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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 helium1,2. Several mechanisms compete with gravitational settling to change a white dwarf’s surface composition as it cools3, 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 kelvin4,5,6,7,8; 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 surface9,10,11. 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.

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Fig. 1: Janus HiPERCAM light curve.
Fig. 2: LRIS spectra.
Fig. 3: SED fitting of the two faces.
Fig. 4: Gaia colour–magnitude diagram.

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, 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 ( and the 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.


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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 (, processed by the Gaia Data Processing and Analysis Consortium (DPAC; 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.

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Authors and Affiliations



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.

Corresponding author

Correspondence to Ilaria Caiazzo.

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Extended data figures and tables

Extended Data Fig. 1 Janus ZTF and CHIMERA light curve.

The left panels show the binned CHIMERA light curve phase-folded at a period of 14.97 minutes in the g′ band (a) and in the r′ band (b). The flux has been normalised to the maximum of the light curve in each band. The amplitude of the photometric variation is about 15% peak-to-trough in both bands. The right panels show the similarly normalised ZTF discovery light curve in the ZTF g-band (c) and r − band (d). The error bars indicate 1σ errors.

Extended Data Fig. 2 Janus Swift UVOT light curve.

The plot shows the binned Swift UVOT light curve phase-folded at a period of 14.97 minutes in the UVW2 filter, the red solid line shows a sinusoidal fit with a peak-to-peak amplitude of 46 ± 8%. The error bars indicate 1σ errors.

Extended Data Fig. 3 Photometric fit.

The black solid line shows the best-fitting model spectrum, fitted to Pan-STARRS and Swift photometry to determine Teff, R* and E(B − V). The synthetic photometric values (obtained from the black line) are shown in red, while the Swift values are shown in green and the Pan-STARRS values in blue. Left: hydrogen atmosphere model; right: helium atmosphere model.

Extended Data Fig. 4 Pressure inside the white dwarf.

The pressure from gas, magnetic field, and convective motions as a function of depth within a white dwarf of 1.259 solar masses with a ONe core and a pure helium atmosphere. The x-axis indicates the exterior mass (the photosphere is on the right), and the right-hand y-axis indicates the optical depth. The shaded blue vertical strip indicates an approximate plausible range of H masses, while the shaded green horizontal strip is a plausible range of magnetic field pressure.

Extended Data Fig. 5 Scheme of the hydrogen diffusion scenario in the presence of a magnetic field.

Hydrogen diffuses upwards (due to its low mass) and toward the magnetic pole (due to the ion pressure gradient and its low charge). If there is sufficiently little hydrogen within the white dwarf, it will only cover the photospheric layers near the magnetic pole, possibly explaining the variable surface composition of Janus.

Extended Data Fig. 6 Comparison of phase-resolved spectra with models.

Upper panel: a comparison of the spectrum at phase 0 (the hydrogen phase, shown in black in the middle) with two synthetic models: on top in blue is a pure hydrogen model45 at 34, 900 K (the temperature inferred for the hydrogen phase from the SED) and at the bottom a pure hydrogen model at 50, 000 K. The lines in the observed spectrum are too weak compared to the model at 34, 900 K and resemble the weak lines of the model at 50, 000 K. Lower panel from top to bottom: a pure helium model46 in blue, the observed spectrum at phase 0.5 in black (the helium phase), a homogeneously mixed atmosphere model with a hydrogen-to-helium mass ratio of 1 in orange46, and a composite atmosphere in green in which 50% of the flux is from a pure DA atmosphere45 and 50% from a DB46. The models are calculated for a temperature of 36, 700 K (the temperature inferred for the helium phase from the SED). Neither a mixed nor a composite atmosphere can explain the weakness of both the hydrogen and helium lines.

Extended Data Fig. 7 Fitting of the lines with a featureless component.

a) An atmospheric model fit of the Balmer lines at phase 0 (in black), from bottom to top: Hα, Hβ, Hγ, Hδ, Hϵ. The blue dashed line shows the best-fitting model: Teff = 34, 900 K, log g = 9.1 and fraction of surface emitting as a black body: 36%. b) Fitting of the He I absorption lines at phase 0.5 (in black). The 0 on the x-axis corresponds to, from bottom to top: 5925 Å, 4880 Å, 4430 Å and 3975 Å. The green line shows the best-fitting model: Teff = 36, 700 K, log g = 9.1 and fraction of surface emitting as a black body: 40%.

Extended Data Table 1 Janus parameters
Extended Data Table 2 Additional photometric data for Janus

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Caiazzo, I., Burdge, K.B., Tremblay, PE. et al. A rotating white dwarf shows different compositions on its opposite faces. Nature 620, 61–66 (2023).

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