Sandblasting the $\textit{r}$-Process: Spallation of Ejecta from Neutron Star Mergers

TL;DR

This paper investigates how high-velocity ejecta from neutron star mergers undergo spallation reactions in space, potentially altering observed isotopic abundances and providing insights into the merger conditions.

Contribution

It introduces a thick-target spallation model for NSM ejecta in the interstellar medium, highlighting how spallation impacts r-process nucleosynthesis signatures.

Findings

Spallation can modify isotopic abundances near r-process peaks.

The extent of modification depends on ejecta velocity, initial abundances, and cross-sections.

Observed yields can constrain neutron star merger properties.

Abstract

Neutron star mergers (NSMs) are rapid neutron capture ($\textit{r}$-process) nucleosynthesis sites that expel matter at high velocities, from $0.1c$ to as high as $0.6c$. Nuclei ejected at these speeds are sufficiently energetic to initiate spallation nuclear reactions with interstellar medium particles. We adopt a thick-target model for the propagation of high-speed heavy nuclei in the interstellar medium, similar to the transport of cosmic rays. We find that spallation may create observable perturbations to NSM isotopic abundances, particularly around the low-mass edges of the $\textit{r}$-process peaks where neighboring nuclei have very different abundances. The extent to which spallation modifies the final NSM isotopic yields depends on: (1) the ejected abundances, which are determined by the NSM astrophysical conditions and the properties of nuclei far from stability, (2) the ejecta velocity distribution and propagation in interstellar matter, and (3) the spallation cross-sections. Observed solar and stellar $\textit{r}$-process yields could thus constrain the velocity distribution of ejected neutron star matter, assuming NSMs are the dominant $\textit{r}$-process source. We suggest avenues for future work, including measurement of relevant cross sections.