Selected for a Viewpoint in Physics week ending PRL 119, 061101 (2017) PHYSICAL REVIEW LETTERS 11 AUGUST 2017
Primordial Black Holes and r-Process Nucleosynthesis
† ‡ George M. Fuller,1,* Alexander Kusenko,2,3, and Volodymyr Takhistov2, 1Department of Physics, University of California, San Diego, La Jolla, California 92093-0424, USA 2Department of Physics and Astronomy, University of California, Los Angeles, Los Angeles, California 90095-1547, USA 3Kavli Institute for the Physics and Mathematics of the Universe (WPI), UTIAS, University of Tokyo, Kashiwa, Chiba 277-8583, Japan (Received 6 April 2017; revised manuscript received 11 June 2017; published 7 August 2017) We show that some or all of the inventory of r-process nucleosynthesis can be produced in interactions 10−14 M
DOI: 10.1103/PhysRevLett.119.061101
Primordial black holes (PBHs) can account for all or part by accretion of particle dark matter onto the NS [24], of dark matter (DM) [1–13]. If a PBH is captured by a although the rates and the implications for dark-matter neutron star (NS), it settles into the center and grows until properties are, of course, different. The probability of the supply of nuclear matter is exhausted by accretion and PBH capture depends on both the PBH and the NS densities. ejection. DM-rich environments, such as the GC and dwarf spheroidal In this Letter we show that NS disruptions by PBHs in galaxies, are not known to host NSs. An exception is the DM-rich environments, such as the Galactic center (GC) young magnetar [25,26], whose age is small compared to the and dwarf spheroidal galaxies, provide a viable site for time scales of PBH capture. NSs are found in the disk and r-process nucleosynthesis, thus offering a solution to a long- the halo, as well as in the globular clusters, where the dark- standing puzzle [14–18]. The transients accompanying matter density [27,28] is too low to cause a substantial NS disruption events and the positrons produced in these decrease in the pulsar population. Positrons emitted from events are consistent with present observations, and they the heated neutron-rich ejecta can account for the observed offer a way of testing the NS-PBH scenario in the future. 511-keV line from the GC [29,30]. The final stages of We will demonstrate that, when a PBH accretes matter neutron-star demise can be the origin [31,32] of some of the inside a rapidly rotating millisecond pulsar (MSP), the recently observed [33] fast radio bursts (FRBs), as well as resulting pulsar spin-up causes ∼0.1 M⊙–0.5 M⊙ of neu- x-ray and γ-ray transients. A kilonova-type [34–41] after- tron-rich material to be ejected without significant heating glow can accompany the decompressing nuclear matter and only modest neutrino emission. This provides a favor- ejecta, but unlike COMs, these events are not associated with able setting for r-process nucleosynthesis, occurring on the a significant release of neutrinos or gravitational radiation. Galactic time scales, which can evade several problems that Therefore, future observations of gravitational waves and have challenged the leading proposed r-process production kilonovae will be able to distinguish between r-process sites, such as neutrino-heated winds from core-collapse scenarios. supernovae or binary compact object mergers (COMs) Millisecond pulsars are responsible for the predominant [19,20]. The unusual distribution of r-process abundances contribution to the nucleosynthesis initiated by PBH- within the ultrafaint dwarf spheroidal galaxies (UFDs) induced centrifugal ejection of neutron-rich material, since [21,22] is naturally explained by the rates of PBH capture MSPs have the highest angular velocities at the time of in these systems. The rates are also consistent with the PBH capture. The most prominent sites of r-process paucity of pulsars in the GC [23]. A similar distribution of production must have a high density of MSPs as well as r-process material in UFDs can be expected from NS PBHs. The latter trace the DM spatial distribution. The DM disruptions due to black holes produced in the NS interiors density is high in the GC and in the UFDs. On the other
0031-9007=17=119(6)=061101(6) 061101-1 © 2017 American Physical Society PHYSICAL REVIEW LETTERS week ending PRL 119, 061101 (2017) 11 AUGUST 2017 hand, the MSP density is high in molecular clouds, including binaries, a higher binary gravitational potential causes an the central molecular zone (CMZ), and the globular clusters. increase in the capture rate. We, therefore, assume that the While the CMZ is located within the GC with an extremely capture rate for MSPs is a factor of 2 higher than for isolated high DM density, observations imply that the DM content of NSs. For example, for typical values of parameters and m ¼ 1019 globular clusters is fairly low [27,28]. The product of the PBH g, one obtains DM density and the MSP density is still sufficient to allow r MW −11 for some -process nucleosynthesis in both the UFDs and the F0 ¼ 1.5 × 10 =yr globular clusters, but we estimate that CMZ accounts for ð2Þ FUFD ¼ 6.0 × 10−10=yr: 10% to 50% of the total Galactic production. We include 0 contributions from the GC (CMZ) and the rest of the halo 48 = (which may be comparable, within uncertainties) [42,43]. For the MW we have used velocity dispersions of km s 105 = The CMZ has an approximate size of ∼200 pc and is and km s for the NS and DM, respectively, as well as 8 8 102 = 3 located near the GC, where supernova rates are the highest. DM density . × GeV cm . The pulsar and DM Since the DM density peaks at the GC, the CMZ is a site of velocity dispersions are simultaneously taken into account frequent PBH-MSP interactions. The pulsar formation rate for the MW as described below. For UFD we have used 2 5 = [44] in the CMZ is 1.5 × 10−3 yr−1 (7% of the Galactic the DM velocity dispersion . km s and the DM den- 10 = 3 formation rate), consistent with the GeV γ-ray flux observed sity GeV cm . [45] from the GC by the Fermi Large Area Telescope. A PBH could also be captured by a NS progenitor prior GC 7 to supernova core collapse [50], but this does not increase Hence, we expect Np ≈ 1.5 × 10 NSs to be produced at 10 the capture rates significantly. the GC during the lifetime of the Galaxy, tG ∼ 10 yr. Natal pulsar kicks can enable pulsars to escape from the Roughly 30%–50% of these NSs become MSPs [46], and region of interest. We include this effect in our calculations the number of MSPs with a particular rotation period can (see Supplemental Material [51]). then be estimated from a population model [46–48]. Pulsar lifetimes in the presence of PBHs with a given Simulations and observations of UFDs imply [21] that ht i¼1=Fþt þ ∼2000 number density can be estimated as NS loss core-collapse supernovae have occurred in 10 UFDs t t ∼ 5 108 NUFD ∼ con, where the first term describes the mean BH capture during UFD × yr. Hence, we expect that p t 2 time, loss is the time for the PBH to be brought within the NS 10 pulsars have been produced in each of these systems, t once it is gravitationally captured, and con is the time for the on average, and we estimate the fraction of fast-rotating black hole to consume the NS. For a typical NS one finds MSPs using a population model [47]. [49] that t ≃4.1×104ðm =1022gÞ−3=2 yr. The spherical The black-hole capture rate can be calculated using the loss PBH ðm Þ accretion rate of NS matter onto the PBH is described by the initial PBH mass PBH , DM density and velocity dm =dt¼4πλ G2m2 ρ =v3 ¼C m2 Bondi equation BH s BH c s 0 BH, dispersion (assuming a Maxwellian distribution). With m ðtÞ where BH is the time-dependent mass of the central the base Milky Way (MW) and UFD capture rates denoted v ρ MW UFD black hole, s is the sound speed, c is the central density, as F0 and F0 , one obtains [49] the full PBH-NS capture λs MW UFD and is a density profile parameter. For typical NS values of rates F¼ðΩ =Ω ÞF0 and F¼ðΩ =Ω ÞF0 in 15 3 PBH DM PBH DM [52] vs ¼ 0.17, ρc ¼ 10 g=cm , and λs ¼ 0.707 (for a star the MW and UFD, respectively. The number of PBHs n ¼ 3 t ¼ Ft t t described by an polytrope) we obtain that con captured by a NS is , where the time is either G or 10ð1019 =m Þ t g PBH yr. If PBHs make up all of the DM, we UFD for the MW or the UFD, respectively. For our analysis ht i < 1012 1017