Trans-Fe elements from type Ia supernovae

I. Heavy element nucleosynthesis during the formation of near-Chandrasekhar white dwarfs

¹ Department of Physics, University of Naples Federico II, Via Cintia, Napoli, 80126 NA, Italy
² INAF – Osservatorio Astronomico d’Abruzzo, Via M. Maggini, 64100 Teramo, Italy
³ The NuGrid Collaboration, http://www.nugridstars.org
⁴ E. A. Milne Centre for Astrophysics, University of Hull, Cottingham Road, Kingston upon Hull HU6 7RX, UK
⁵ School of Mathematical and Physical Sciences, Hicks Building, Hounsfield Road, Sheffield S3 7RH, UK
⁶ Heidelberg Institute for Theoretical Studies, Schloss-Wolfsbrunnenweg 35, 69118 Heidelberg, Germany
⁷ Zentrum für Astronomie der Universität Heidelberg, Institut für Theoretische Astrophysik, Philosophenweg 12, 69120 Heidelberg, Germany
⁸ Zentrum für Astronomie der Universität Heidelberg, Astronomisches Recheninstitut, Mönchhofstr. 12–14, 69120 Heidelberg, Germany
⁹ Department of Physics & Astronomy, University of Victoria, Victoria, BC V8W 2Y2, Canada
¹⁰ Joint Institute for Nuclear Astrophysics – Center for the Evolution of the Elements (JINA-CEE), Michigan State University, 640 South Shaw Lane, East Lansing, MI 48824, USA
¹¹ INFN – Sezione di Napoli, Via Cintia, 80126 Naples, Italy
¹² School of Chemical and Physical Sciences, Keele University, Lennard-Jones Laboratories, Keele ST5 5BG, UK
¹³ INFN – Sezione di Roma, Piazzale Aldo Moro 2, 00185 Rome, Italy
¹⁴ School of Mathematics and Physics, Queen’s University Belfast, Belfast BT7 1NN, UK
¹⁵ INAF – Osservatorio Astrofisico di Torino, Via Osservatorio, 20, 10025 Pino Torinese, TO, Italy

^⋆ Corresponding author: umberto.battino@unina.it, umberto.battino@inaf.it

Received: 30 May 2025
Accepted: 24 July 2025

Abstract

Context. A type Ia supernova (SNIa) marks the catastrophic explosion of a white dwarf in a binary system. These events play a crucial role in galactic chemical evolution and serve as pivotal standardisable candles for measuring cosmic distances, underpinning the discovery of the Universe’s accelerated expansion. However, the progenitors of SNIa remain uncertain, introducing challenges to their use in cosmology and nucleosynthesis predictions.

Aims. In this work, we present a grid of five models detailing the evolution and nucleosynthesis of slowly merging carbon-oxygen white dwarfs approaching the Chandrasekhar mass.

Methods. These models test a variety of physics input settings, including accretion rates, nuclear reaction rates, convection parameters, and the composition of the accreted material. During the merger process, as the mass of the primary white dwarf approaches the Chandrasekhar limit, carbon burning is initiated first on the surface before eventually igniting explosively at the centre. As a consequence, the ²²Ne(α,n)²⁵Mg reaction activates in the outer layers of all models.

Results. The neutrons released in this way produce a weak s-process-like abundance distribution peaking at Kr, which is overproduced by more than a factor of ∼1000 compared to solar. The trans-Fe elements-enriched outer layer mass varies from ∼0.04 M_⊙ to ∼0.11 M_⊙, depending on the accretion rate. Our explosion simulation of these progenitor models ejects a significant amount of first-peak elements (e.g. Kr, Sr) as well as some long-lived radioactive species, such as ⁶⁰Fe.

Conclusions. In a previous theoretical study, we found that a similar nucleosynthesis process during the progenitor phase may also occur on the surface of near-Chandrasekhar white dwarfs formed through the accretion of H-rich material via the single-degenerate scenario. Therefore, these results suggest that trans-Fe enrichment might be a hallmark of near-Chandrasekhar SNIa ejecta, regardless of the specific progenitor channel, and could provide a new spectral signature that distinguishes them from sub-Chandrasekhar explosions.