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Published April 2023 | Published
Journal Article Open

Linking the interiors and surfaces of magnetic stars


Strong magnetic fields are observed in a substantial fraction of upper main sequence stars and white dwarfs. Many such stars are observed to exhibit photometric modulations as the magnetic poles rotate in and out of view, which could be a consequence of magnetic perturbations to the star's thermal structure. The magnetic pressure is typically larger than the gas pressure at the star's photosphere, but much smaller than the gas pressure in the star's interior, so the expected surface flux perturbations are not clear. We compute magnetically perturbed stellar structures of young 3 M_⊙ stars that are in both hydrostatic and thermal equilibrium, and which contain both poloidal and toroidal components of a dipolar magnetic field as expected for stable fossil fields. This provides semi-analytical models of such fields in baroclinic stably stratified regions. The star's internal pressure, temperature, and flux perturbations can have a range of magnitudes, though we argue the most likely configurations exhibit flux perturbations much smaller than the ratio of surface magnetic pressure to surface gas pressure, but much larger than the ratio of surface magnetic pressure to central gas pressure. The magnetic pole is hotter than the equator in our models, but a cooler magnetic pole is possible depending on the magnetic field configuration. The expected flux variations for observed field strengths are δL/L ≲ 10⁻⁶, much smaller than those observed in magnetic stars, suggesting that observed perturbations stem from changes to the emergent spectrum rather than changes to the bolometric flux.

Additional Information

© 2023 The Author(s) Published by Oxford University Press on behalf of Royal Astronomical Society. This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/open_access/funder_policies/chorus/standard_publication_model). JF is thankful for support through an Innovator Grant from The Rose Hills Foundation. SM acknowledges support from CNES SOHO, PLATO, and LISA grants at CEA/IRFU/DAp. The authors are grateful to the referee, Pr. J. Landstreet, for constructive comments that have allowed us to improve the article. DATA AVAILABILITY. Source code for computing magnetically perturbed structures is available upon request. The relaxation code to compute magnetic perturbations is available upon request.

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August 22, 2023
August 22, 2023