R-process Rain from Binary Neutron Star Mergers in the Galactic Halo
  Compact binary mergers involving at least one neutron star are promising
sites for the synthesis of r-process elements found in stars and planets.
However, mergers are generally thought to take place at high galactic
latitudes, far from any star-forming regions of the host galaxy. It is thus
important to understand the physical mechanisms involved in transporting
enriched material from the gas environments of the galactic halo to the
star-forming disk. We investigate these processes, starting from an explosive
injection event and its interaction with the halo gas medium. We show that the
total outflow mass in compact binary mergers is far 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 $1-10$ pc scales, and becomes
corrugated by the Rayleigh-Taylor instability. The corrugated shell is denser
than the ambient medium, and breaks into clouds which then sink toward the
disk. The sinking clouds lose thermal energy through radiative cooling, and are
also ablated by shearing instabilities. We present a dynamical heuristic that
models these effects, and would 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 r-process clouds over $\sim 10^5$
yr. 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 by turbulent diffusion or large-scale inflowing
mass currents.
