Angular momentum drives proton-rich nucleosynthesis in hyperaccreting neutron stars in common envelopes

Interacting binaries, especially for massive star systems, produce a wide range of exotic systems including X-ray binaries and merging neutron star systems. In the rapid expansion as a star moves off the main sequence (core hydrogen exhaustion), the star can engulf its companion, forming a Common Envelope (CE) evolutionary phase. A CE phase where the engulfed star is a compact object (e.g. neutron star) can lead to rapid accretion onto the neutron star. Past work has focused on systems in which the accreting material has low angular momentum, studying turbulent outflows. This study instead focuses on accreting conditions where the accreting material has sufficient angular momentum to form a disk. Disk accretion systems lead to very different nuclear burning conditions, e.g. an increase in the timescale on which nuclear burning can occur. This paper presents the results of nucleosynthesis modelling of material ejected from an accretion disk surrounding a 1.5 M⊙ neutron star in a CE with a 15 M⊙ companion. As material is accreted towards the neutron star sufficient heating will occur to eject a fraction of the material back into the surrounding envelope, producing a nucleosynthetic yield signature that differs from other explosion. We find that significant mass fractions of rp-process products are synthesised, thereby providing another mechanism for rp-process contribution to galactic chemical evolution, following ejection of the companion envelope. Furthermore, for later stages of the companion evolution, the accretion of helium leads to alpha-rich nucleosynthesis, resulting in supernova-like nucleosynthesis, producing in particular 44Ti and 56Ni. Further work on modelling both the accretion disk wind, and the companion envelope ejection, is vital to understand the contributions of these scenarios to chemical evolution.