r-Process nucleosynthesis from hyperaccreting neutron stars in common envelopes

TL;DR

This work investigates heavy-element nucleosynthesis in hyperaccreting neutron-star atmospheres formed during common-envelope evolution, covering accretion rates from $0.3$ to $3\times10^4$ $M_\odot$ yr$^{-1}$. Using the Cosmos++ framework to solve general-relativistic hydrodynamics with inline nuclear networks and neutrino cooling, the authors model convective, neutrino-cooling–driven atmospheres that cyclically heat and cool, producing NSE freezeout and diverse $r$-, $p$-, and $\gamma$-process products. A novel reheating pathway on reprocessed trajectories yields neutron-deficient isotopes, and high-entropy infall allows strong nucleosynthesis signatures without a conventional neutron excess, highlighting a potential CE site for significant heavy-element production. The results provide constraints on the role of common envelopes in galactic chemical evolution and establish a detailed, high-resolution framework for multi-physics modeling of accretion-atmosphere nucleosynthesis in extreme environments.

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 $α$-particles that does not require an excess of neutrons.

Figures (1)

• Figure 1: The neutrino optical depth attributed to elastic scattering is integrated radially across the computational domain and plotted as a function of time for all the calculations.