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The origin and evolution of magnetic white dwarfs in close binary stars

Abstract

The origin of magnetic fields in white dwarfs remains a fundamental unresolved problem in stellar astrophysics. In particular, the very different fractions of strongly (more than about a megagauss) magnetic white dwarfs in evolutionarily linked populations of close white dwarf binary stars cannot be reproduced by any scenario suggested so far. Strongly magnetic white dwarfs are absent among detached white dwarf binary stars that are younger than approximately a billion years. In contrast, of cataclysmic variables (semi-detached binary star systems that contain a white dwarf) in which the white dwarf accretes from a low-mass star companion, more than a third host a strongly magnetic white dwarf1. Here we present binary star evolutionary models that include the spin evolution of accreting white dwarfs and crystallization of their cores, as well as magnetic field interactions between the stars. We show that a crystallization- and rotation-driven dynamo similar to those working in planets and low-mass stars2 can generate strong magnetic fields in the white dwarfs in cataclysmic variables, which explains their large fraction among the observed population. When the magnetic field generated in the white dwarf connects with that of the secondary star in the binary system, synchronization torques and reduced angular momentum loss cause the binary to detach for a relatively short period of time. The few known strongly magnetic white dwarfs in detached binaries, such as AR Scorpii3, are in this detached phase.

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Fig. 1: Spin period, Roche-lobe-filling factor and mass transfer rate as a function of orbital period.
Fig. 2: Observed white dwarf effective temperatures versus their masses.

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Data availability

The data presented in this work is available upon request. In addition, the MESA files required to reproduce our simulations will be publicly available at the MESA Zenodo community (https://zenodo.org/communities/mesa/).

Code availability

MESA is publicly available (http://mesa.sourceforge.net/).

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Acknowledgements

We thank C. Tout, J. Isern and L. Ferrario for helpful comments. M.R.S. acknowledges support from the ANID Millennium Science Initiative programme NCN19\_171 and Fondecyt (grant 1181404). D.B. was supported by grant 2017/14289-3, the São Paulo Research Foundation (FAPESP) and ESO/Gobierno de Chile. B.T.G. was supported by a Leverhulme Research Fellowship and the UK STFC grant ST/T000406/1. S.G.P. acknowledges the support of a STFC Ernest Rutherford Fellowship. M.Z. acknowledges support from CONICYT PAI (Concurso Nacional de Inserción en la Academia 2017, Folio 79170121) and CONICYT/FONDECYT (Programa de Iniciación, Folio 11170559).

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All authors contributed to the discussion and writing of this article. M.R.S. and D.B. developed the idea and carried out the simulations. S.G.P. and B.T.G. provided an observational overview of magnetic white dwarfs and recent observations of pre-polars. M.Z. provided crystallization temperatures for different white dwarf masses based on published white dwarf evolutionary sequences and estimated relative numbers of magnetic and non-magnetic CVs using binary population models.

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Correspondence to Matthias R. Schreiber.

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Peer review information Nature Astronomy thanks Lilia Ferrario, Jordi Isern and Christopher Tout for their contribution to the peer review of this work.

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Schreiber, M.R., Belloni, D., Gänsicke, B.T. et al. The origin and evolution of magnetic white dwarfs in close binary stars. Nat Astron 5, 648–654 (2021). https://doi.org/10.1038/s41550-021-01346-8

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