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# A highly magnetized and rapidly rotating white dwarf as small as the Moon

### Subjects

A Publisher Correction to this article was published on 02 August 2021

## Abstract

White dwarfs represent the last stage of evolution of stars with mass less than about eight times that of the Sun and, like other stars, are often found in binaries1,2. If the orbital period of the binary is short enough, energy losses from gravitational-wave radiation can shrink the orbit until the two white dwarfs come into contact and merge3. Depending on the component masses, the merger can lead to a supernova of type Ia or result in a massive white dwarf4. In the latter case, the white dwarf remnant is expected to be highly magnetized5,6 because of the strong magnetic dynamo that should arise during the merger, and be rapidly spinning from the conservation of the orbital angular momentum7. Here we report observations of a white dwarf, ZTF J190132.9+145808.7, that exhibits these properties, but to an extreme: a rotation period of 6.94 minutes, a magnetic field ranging between 600 megagauss and 900 megagauss over its surface, and a stellar radius of $${2140}_{-230}^{+160}$$ kilometres, only slightly larger than the radius of the Moon. Such a small radius implies that the star’s mass is close to the maximum white dwarf mass, or Chandrasekhar mass. ZTF J190132.9+145808.7 is likely to be cooling through the Urca processes (neutrino emission from electron capture on sodium) because of the high densities reached in its core.

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## Data availability

Upon request, I.C. will provide the reduced photometric lightcurves and spectroscopic data, and available ZTF data for the object. The spectroscopic data and photometric lightcurves are also available in the GitHub repository https://github.com/ilac/ZTF-J1901-1458. ZTF data are accessible in the ZTF database. The astrometric and photometric data are already in the public domain, and they are readily accessible in the Gaia and Pan-STARSS catalogues and in the Swift database.

## Code availability

We used the pyphot package (https://mfouesneau.github.io/docs/pyphot/) and the corner.py package70. The LRIS spectra were reduced using the Lpipe pipeline71. Upon request, I.C. will provide the code used to analyse the spectroscopic and photometric data.

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## Acknowledgements

The authors thank S.-C. Leung and S. Phinney for discussions, and N. Reindl and M. Kilic for comments. I.C. is a Sherman Fairchild Fellow at Caltech and thanks the Burke Institute at Caltech for supporting her research. J.F. acknowledges support through an Innovator Grant from the Rose Hills Foundation, and the Sloan Foundation through grant FG-2018-10515. K.B.B. thanks NASA and the Heising Simons Foundation for supporting his research. J.S. is supported by the A. F. Morrison Fellowship in Lick Observatory and by the US National Science Foundation (NSF) through grant ACI-1663688. This work is based on observations obtained with the Samuel Oschin 48-inch telescope and the Palomar Observatory 60-inch telescope as part of the ZTF project. ZTF is supported by the NSF under grant no. AST-1440341 and a collaboration including Caltech, IPAC, the Weizmann Institute for Science, the Oskar Klein Center at Stockholm University, the University of Maryland, the University of Washington, Deutsches Elektronen-Synchrotron and Humboldt University, Los Alamos National Laboratories, the TANGO Consortium of Taiwan, the University of Wisconsin at Milwaukee, and Lawrence Berkeley National Laboratories. Operations are conducted by Caltech Optical Observatories, IPAC and the University of Washington. 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 NASA. 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, The Johns Hopkins University, Durham University, the University of Edinburgh, the 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, NASA under grant no. NNX08AR22G issued through the Planetary Science Division of the NASA Science Mission Directorate, NSF grant no. AST-1238877, the University of Maryland, Eotvos Lorand University (ELTE), Los Alamos National Laboratory, and the Gordon and Betty Moore Foundation. This work was supported by the Natural Sciences and Engineering Research Council of Canada.

## Author information

Authors

### Contributions

I.C. reduced the ultraviolet data, conducted the spectral and photometric analysis, identified the magnetic field and is the primary author of the manuscript. K.B.B. performed the period search on ZTF data and reduced the optical data. I.C. and J.H. conducted the mass–radius analysis. I.C., K.B.B., J.F., J.H., S.R.K., T.A.P., H.B.R. and J.S. contributed to the physical interpretation of the object. J.S. constructed preliminary MESA models for the object. I.A., A.D., D.A.D., A.A.M., F.J.M., R.S. and M.T.S. contributed to the implementation of ZTF. G.H. is a co-PI of the ZTF Mid-Scale Innovations Program (MSIP). M.J.G. is the project scientist, E.C.B. is the survey scientist, T.A.P. is the co-PI and S.R.K. is the PI of ZTF.

### Corresponding author

Correspondence to Ilaria Caiazzo.

## Ethics declarations

### Competing interests

The authors declare no competing interests.

Peer review information Nature thanks the anonymous reviewers for their contribution to the peer review of this work.

Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

## Extended data figures and tables

### Extended Data Fig. 1 Photometric fit.

The blue 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. The Swift values are shown in green with 1σ error bars and the Pan-STARRS values in blue with the error that we chose to account for the photometric variability (0.02 mag).

### Extended Data Fig. 2 Gaia E(B − V) of the closest stars.

The plot shows the extinction AG as measured by Gaia of the stars within 5 degrees of ZTF 1901+1458 as a function of distance and converted to E(B − V) assuming a reddening law with RV = 3.1. We use the average reddening or the closest stars as a prior for the fitting. The error bars show 1σ errors.

### Extended Data Fig. 3 Corner plots.

Corner plots for the photometric fitting: results for the model atmospheres of Tremblay et al.43 (left) and Bohlin et al.45 (right).

### Extended Data Fig. 4 Phase-resolved spectra, blue side.

The LRIS phase-resolved spectra of ZTF J1901+1458 in the blue side. Some small variations can be observed in the spectral features with phase, especially in features at ~4,600 Å and at ~3,800 Å.

### Extended Data Fig. 5 Phase-resolved spectra, red side.

The LRIS phase-resolved spectra of ZTF J1901+1458 in the red side. Some small variations can be observed in the spectral features with phase: in particular, the feature at ~6,620 Å becomes broader and narrower with phase.

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Caiazzo, I., Burdge, K.B., Fuller, J. et al. A highly magnetized and rapidly rotating white dwarf as small as the Moon. Nature 595, 39–42 (2021). https://doi.org/10.1038/s41586-021-03615-y

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• DOI: https://doi.org/10.1038/s41586-021-03615-y

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