A thermonuclear supernova interacting with hydrogen- and helium-deficient circumstellar material

SN 2020aeuh as a SN Ia-CSM-C/O?

¹ The Oskar Klein Centre, Department of Astronomy, Stockholm University, AlbaNova, SE-10691 Stockholm, Sweden
² Graduate Institute for Advanced Studies, SOKENDAI, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
³ National Astronomical Observatory of Japan, National Institutes of Natural Sciences, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
⁴ The Oskar Klein Centre, Department of Physics, Stockholm University, AlbaNova, SE-10691 Stockholm, Sweden
⁵ Hamburger Sternwarte, University of Hamburg, Gojenbergsweg 112, D-21029 Hamburg, Germany
⁶ Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA), Northwestern University, 1800 Sherman Ave., Evanston, IL 60201, USA
⁷ School of Physics and Astronomy, Monash University, Clayton, VIC 3800, Australia
⁸ University of North Carolina at Chapel Hill, 120 E. Cameron Ave., Chapel Hill, NC 27514, USA
⁹ Department of Astronomy, University of California, Berkeley, CA 94720-3411, USA
¹⁰ IPAC, California Institute of Technology, 1200 E. California Blvd, Pasadena, CA 91125, USA
¹¹ Division of Physics, Mathematics and Astronomy, California Institute of Technology, Pasadena, CA 91125, USA
¹² MIT Kavli Institute for Astrophysics and Space Research, 70 Vassar St., Cambridge, MA 02139, USA
¹³ Department of Physics, Lancaster University, Lancaster LA1 4YB, UK
¹⁴ Caltech Optical Observatories, California Institute of Technology, Pasadena, CA 91125, USA
¹⁵ School of Physics, Trinity College Dublin, The University of Dublin, Dublin 2, Ireland
¹⁶ Department of Physics, Drexel University, Philadelphia, PA 19104, USA
¹⁷ Department of Physics, Tsinghua University, Qinghua Yuan, Beijing 100084, China

^⋆ Corresponding author: konstantinos.tsalapatas@astro.su.se

Received: 11 July 2025
Accepted: 24 September 2025

Abstract

Identifying the progenitors of thermonuclear supernovae (Type Ia supernovae; SNe Ia) remains a key objective in contemporary astronomy. The rare sub-class of SNe Ia-CSM that interacts with circumstellar material (CSM) allows for studies of the progenitor’s environment before explosion, and generally favours single-degenerate progenitor channels. The case of SN Ia-CSM PTF11kx clearly connected thermonuclear explosions with hydrogen-rich CSM-interacting events, and the more recent SN 2020eyj connected SNe Ia with helium-rich companion progenitors. Both of these objects displayed delayed CSM interaction which established their thermonuclear nature. Here we present a study of SN 2020aeuh, a Type Ia-CSM with delayed interaction. We analyse photometric and spectroscopic data that monitor the evolution of SN 2020aeuh and compare its properties with those of peculiar SNe Ia and core-collapse SNe. At early times, the evolution of SN 2020aeuh resembles a slightly overluminous SN Ia. Later, the interaction-dominated spectra develop the same pseudocontinuum seen in Type Ia-CSM PTF11kx and SN 2020eyj. However, the later-time spectra of SN 2020aeuh lack hydrogen and helium narrow lines. Instead, a few narrow lines could be attributed to carbon and oxygen. We fit the pseudobolometric light curve with a CSM-interaction model, yielding a CSM mass of 1 − 2 M_⊙. We propose that SN 2020aeuh was a Type Ia supernova that eventually interacted with a dense medium that was deficient in both hydrogen and helium. Whereas previous SNe Ia-CSM constitute our best evidence of non-degenerate companion progenitors, the CSM around SN 2020aeuh is more difficult to understand. We include a hydrodynamical simulation for a double-degenerate dynamical collision to showcase that such a progenitor scenario could produce significant amounts of hydrogen-poor CSM, although likely not as much as the inferred CSM mass around SN 2020aeuh. It is clear that SN 2020aeuh challenges current models of stellar evolution leading up to a SN Ia explosion.