The decompression of the outer neutron star crust and r-process nucleosynthesis

TL;DR

This study investigates how decompression of neutron star outer crust material can produce r-process nucleosynthesis, resulting in stable neutron-rich nuclei distributions that closely match the solar system's observed abundance patterns.

Contribution

It demonstrates that decompression of neutron star outer crust at specific initial conditions can replicate the solar r-process abundance distribution, highlighting a potential astrophysical site.

Findings

Decompression leads to stable neutron-rich nuclei matching solar r-process distribution.

Initial temperature around 8 GK is crucial for producing solar-like abundance patterns.

Outer crust ejection results in robust r-process nucleosynthesis for nuclei with A < 140.

Abstract

The rapid neutron-capture process, or r-process, is known to be fundamental for explaining the origin of approximately half of the A>60 stable nuclei observed in nature. In recent years nuclear astrophysicists have developed more and more sophisticated r-process models, by adding new astrophysical or nuclear physics ingredients to explain the solar system composition in a satisfactory way. Despite these efforts, the astrophysical site of the r-process remains unidentified. The composition of the neutron star outer crust material is investigated after the decompression that follows its possible ejection. The composition of the outer crust of a neutron star is estimated before and after decompression. Two different possible initial conditions are considered, namely an idealized crust composed of cold catalyzed matter and a crust initially in nuclear statistical equilibrium at temperatures around 10 GK. We show that in this second case before decompression and at temperatures typically corresponding to 8 GK, the Coulomb effect due to the high densities in the crust leads to an overall composition of the outer crust in neutron-rich nuclei with a mass distribution close to the solar system r-abundance distribution. Such distributions differ, however, from the solar one due to a systematic shift in the second peak to lower values. After decompression, the capture of the few neutrons per seed nucleus available in the hot outer crust leads to a final distribution of stable neutron-rich nuclei with a mass distribution of 80 < A < 140 nuclei in excellent agreement with the solar distribution, provided the outer crust is initially at temperatures around 8 GK and all layers of the outer crust are ejected. The decompression of the neutron star matter from the outer crust provides suitable conditions for a robust r-processing of the light species, i.e., r-nuclei with A < 140.