A dense 0.1-solar-mass star in a 51-minute-orbital-period eclipsing binary

Creators: Burdge, Kevin B.; El-Badry, Kareem; Marsh, Thomas R.; Rappaport, Saul; Brown, Warren R.; Caiazzo, Ilaria; Chakrabarty, Deepto; Dhillon, V. S.; Fuller, Jim; Gänsicke, Boris T.; Graham, Matthew J.; Kara, Erin; Kulkarni, S. R.; Littlefair, S. P.; Mróz, Przemek; Rodríguez-Gil, Pablo; van Roestel, Jan; Simcoe, Robert A.; Bellm, Eric C.; Drake, Andrew J.; Dekany, Richard G.; Groom, Steven L.; Laher, Russ R.; Masci, Frank J.; Riddle, Reed; Smith, Roger M.; Prince, Thomas A.

Abstract

Of more than a thousand known cataclysmic variables (CVs), where a white dwarf is accreting from a hydrogen-rich star, only a dozen have orbital periods below 75 minutes. One way to achieve these short periods requires the donor star to have undergone substantial nuclear evolution before interacting with the white dwarf, and it is expected that these objects will transition to helium accretion. These transitional CVs have been proposed as progenitors of helium CVs. However, no known transitional CV is expected to reach an orbital period short enough to account for most of the helium CV population, leaving the role of this evolutionary pathway unclear. Here we report observations of ZTF J1813+4251, a 51-minute-orbital-period, fully eclipsing binary system consisting of a star with a temperature comparable to that of the Sun but a density 100 times greater owing to its helium-rich composition, accreting onto a white dwarf. Phase-resolved spectra, multi-band light curves and the broadband spectral energy distribution allow us to obtain precise and robust constraints on the masses, radii and temperatures of both components. Evolutionary modelling shows that ZTF J1813+4251 is destined to become a helium CV binary, reaching an orbital period under 20 minutes, rendering ZTF J1813+4251 a previously missing link between helium CV binaries and hydrogen-rich CVs.

Additional Information

© 2022 Nature Publishing Group. K.B.B. is a Pappalardo Postdoctoral Fellow in Physics at MIT and thanks the Pappalardo fellowship programme for supporting his research. 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). V.S.D. and HiPERCAM operations are supported by STFC grant ST/V000853/1. T.R.M. and B.T.G. acknowledge support from the UK's Science and Technology Facilities Council (STFC), grant ST/T000406/1. This project has received funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation programme (grant agreement number 101020057). For the purpose of open access, the authors have applied a creative commons attribution (CC BY) licence to any author accepted manuscript version arising. Based on observations made with the Gran Telescopio Canarias, installed at the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias, on the island of La Palma. Based on observations obtained with the Samuel Oschin Telescope 48-inch Telescope at the Palomar Observatory as part of the Zwicky Transient Facility project. ZTF is supported by the National Science Foundation under grant number 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 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. The observatory was made possible by the generous financial support of the W.M. Keck Foundation. We recognize and acknowledge the very significant cultural role and reverence that the summit of Mauna Kea has always had within the Indigenous Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain. Contributions. K.B.B. discovered the object, conducted the LCURVE light-curve analysis, spectroscopic data reduction and analysis of Keck LRIS and ESI data, the reduction and analysis of the Swift UVOT and X-ray telescope data, and was the primary author of the manuscript. K.E.-B., T.R.M. and S.R. contributed to the interpretation of the object and the implications of its evolutionary history, and helped in responding to the referees reports. K.E.-B. performed the MESA modelling used to construct Fig. 4, and supplied the section and figure on this in the extended data. T.R.M. conducted the HiPERCAM data reduction. S.R. constructed independent models of the system as a sanity check of those used in this paper. W.R.B. performed the cross-correlation radial velocity measurement as a sanity check of that obtained by fitting Voigt profiles to the absorption lines. K.B.B., K.E.-B., T.R.M., S.R., W.R.B., I.C., D.C., V.S.D., J.F., B.T.G., M.J.G., E.K., S.R.K., S.P.L., P.M., P.R.-G., J.V.R. and R.A.S. contributed comments and edits to the paper. V.S.D. is the principal investigator of HiPERCAM. P.R.-G. was the principal investigator of the HiPERCAM proposal that observed the object. A.J.D., R.G.D., S.L.G., R.R.L., F.J.M., R.R. and R.M.S. are ZTF builders. S.R.K., T.A.P., M.J.G. and E.C.B. are the principal investigator, the co-investigator, the project scientist and the survey scientist of ZTF, respectively. Data availability. Reduced HiPERCAM photometric data, LRIS spectroscopic data and MESA tracks resulting from the models are available at https://github.com/kburdge/ZTFJ1813-4251.git. The ZTF data used are all in the public domain. The proprietary period for the spectroscopic data will expire at the start of 2022, at which point the raw spectroscopic images will also be accessible via the Keck observatory archive. Code availability. Upon request, the corresponding author will provide the code (primarily in Python) used to analyse the observations, create the MESA models and any data used to generate the figures (MATLAB was used to generate most of the figures). The LCURVE modelling code can be found at https://github.com/trmrsh/cpp-lcurve. The authors declare no competing interests. Nature thanks Taichi Kato, Youngdae Lee, Lorne Nelson and the other, anonymous, reviewer(s) for their contribution to the peer review of this work. Peer reviewer reports are available.

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Resource type: Journal Article
Publisher: Nature Publishing Group
Published in: Nature, 610(7932), 467-471, ISSN: 0028-0836.