Trans-Fe elements from Type Ia Supernovae. I. Heavy element nucleosynthesis during the formation of near-Chandrasekhar white dwarfs

TL;DR

This study models the nucleosynthesis in near-Chandrasekhar white dwarf progenitors of Type Ia supernovae, revealing a weak s-process enrichment of trans-Fe elements that could serve as a spectral signature for these explosions.

Contribution

It introduces detailed models of white dwarf mergers approaching Chandrasekhar mass, highlighting the nucleosynthesis of trans-Fe elements and potential observational signatures.

Findings

Weak s-process-like abundance distribution peaking at Kr

Enrichment of trans-Fe elements in outer layers

Ejection of radioactive species like 60Fe

Abstract

Type Ia supernovae (SNIa) are thermonuclear explosions of white dwarfs in binary systems. They are central to galactic chemical evolution and serve as standardizable candles in cosmology, yet their progenitors remain uncertain. 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. 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 center. As a consequence, the 22Ne(a,n)25Mg reaction activates in the outer layers of all models. 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 Msun to 0.11 Msun, depending on the accretion rate. Our explosion simulation of these progenitor models ejects significant amount of first-peak elements (e.g., Kr, Sr) as well as of some long-lived radioactive species, such as 60Fe. 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 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 distinguishing them from sub-Chandrasekhar explosions.