Nucleosynthesis in Thermonuclear Supernovae

Abstract

The explosion energy of thermonuclear (Type Ia) supernovae is derived from the difference in nuclear binding energy liberated in the explosive fusion of light "fuel" nuclei, predominantly carbon and oxygen, into more tightly bound nuclear "ash" dominated by iron and silicon group elements. The very same explosive thermonuclear fusion event is also one of the major processes contributing to the nucleosynthesis of the heavy elements, in particular the iron-group elements. For example, most of the iron and manganese in the Sun and its planetary system was produced in thermonuclear supernovae. Here, we review the physics of explosive thermonuclear burning in carbon-oxygen white dwarf material and the methodologies utilized in calculating predicted nucleosynthesis from hydrodynamic explosion models. While the dominant explosion scenario remains unclear, many aspects of the nuclear combustion and nucleosynthesis are common to all models and must occur in some form in order to produce the observed yields. We summarize the predicted nucleosynthetic yields for existing explosions models, placing particular emphasis on characteristic differences in the nucleosynthetic signatures of the different suggested scenarios leading to Type Ia supernovae. Following this, we discuss how these signatures compare with observations of several individual supernovae, remnants, and the composition of material in our Galaxy and galaxy clusters.

Electron capture in stars

“…Seitenzahl and collaborators made the important observation that 55 Mn must be overproduced relative to iron in type Ia supernovae to explain the solar [Mn/Fe] ratio, as all modern core-collapse simulations predict a [Mn/Fe] ratio noticeably smaller than the solar one [185]. As the high-density regime needed to produce manganese in thermonuclear supernovae is only achieved in near M Ch scenarios, 55 Mn nucleosynthesis and its evolution over time in our Galaxy also places valuable constraints on the WD mass in the type Ia scenarios [179,185,186].…”

Horizons: nuclear astrophysics in the 2020s and beyond

“…Thermonuclear supernovae are modeled as thermonuclear explosions of white dwarfs. The evolution of the progenitor system toward explosion, the ignition of the thermonuclear burning, and details of the explosion mechanism are still uncertain [368]. This is of particular importance because of the prominent role these events play in observational cosmology and in the cosmic cycle of matter [467][468][469].…”

“…Novae are thought to be important sources of specific light element isotopes such as 7 Li, 13 C, 15 N, and 17 O, as well as γ-ray emitters such as 26 Al and 22 Na [366,367]. Thermonuclear supernovae are major contributors to nucleosynthesis, providing about half of the iron-group elements Cr, Mn, Fe, and Ni, in the Galaxy [368], and, potentially, also contribute to proton-rich isotopes of heavy elements beyond iron through the p-process (section 3). Furthermore, the use of these objects as so-called standard candles to inform, e.g., cosmological expansion, necessitates an improved understanding of such transients [369].…”

Evidence for sub-Chandrasekhar Type Ia supernovae from the last major merger

“…The commonality between most observed SNe Ia is they likely arise from the explosion of a carbon-oxygen (CO) white dwarf (WD) in a binary system (Whelan & Iben 1973;Iben & Tutukov 1984;Webbink 1984). Beyond this, there are many suggested scenarios for the nature of the progenitor systems and the explosion mechanisms (see the reviews of Hillebrandt & Niemeyer 2000;Maoz et al 2014;Seitenzahl & Townsley 2017;Ruiter 2020).…”

“…Mn is mostly produced through the decay of 55 Co which is made in large quantities in normal freeze-out from nuclear statistical equilibrium achieved in 𝑀 Ch WDs, or in lower quantities in incomplete silicon burning at the lower densities typical in sub-𝑀 Ch WDs, and is destroyed during alpha-rich freeze-out which occurs at lower densities (see Lach et al 2020). Stable Ni is produced as 58 Ni which arises from the neutron-rich environments produced by electron-capture in the high density environments of 𝑀 Ch WDs (Seitenzahl & Townsley 2017). For these reasons, typically sub-𝑀 Ch explosions lead to subsolar [Mn/Fe] and [Ni/Fe], whilst 𝑀 Ch events produce super-solar yields.…”