On the R-Process Enrichment of Dwarf Spheroidal Galaxies

TL;DR

This paper explores alternative dark matter-related mechanisms for r-process element enrichment in dwarf spheroidal galaxies, challenging the neutron star merger hypothesis and proposing a novel dark matter-induced neutron star implosion model.

Contribution

It introduces a new dark matter-induced neutron star implosion mechanism as an alternative to neutron star mergers for r-process enrichment in UFDs.

Findings

Dark matter-induced neutron star implosions can produce r-process elements at observed levels.

This mechanism also addresses the missing pulsar problem in the Milky Way's center.

Proposes specific tests to validate or falsify the dark matter r-process model.

Abstract

Recent observations of Reticulum II have uncovered an overabundance of r-process elements, compared to similar ultra-faint dwarf spheroidal galaxies (UFDs). Because the metallicity and star formation history of Reticulum II appear consistent with all known UFDs, the high r-process abundance of Reticulum II suggests enrichment through a single, rare event, such as a double neutron star (NS) merger. However, we note that this scenario is extremely unlikely, as binary stellar evolution models require significant supernova natal kicks to produce NS-NS or NS-black hole mergers, and these kicks would efficiently remove compact binary systems from the weak gravitational potentials of UFDs. We examine alternative mechanisms for the production of r-process elements in UFDs, including a novel mechanism wherein NSs in regions of high dark matter density implode after accumulating a black-hole-forming mass of dark matter. We find that r-process proto-material ejection by tidal forces, when a single neutron star implodes into a black hole, can occur at a rate matching the r-process abundance of both Reticulum II and the Milky Way. Remarkably, dark matter models which collapse a single neutron star in observed UFDs also solve the missing pulsar problem in the Milky Way Galactic center. We propose tests specific to dark matter r-process production which may uncover, or rule out, this model.