r-Process nucleosynthesis from hyperaccreting neutron stars in common envelopes

TL;DR

This paper models nuclear reactions in hyperaccreting neutron stars within common envelopes, revealing complex nucleosynthesis processes including r-, p-, and gamma processes, influenced by accretion rates and convective dynamics.

Contribution

It introduces a comprehensive model of nuclear reactions in hyperaccreting neutron stars, highlighting new pathways for element synthesis without requiring neutron excess.

Findings

Convection drives diverse nucleosynthesis during accretion.

High entropy conditions enable r-process element formation.

Reheated trajectories produce neutron-deficient isotopes.

Abstract

We investigate nuclear reactions and feedback in hyperaccreting neutron star environments, considering accretion rates in the range 0.3 - $3\times10^4$ $M_\odot$ yr$^{-1}$, typical of short-period compact object binaries in common envelopes. Our mode ls account for weak reactions, neutrino energy loss, nuclear energy release, pair production, degenerate equations of state, and general relativistic hydrodynamics. Depending on accretion rates, these systems can develop both proton and neutron-rich atmospheres with strong convective instabilities linking the neutrino sphere to the outgoing accretion shock inside the radia tion trapping zone. Convection drives nucleons through multiple heating and cooling cycles, with photodisintegration dominat ing during the heating phase and heavy element synthesis during the cooling phase, ejecting material with abundances that dep end on the accretion rate and depth of the final decompression trajectory. The turbulent nature of convective currents is con ducive to creating a wide range of nuclear products through a variety of effects, including NSE freezeout and $r$, $p$ and $\ gamma$ processes. We also observe a novel multi-step process in reheated trajectories, consisting of proton-capture and photo-dissociation reactions operating on $r$-process seeds, producing overall neutron-deficient isotopes. A significant amount of infalling gas experiences high entropy and short (millisecond) freezeout timescales capable of making $r$-process elements w ith high over-abundances through a disequilibrium effect between neutrons and $\alpha$-particles that does not require an excess of neutrons.