A radio-detected type Ia supernova with helium-rich circumstellar material

Abstract

<p>Type Ia supernovae (SNe Ia) are thermonuclear explosions of degenerate white dwarf stars destabilized by mass accretion from a companion star^{<a href="https://www.nature.com/articles/s41586-023-05916-w#ref-CR1" title="Whelan, J. & Iben, I.Jr Binaries and supernovae of Type I. Astrophys. J. 186, 1007–1014 (1973).">1</a>}, but the nature of their progenitors remains poorly understood. A way to discriminate between progenitor systems is through radio observations; a non-degenerate companion star is expected to lose material through winds^{<a href="https://www.nature.com/articles/s41586-023-05916-w#ref-CR2" title="Seaquist, E. R. & Taylor, A. R. The collective radio properties of symbiotic stars. Astrophys. J. 349, 313–327 (1990).">2</a>} or binary interaction^{<a href="https://www.nature.com/articles/s41586-023-05916-w#ref-CR3" title="Hachisu, I., Kato, M. & Nomoto, K. A new model for progenitor systems of Type Ia supernovae. Astrophys. J. Lett. 470, L97 (1996).">3</a>} before explosion, and the supernova ejecta crashing into this nearby circumstellar material should result in radio synchrotron emission. However, despite extensive efforts, no type Ia supernova (SN Ia) has ever been detected at radio wavelengths, which suggests a clean environment and a companion star that is itself a degenerate white dwarf star^{<a href="https://www.nature.com/articles/s41586-023-05916-w#ref-CR4" title="Horesh, A. et al. Early radio and X-ray observations of the youngest nearby Type Ia supernova PTF 11kly (SN 2011fe). Astrophys. J. 746, 21 (2012).">4</a>,<a href="https://www.nature.com/articles/s41586-023-05916-w#ref-CR5" title="Pérez-Torres, M. A. et al. Constraints on the progenitor system and the environs of SN 2014J from deep radio observations. Astrophys. J. 792, 38 (2014).">5</a>}. Here we report on the study of SN 2020eyj, a SN Ia showing helium-rich circumstellar material, as demonstrated by its spectral features, infrared emission and, for the first time in a SN Ia to our knowledge, a radio counterpart. On the basis of our modelling, we conclude that the circumstellar material probably originates from a single-degenerate binary system in which a white dwarf accretes material from a helium donor star, an often proposed formation channel for SNe Ia (refs. ^{<a href="https://www.nature.com/articles/s41586-023-05916-w#ref-CR6" title="Wang, B., Meng, X., Chen, X. & Han, Z. The helium star donor channel for the progenitors of Type Ia supernovae. Mon. Not. R. Astron. Soc. 395, 847–854 (2009).">6</a>,<a href="https://www.nature.com/articles/s41586-023-05916-w#ref-CR7" title="Ruiter, A. J., Belczynski, K. & Fryer, C. Rates and delay times of Type Ia supernovae. Astrophys. J. 699, 2026–2036 (2009).">7</a>}). We describe how comprehensive radio follow-up of SN 2020eyj-like SNe Ia can improve the constraints on their progenitor systems.</p>