Low Mass Black Holes from Dark Core Collapse

Unusual masses of black holes being discovered by gravitational wave experiments pose fundamental questions about the origin of these black holes. Black holes with masses smaller than the Chandrasekhar limit ≈1.4  M_{⊙} are essentially impossible to produce through stellar evolution. We propose a new channel for production of low mass black holes: stellar objects catastrophically accrete nonannihilating dark matter, and the small dark core subsequently collapses, eating up the host star and transmuting it into a black hole. The wide range of allowed dark matter masses allows a smaller effective Chandrasekhar limit and thus smaller mass black holes. We point out several avenues to test our proposal, focusing on the redshift dependence of the merger rate. We show that redshift dependence of the merger rate can be used as a probe of the transmuted origin of low mass black holes.

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_{⊙}<M_{PBH}<10^{-8}  M_{⊙} 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 ∼0.1  M_{⊙}-0.5  M_{⊙} 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.

R-process enrichment from a single event in an ancient dwarf galaxy

Elements heavier than zinc are synthesized through the rapid (r) and slow (s) neutron-capture processes. The main site of production of the r-process elements (such as europium) has been debated for nearly 60 years. Initial studies of trends in chemical abundances in old Milky Way halo stars suggested that these elements are produced continually, in sites such as core-collapse supernovae. But evidence from the local Universe favours the idea that r-process production occurs mainly during rare events, such as neutron star mergers. The appearance of a plateau of europium abundance in some dwarf spheroidal galaxies has been suggested as evidence for rare r-process enrichment in the early Universe, but only under the assumption that no gas accretes into those dwarf galaxies; gas accretion favours continual r-process enrichment in these systems. Furthermore, the universal r-process pattern has not been cleanly identified in dwarf spheroidals. The smaller, chemically simpler, and more ancient ultrafaint dwarf galaxies assembled shortly after the first stars formed, and are ideal systems with which to study nucleosynthesis events such as the r-process. Reticulum II is one such galaxy. The abundances of non-neutron-capture elements in this galaxy (and others like it) are similar to those in other old stars. Here, we report that seven of the nine brightest stars in Reticulum II, observed with high-resolution spectroscopy, show strong enhancements in heavy neutron-capture elements, with abundances that follow the universal r-process pattern beyond barium. The enhancement seen in this 'r-process galaxy' is two to three orders of magnitude higher than that detected in any other ultrafaint dwarf galaxy. This implies that a single, rare event produced the r-process material in Reticulum II. The r-process yield and event rate are incompatible with the source being ordinary core-collapse supernovae, but consistent with other possible sources, such as neutron star mergers.