Slow White Dwarf Mergers as a New Galactic Source of Trans-Iron Elements

¹ E.A. Milne Centre for Astrophysics, University of Hull, Hull, HU6 7RX, UK
² School of Physics and Astronomy, University of Edinburgh, Edinburgh, EH9 3FD, UK
³ INFN -Istituto Nazionale Fisica Nucleare, 10125 Turin, Italy
⁴ Heidelberg Institute for Theoretical Studies, 69118 Heidelberg, Germany

^* e-mail: µU.Battino@hull.ac.uk

Published online: 24 February 2022

Abstract

The astrophysical origins of the heaviest stable elements that we observe today in the Solar System are still not fully understood. Recent studies have demonstrated that H-accreting white dwarfs (WDs) in a binary system exploding as type Ia supernovae could be an efficient p-process source beyond iron. However, both observational evidence and stellar models challenge the required frequency of these events. In this work, we calculate the evolution and nucleosynthesis in slowly merging carbon-oxygen WDs. As our models approach the Chandrasekhar mass during the merger phase, the ²²Ne(α,n)²⁵Mg neutron source reaction is activated in the external layers of the primary WD, where the carbon-rich material accreted from the secondary WD is burned via the ¹²C+¹²C reaction, which provides the necessary α-particles via the ¹²C(¹²C,α)²⁰Ne channel. The resulting neutron capture abundance distribution closely resembles a weak s-process one and peaks at Zr, which is overproduced by a factor of 30 compared to solar. The mass of the most external layers enriched in first-peak s-process elements crucially depends on the ¹²C+¹²C reaction rate, ranging between 0.05 M_⊙ and ~0.1 M_⊙. These results indicate that slow white dwarf mergers can efficiently produce the lightest p-process isotopes (such as ⁷⁴Se, ⁷⁸Kr, ⁸⁴Sr, ⁹²Mo and ⁹⁴Mo) via γ-induced reactions if they explode via a delayed detonation mechanism, or eject the unburned external layers highly enriched in first peak s-process elements in the case of a pure deflagration. In both cases, we propose for the first time that slow WD mergers in binary systems may be a new relevant source for elements heavier than iron.