R-process Rain from Binary Neutron Star Mergers in the Galactic Halo

Abstract

Compact binary mergers involving at least one neutron star are promising sites for the synthesis of $\textit{r}$-process elements found in stars and planets. However, mergers can take place at significant offsets from their host galaxies, with many occurring several kpc from star-forming regions. It is thus important to understand the physical mechanisms involved in transporting enriched material from merger sites in the galactic halo to the star-forming disk. We investigate these processes, starting from an explosive injection event and its interaction with the halo medium. We show that the total outflow mass in compact binary mergers is too low for the material to travel to the disk in a ballistic fashion. Instead, the enriched ejecta is swept into a shell, which decelerates over $\lesssim 10$ pc scales and becomes corrugated by the Rayleigh-Taylor instability. The corrugated shell is denser than the ambient medium, and breaks into clouds which sink toward the disk. These sinking clouds lose thermal energy through radiative cooling, and are also ablated by shearing instabilities. We present a dynamical heuristic that models these effects to predict the delay times for delivery to the disk. However, we find that turbulent mass ablation is extremely efficient, and leads to the total fragmentation of sinking $\textit{r}$-process clouds over $10-100$ pc scales. We thus predict that enriched material from halo injection events quickly assimilates into the gas medium of the halo, and that enriched mass flow to the disk could only be accomplished through turbulent diffusion or large-scale inflowing mass currents.