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Suppression of cooling by strong magnetic fields in white dwarf stars

Nature volume 515, pages 88–91 (2014)Cite this article

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Abstract

Isolated cool white dwarf stars more often have strong magnetic fields than young, hotter white dwarfs1,2,3,4, which has been a puzzle because magnetic fields are expected to decay with time5,6 but a cool surface suggests that the star is old. In addition, some white dwarfs with strong fields vary in brightness as they rotate7,8,9,10, which has been variously attributed to surface brightness inhomogeneities similar to sunspots8,9,10,11,12, chemical inhomogeneities13,14 and other magneto-optical effects15,16,17. Here we describe optical observations of the brightness and magnetic field of the cool white dwarf WD 1953-011 taken over about eight years, and the results of an analysis of its surface temperature and magnetic field distribution. We find that the magnetic field suppresses atmospheric convection, leading to dark spots in the most magnetized areas. We also find that strong fields are sufficient to suppress convection over the entire surface in cool magnetic white dwarfs, which inhibits their cooling evolution relative to weakly magnetic and non-magnetic white dwarfs, making them appear younger than they truly are. This explains the long-standing mystery of why magnetic fields are more common amongst cool white dwarfs, and implies that the currently accepted ages of strongly magnetic white dwarfs are systematically too young.

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Figure 1: Modelling results for WD 1953-011.
Figure 2: Dependence of amplitude of the photometric variability of MWDs on their surface magnetic field strength Bs.
Figure 3: Some characteristic features in statistical distributions of MWDs.
Figure 4: Absolute values of spatial velocities |V| of MWDs versus their surface temperatures.

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Acknowledgements

G.V. thanks J. Landstreet and S. Fabrika for discussions and practical help in interpreting the results. G.V. also thanks E. Kaisina for help in the preparation of the manuscript. G.V. and G.A.G. acknowledge the support of Chilean fund FONDECYT-regular (project 1120190). G.V. and D.H. acknowledge financial support from CONACyT, Mexico (grant 180817). G.V., T.B. and A.B. acknowledge financial support from the ministry of science and education of the Russian Federation (contracts 14.518.11.7070 and 16.518.11.7073). D.S. acknowledges financial support from CRC963 Astrophysical Flow Instabilities and Turbulence (project A16-A17). G.A.W. is supported by a Natural Sciences and Engineering Research Council (NSERC Canada) Discovery Grant. S.V.Z. acknowledges support from DGAPA/PAPIIT IN100614 and CONACYT 151858 projects. L.F.M. acknowledges financial support from the Universidad Nacional Autónoma de México under grant PAPIIT IN104612.

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Authors and Affiliations

  1. Special Astrophysical Observatory of Russian Academy of Science, Nizhnij Arkhyz, Zelenchukskiy region, Karachai-Cherkessian Republic 369167, Russia,

    G. Valyavin, T. Burlakova & A. Burenkov

  2. Institute for Astrophysics, Georg-August-University, Friedrich-Hund-Platz 1, D-37077, Göttingen, Germany,

    D. Shulyak

  3. Department of Physics, Royal Military College of Canada, PO Box 17000 Station Forces, Kingston, Ontario, K7K 7B4, Canada,

    G. A. Wade

  4. Crimean Astrophysical Observatory, Nauchny, Crimea 98409, Ukraine,

    K. Antonyuk & S. Plachinda

  5. Instituto de Astronomía, Observatorio Astronómico Nacional San Pedro Martir (SPM), Universidad Nacional Autónoma de México, 22860 Ensenada, Baja California, Mexico,

    S. V. Zharikov, L. Fox Machado, M. Alvarez, D. M. Clark, J. M. Lopez & D. Hiriart

  6. Instituto de Astronomía, Universidad Catolica del Norte, Avenida Angamos 0610, Antofagasta, Chile,

    G. A. Galazutdinov

  7. Pulkovo Observatory, Pulkovskoe Shosse 65/1, Saint-Petersburg 196140, Russia,

    G. A. Galazutdinov

  8. Armagh Observatory, College Hill, Armagh BT61 9DG, UK,

    S. Bagnulo

  9. Korea Astronomy and Space Science Institute 776, Daedeokdae-ro, Yuseong-gu, Daejeon 305-348, South Korea,

    Inwoo Han & Young-Beom Jeon

  10. Instituto de Astrofísica de Canarias, 38200, La Laguna, Tenerife, Spain,

    C. Zurita

  11. Departamento de Astrofísica, Universidad de La Laguna, 38206 La Laguna, Tenerife, Spain,

    C. Zurita

  12. Instituto Nacional de Astrofísica, Óptica y Electróníca, Apartado Postal 51 y 216, 72000 Tonantzintla, Pueblo, Mexico,

    R. Mujica

  13. European Southern Observatory, Alonso de Córdova 3107, Santiago, Chile,

    T. Szeifert

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  1. G. Valyavin
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Contributions

G.V, D.S. and G.A.W. analysed the main ideas, drew the basic conclusions presented in this study and wrote the text. G.V., D.S. and S.B. modelled the magnetic and atmospheric properties of WD 1953-011. All authors participated in the organization, conduct and reduction of spectral, spectropolarimetric and photometric observations of WD 1953-011 at different European, Russian, Ukrainian and Mexican observatories between 2002 and 2014.

Corresponding author

Correspondence to G. Valyavin.

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The authors declare no competing financial interests.

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Valyavin, G., Shulyak, D., Wade, G. et al. Suppression of cooling by strong magnetic fields in white dwarf stars. Nature 515, 88–91 (2014). https://doi.org/10.1038/nature13836

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