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Published August 3, 2023 | Supplemental Material
Journal Article Open

A rotating white dwarf shows different compositions on its opposite faces


White dwarfs, the extremely dense remnants left behind by most stars after their death, are characterized by a mass comparable to that of the Sun compressed into the size of an Earth-like planet. In the resulting strong gravity, heavy elements sink towards the centre and the upper layer of the atmosphere contains only the lightest element present, usually hydrogen or helium. Several mechanisms compete with gravitational settling to change a white dwarf's surface composition as it cools, and the fraction of white dwarfs with helium atmospheres is known to increase by a factor of about 2.5 below a temperature of about 30,000 kelvin; therefore, some white dwarfs that appear to have hydrogen-dominated atmospheres above 30,000 kelvin are bound to transition to be helium-dominated as they cool below it. Here we report observations of ZTF J203349.8+322901.1, a transitioning white dwarf with two faces: one side of its atmosphere is dominated by hydrogen and the other one by helium. This peculiar nature is probably caused by the presence of a small magnetic field, which creates an inhomogeneity in temperature, pressure or mixing strength over the surface. ZTF J203349.8+322901.1 might be the most extreme member of a class of magnetic, transitioning white dwarfs—together with GD 323 (ref. 12), a white dwarf that shows similar but much more subtle variations. This class of white dwarfs could help shed light on the physical mechanisms behind the spectral evolution of white dwarfs.

Additional Information

© 2023 Springer Nature Limited. We would like to dedicate this work to the memory of our good friend and colleague T. R. Marsh. We thank D. Veras and T. Cunningham for discussions. I.C. thanks the Burke Institute at Caltech for supporting her research. P.-E.T. received funding from the European Research Council under the European Union's Horizon 2020 research and innovation programme number 101002408 (MOS100PC), the Leverhulme Trust Grant (ID RPG-2020-366) and the UK STFC consolidated grant ST/T000406/1. T.R.M. and I.P. were funded by STFC grant ST/T000406/1. S.G.P. acknowledges the support of a STFC Ernest Rutherford Fellowship. This research was supported in part by the National Science Foundation under grant number NSF PHY-1748958. This work is based on observations obtained with the 48-inch Samuel Oschin Telescope and the 60-inch Telescope at the Palomar Observatory as part of the Zwicky Transient Facility project (ZTF). ZTF is supported by the National Science Foundation under grant number AST-2034437 and a collaboration including Caltech, IPAC, the Weizman Institute of Science, the Oskar Klein Center at Stockholm University, the University of Maryland, Deutsches Elektronen-Synchrotron and Humboldt University, the TANGO Consortium of Taiwan, the University of Wisconsin at Milwaukee, Trinity College Dublin, Lawrence Livermore National Laboratories, IN2P3, France, the University of Warwick, the University of Bochum and Northwestern University. Operations are conducted by COO, IPAC and UW. Some of the data presented herein were obtained at the W.M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California and the National Aeronautics and Space Administration. This work has 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 has been provided by national institutions, in particular the institutions participating in the Gaia Multilateral Agreement. The Pan-STARRS1 Surveys (PS1) and the PS1 public science archive have been made possible through contributions by the Institute for Astronomy, the University of Hawaii, the Pan-STARRS Project Office, the Max Planck Society and its participating institutes, the Max Planck Institute for Astronomy, Heidelberg and the Max Planck Institute for Extraterrestrial Physics, Garching, Johns Hopkins University, Durham University, the University of Edinburgh, Queen's University Belfast, the Harvard-Smithsonian Center for Astrophysics, the Las Cumbres Observatory Global Telescope Network Incorporated, the National Central University of Taiwan, the Space Telescope Science Institute, the National Aeronautics and Space Administration under grant number NNX08AR22G issued through the Planetary Science Division of the NASA Science Mission Directorate, the National Science Foundation grant number AST-1238877, the University of Maryland, Eotvos Lorand University (ELTE), the Los Alamos National Laboratory and the Gordon and Betty Moore Foundation. The design and construction of HiPERCAM was funded by the European Research Council under the European Union's Seventh Framework Programme (FP/2007-2013) under ERC-2013-ADG grant agreement number 340040 (HiPERCAM). VSD and HiPERCAM operations are funded by the Science and Technology Facilities Council (grant ST/V000853/1). This research has made use of NASA's Astrophysics Data System and of astropy. Data availability: Upon request, the corresponding author will provide the reduced photometric light curves and spectroscopic data, and available ZTF data for the object. The spectroscopic data and the optical photometric light curves are also available in the GitHub repository https://github.com/ilac/Janus, and the ZTF data are accessible in the ZTF database. The astrometric data from Gaia and photometric data from Gaia, Pan-STARRS and Swift are already in the public domain, and they are readily accessible in the Gaia and Pan-STARRS catalogues and in the Swift database. Code availability: We used astropy67, the pyphot package (https://mfouesneau.github.io/pyphot/) and the corner.py package68. The LRIS spectra were reduced using the LPIPE pipeline42. Upon request, the corresponding author will provide the code used to analyse the spectroscopic and photometric data. Contributions: I.C. reduced the UV and optical data, conducted the spectral and photometric analysis, and is the primary author of the paper. K.B.B. performed the period search on ZTF data. I.C., K.B.B., P.M., A.C.R., J.v.R. and Z.P.V. performed the observations with LRIS and CHIMERA. I.C., K.B.B., P.-E.T., L.F., J.F., B.T.G., J.J.H., J.H., A.K, S.R.K., T.R.M., T.A.P., H.B.R., A.C.R., J.v.R., Z.P.V, S.V. and D.W. contributed to the physical interpretation of the object. P.-E.T. contributed the synthetic spectral models and conducted the analysis to estimate the minimum field strength needed to suppress convection in the white dwarf's atmosphere. J.F. constructed MESA models for the object. P.M. developed a reduction pipeline for the ZTF data and contributed to the analysis. D.P. developed a reduction pipeline for the LRIS data. T.R.M., V.S.D., S.P.L., J.M., E.B., A.J.B., M.J.D., M.J. Green, P.K., S.G.P., I.P. and D.I.S. were responsible for the operation of HiPERCAM. R.D., A.D., R.R.L., R.L.R. and B.R. contributed to the implementation of ZTF. M.J. Graham is the project scientist, E.C.B. is the survey scientist, T.A.P. is the co-principal investigator and S.R.K. is the principal investigator of ZTF. The authors declare no competing interests.

Attached Files

Supplemental Material - 41586_2023_6171_Fig10_ESM.webp

Supplemental Material - 41586_2023_6171_Fig11_ESM.webp

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Supplemental Material - 41586_2023_6171_Fig9_ESM.webp

Supplemental Material - 41586_2023_6171_Tab1_ESM.jpg

Supplemental Material - 41586_2023_6171_Tab2_ESM.jpg


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August 22, 2023
November 21, 2023