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.
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This research made extensive use of Astropy (http://www.astropy.org), a community-developed core Python package for Astronomy105,106. 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/.
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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.
The authors declare no competing interests.
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Pelisoli, I., Neunteufel, P., Geier, S. et al. A hot subdwarf–white dwarf super-Chandrasekhar candidate supernova Ia progenitor. Nat Astron 5, 1052–1061 (2021). https://doi.org/10.1038/s41550-021-01413-0