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Published October 2021 | Supplemental Material
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A hot subdwarf–white dwarf super-Chandrasekhar candidate supernova Ia progenitor


Supernovae Ia are bright explosive events that can be used to estimate cosmological distances, allowing us to study the expansion of the Universe. They are understood to result from a thermonuclear detonation in a white dwarf that formed from the exhausted core of a star more massive than the Sun. However, the possible progenitor channels leading to an explosion are a long-standing debate, limiting the precision and accuracy of supernovae Ia as distance indicators. Here we present HD 265435, a binary system with an orbital period of less than a hundred minutes that consists of a white dwarf and a hot subdwarf, which is a stripped core-helium-burning star. The total mass of the system is 1.65 ± 0.25 solar masses, exceeding the Chandrasekhar limit (the maximum mass of a stable white dwarf). The system will merge owing to gravitational wave emission in 70 million years, likely triggering a supernova Ia event. We use this detection to place constraints on the contribution of hot subdwarf–white dwarf binaries to supernova Ia progenitors.

Additional Information

© 2021 Nature Publishing Group. Received 02 February 2021; Accepted 28 May 2021; Published 12 July 2021. I.P. and V.S. were partially funded by the Deutsche Forschungsgemeinschaft (DFG) under grant no. GE2506/12-1. I.P. also acknowledges funding by the United Kingdom's Science and Technology Facilities Council, grant no. ST/T000406/1. P.N. gratefully acknowledges funding provided by the Max Planck Society. A.I. acknowledges funding by the DFG through grant no. HE1356/71-1. D.S. was supported by the DFG under grant nos. HE1356/70-1 and IR190/1-1. B.B. acknowledges support from the National Aeronautics and Space Administration (NASA) under the TESS Guest Investigator program, grant no. 80NSSC19K1720. T.K. acknowledges support by the United States National Science Foundation through grant no. NSF PHY-1748958. We thank T. R. Marsh for enlightening discussions and for providing an MCMC wrapper to be used with LCURVE. We are grateful to A. S. Baran and D. Jones for providing helpful comments to an earlier version of this manuscript. This article includes data collected by the TESS mission; funding for this mission is provided by the NASA Explorer Program. This work has also made use of data from the European Space Agency mission Gaia (https://www.cosmos.esa.int/gaia), processed by the Gaia Data Processing and Analysis Consortium (https://www.cosmos.esa.int/web/gaia/dpac/consortium); funding for this consortium has been provided by national institutions, in particular the institutions participating in the Gaia Multilateral Agreement. Finally, some of the data presented here 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. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation. We wish to recognize and acknowledge the very important cultural role that the summit of Mauna Kea has always had within the indigenous Hawaiian community and the reverence that the community has for it. We are most fortunate to have the opportunity to conduct observations from this mountain. Data availability: The TESS data used in this work are publicly available and can be accessed via the Barbara A. Mikulski Archive for Space Telescopes (https://mast.stsci.edu/). Obtained follow-up spectra, evolutionary models and MESA inlists are available on Zenodo (https://doi.org/10.5281/zenodo.4792304). Code availability: This research made extensive use of Astropy (http://www.astropy.org), a community-developed core Python package for Astronomy. The PyRAF-based pipeline for DBSP spectra reduction is available at https://github.com/ebellm/pyraf-dbsp, and the MAKEE pipeline for ESI spectra can be found at http://www.astro.caltech.edu/~tb/ipac_staff/tab/makee/. The radial velocity determination code RVSAO is available from http://tdc-www.harvard.edu/iraf/rvsao/. The package galpy can be installed following https://docs.galpy.org/en/v1.6.0/. The SED and spectral fitting routines are publicly documented as described above, but not publicly available. The Period04 software employed for pre-whitening the light curve can be obtained from https://www.univie.ac.at/tops/Period04/. LCURVE is available at https://github.com/trmrsh/cpp-lcurve. The stellar evolution code MESA can be downloaded from http://mesa.sourceforge.net/. Author Contributions: I.P. carried out the radial velocity estimates and fitting and the light curve fitting, and led the writing of the manuscript. P.N. calculated the evolution of the system. S.G. and U.H. performed the spectral fitting. T.K. did the spectroscopic reduction and cross-checked the light curve fitting. D.S. and U.H. performed the SED fitting. A.I. wrote the SED fitting tool and calculated the spectral models used for SED and spectral fitting. A.B. calculated the Galactic orbit of the system. J.v.R. performed the spectroscopic observations and contributed to the light curve fit. V.S. and B.N.B. contributed to the analysis of the light curve. All authors reviewed the manuscript. The authors declare no competing interests. Peer review information: Nature Astronomy thanks Zhanwen Han and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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August 22, 2023
October 23, 2023