Primordial Black Holes and $r$-Process Nucleosynthesis

We show that some or all of the inventory of $r$-process nucleosynthesis can be produced in interactions of primordial black holes (PBHs) with neutron stars (NSs) if PBHs with masses ${10}^{-14}{M}_{\odot }<M_{\mathrm{PBH}}<{10}^{-8}{M}_{\odot }$ make up a few percent or more of dark matter. A PBH captured by a NS sinks to the center of the NS and consumes it from the inside. When this occurs in a rotating millisecond-period NS, the resulting spin-up ejects $\sim 0.1M_{\odot }–0.5M_{\odot }$ of relatively cold neutron-rich material. This ejection process and the accompanying decompression and decay of nuclear matter can produce electromagnetic transients, such as a kilonova-type afterglow and fast radio bursts. These transients are not accompanied by significant gravitational radiation or neutrinos, allowing such events to be differentiated from compact object mergers occurring within the distance sensitivity limits of gravitational-wave observatories. The PBH-NS destruction scenario is consistent with pulsar and NS statistics, the dark-matter content, and spatial distributions in the Galaxy and ultrafaint dwarfs, as well as with the $r$-process content and evolution histories in these sites. Ejected matter is heated by beta decay, which leads to emission of positrons in an amount consistent with the observed 511-keV line from the Galactic center.

Figure 1

Total ejected mass ($M_{\text{ej}}$) from a MSP with initial rotation period $P$ disrupted by a PBH. $n=3$ polytrope (red) and $n=1.5$ polytrope (orange) NS density profiles are shown. The black line indicates ejection of $0.1M_{\odot }$. The MSP period-population distribution [47] is displayed with a dashed blue line.

Figure 2

Parameter space where PBHs can account for all or partial $r$-process element production in the Milky Way and the UFDs simultaneously. Constraints from extragalactic $\gamma$-rays from BH evaporation [74] ($\mathrm{EG}\gamma$), femtolensing [75] (FL), white dwarf abundance [76] (WD), Kepler star milli- or microlensing [77] (K), Subaru HSC microlensing [78] (HSC), and MACHO, EROS, and OGLE microlensing [79] (ML) are displayed.