multilateral Agreement. Support for J.J.H. was provided by NASA through Hubble Fellowship grant #HST-HF2-51357.001-A, awarded by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555.

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P.-E.T. and B.H.D. identified and characterized the empirical crystallization sequence. G.F. made the evolutionary white-dwarf models used in this work. N.P.G.F., M.A.H. and T.C. constructed the Gaia white-dwarf sample employed in this study and performed the cross-match with other photometric and spectroscopic surveys. P.-E.T., B.T.G., T.R.M., J.J.H. and G.F. wrote the text and developed the argument for a crystallization sequence. E.C. and T.C. characterized the accuracy of Gaia measurements and derived parameters for white dwarfs.

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Correspondence to Pier-Emmanuel Tremblay.

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Tremblay, P., Fontaine, G., Fusillo, N.P.G. et al. Core crystallization and pile-up in the cooling sequence of evolving white dwarfs. Nature 565, 202–205 (2019). https://doi.org/10.1038/s41586-018-0791-x

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