Access the full text.
Sign up today, get DeepDyve free for 14 days.
References for this paper are not available at this time. We will be adding them shortly, thank you for your patience.
VIEWPOINT Neutron-Star Implosions as Heavy-Element Sources A dramatic scenario in which a compact black hole eats a spinning neutron star from inside might explain a nearby galaxy's unexpectedly high abundance of heavy elements. by Hans-Thomas Janka he lightest of the chemical elements—hydrogen, he- lium, and lithium—were created in the hot, early phase of the Universe, about a minute after the big T bang. Heavier elements were forged later—in the nuclear fires of many generations of stars and during super- nova explosions [1]. But the origin of many rare chemical species, particularly the heaviest elements, remains uncer- tain. In particular, recent observations [2] of a nearby galaxy enriched with heavy elements challenge traditional nucleosynthesis models. George Fuller of the University of California, San Diego, and colleagues [3] now propose a Figure 1: Fuller et al. [3] propose a model for the synthesis of novel scenario for the origin of the heaviest elements, includ- heavy elements in which a rapidly rotating neutron star is ing gold, platinum, and uranium. Their hypothesis involves swallowed from the inside by a tiny black hole. The centrifugally tiny black holes inducing neutron-star implosions and, if deformed star, shown in cross-section, sheds considerable mass at its equator as it spins up and angular momentum is transferred viable, would in one fell swoop offer solutions to other as- outward. Heavy atomic nuclei, including gold and platinum, can trophysical riddles beyond heavy element synthesis. form via the r -process in the neutron-rich matter that's expelled Elements heavier than iron can be assembled only from from the imploding star. (APS/Alan Stonebraker) lighter “seed” nuclei that capture free neutrons or protons [1]. Neutron capture occurs through either a “slow” s pro- cess or a “rapid” r process. In both cases, the neutron-rich [4, 5]—and in mergers between two neutron stars or between nucleus undergoes beta decay, converting neutrons to pro- a neutron star and a black hole [6]. These compact binary tons and advancing to higher atomic numbers. The s process mergers are estimated to be 1000 times less frequent than can proceed at the modest neutron densities available in the supernovae, but they can expel considerably larger amounts outer shells of evolving stars. By contrast, the r process of neutron-rich matter [7, 8]—a low-rate/high-yield scenario requires 10 billion times greater neutron densities (above that’s consistent with the rarity of plutonium-244 in the early 18 3 10 cm ) in order that neutron captures occur much faster Solar System and in deep-sea reservoirs on Earth [9, 10]. than beta decay. The r process is responsible for gold, A wrinkle in this picture is a nearby low-luminosity dwarf platinum, most of the lanthanides, and all of the natural ac- galaxy known as Reticulum II, whose stars are highly en- tinides. The heaviest r-process nuclei—up to and beyond riched with strong-r-process nuclei [2]. Reticulum II is the an atomic mass number of 240—occur through the “strong” only dwarf galaxy (out of ten) with a significant “excess” r process, in which an iron seed captures 100 or more neu- of heavy nuclei, which suggests the nuclei were produced trons. by an infrequent event, but perhaps one not so rare as a compact-object merger [11]. Fuller and co-workers [3] The strong r process requires a high neutron density and therefore envision an alternative scenario in which r-process some combination of a large excess of neutrons over pro- nuclei are generated in the ejected matter of a very rapidly tons, very high temperatures, and rapid expansion. Such spinning neutron star, or “millisecond pulsar,” as it im- extremes are expected in supernovae—but only in rare cases plodes to form a black hole. The researchers imagine that the trigger for this catas- Max Planck Institute for Astrophysics, Karl-Schwarzschild-Str. 1, trophic collapse is a primordial black hole (PBH). Hypotheti- D-85748 Garching, Germany cal relics from the early Universe, PBHs can have the mass of physics.aps.org 2017 American Physical Society 07 August 2017 Physics 10, 89 an asteroid packed into an atom-sized space and collectively with a source that produces r-process nuclei; they would they are one of several candidates for dark matter. PBHs then need to use other observations to identify the source. would roam dwarf galaxies and the center of our Milky Way For example, did the signal come from a region of copi- with a relatively high abundance, so they would collide with ous dark matter, as Fuller et al. and Bramante and Linden neutron stars at a higher rate than that of compact-object propose, or was it accompanied by gravitational waves, as mergers. When a PBH is captured by a neutron star, it sinks expected for inspiralling and merging compact binary stars? towards the center and swallows the star from the inside. Such gravitational waves should be detectable by Advanced Then, as the growing black hole sucks in neutron-star matter, LIGO, VIRGO, and KAGRA, and they may ultimately be the viscous shearing and magnetic fields carry angular momen- smoking gun that allows physicists to solve the mysterious tum to the star ’s outer layers along its equator. Fuller et al. origin of gold. argue that these mechanisms rip off dense nuclear matter in which the strong r process can develop (Fig. 1). This research is published in Physical Review Letters. This scenario is similar to one proposed by Joseph Bra- mante and Tim Linden in 2016 [11]. Instead of PBHs, they proposed that dark matter particles could accumulate in- side an aging neutron star to form a star-consuming black REFERENCES hole. As the black hole accreted mass, it would release [1] E. M. Burbidge, G. R. Burbidge, W. A. Fowler, and F. Hoyle, enough gravitational binding energy to power the ejection ``Synthesis of the Elements in Stars,'' Rev. Mod. Phys. 29, 547 of dense neutron matter for strong-r-process synthesis. Both (1957). teams estimated the parameters required by their models [2] A. P. Ji, A. Frebel, A. Chiti, and J. D. Simon, ``R-process Enrich- to predict implosion rates that are compatible with the r- ment from a Single Event in an Ancient Dwarf Galaxy,'' Nature process-enhancement of Reticulum II and the distribution of 531, 610 (2016). r-process elements in the Milky Way. These calculated pa- [3] G. M. Fuller, A. Kusenko, and V. Takhistov, ``Primordial Black rameters, which include, for example, dark matter density, Holes and r -Process Nucleosynthesis,'' Phys. Rev. Lett. 119, 061101 (2017). appear to be realistic. [4] C. Winteler, R. Käppeli, A. Perego, A. Arcones, N. Vasset, N. What’s attractive about the models presented by Fuller et Nishimura, M. Liebendörfer, and F.-K. Thielemann, ``Magne- al. and by Bramante and Linden is that they might simul- torotationally Driven Supernovae as the Origin of Early Galaxy taneously resolve a number of astrophysical conundrums. r -Process Elements?'' Astrophys. J. Lett. 750, L22 (2012). For example, the possibility that neutron stars are being rou- [5] P. Banerjee, W. C. Haxton, and Y.-Z. Qian, ``Long, Cold, Early r tinely eaten by black holes could explain why there are far Process? Neutrino-Induced Nucleosynthesis in He Shells Re- fewer pulsars at the center of our Galaxy than astrophysicists visited,'' Phys. Rev. Lett. 106, 201104 (2011). expect—though the average collapse time of a star is suf- [6] J. M. Lattimer, F. Mackie, D. G. Ravenhall, and D. N. Schramm, ficiently long that a large population of old pulsars should ``The Decompression of Cold Neutron Star Matter,'' Astrophys. still exist. In addition, both teams refer to a possibility sug- J. 213, 225 (1977). [7] C. Freiburghaus, S. Rosswog, and F.-K. Thielemann, ``r - gested by another group [12]: The final stages of a neutron Process in Neutron Star Mergers,'' Astrophys. J. Lett. 525, star ’s demise, as well as its release of energy via the “re- L121 (1999). connection” of its magnetic field, might be connected to [8] A. Bauswein, R. Ardevol Pulpillo, H.-T. Janka, and S. Goriely, recently discovered extragalactic fast radio bursts. Fuller et ``Nucleosynthesis Constraints on the Neutron Star-Black Hole al. also explain the mysterious 511-keV line in the gamma- Merger Rate,'' Astrophys. J. Lett. 795, L9 (2014). ray emission from our Galaxy’s center, linking it to positron [9] A. Wallner et al., ``Abundance of Live Pu in Deep-Sea Reser- production in the radioactively heated ejecta from a neutron- voirs on Earth Points to Rarity of Actinide Nucleosynthesis,'' star implosion. Nat. Commun. 6, 5956 (2015). But while these phenomena are all consistent with the r- [10] K. Hotokezaka, T. Piran, and M. Paul, ``Short-Lived Pu process scenario proposed by Fuller et al., each could be Points to Compact Binary Mergers as Sites for Heavy r - explained with less speculative (and not necessarily related) Process Nucleosynthesis,'' Nat. Phys. 11, 1042 (2015). [11] J. Bramante and T. Linden, ``On the r -Process Enrichment of ideas. Moreover, the viability of their proposal, and that Dwarf Spheroidal Galaxies,'' Astrophys. J. 826, 57 (2016). by Bramante and Linden, depends on whether the neutron [12] J. Fuller and C. D. Ott, ``Dark Matter-Induced Collapse of Neu- stars eject sufficient mass as they collapse. Assessing this tron Stars: A Possible Link Between Fast Radio Bursts and the fact will require detailed relativistic hydrodynamical calcu- Missing Pulsar Problem,'' Mon. Not. R. Astron. Soc. Lett. 450, lations that go beyond the coarse analytical estimates in both L71 (2015). papers. Researchers might distinguish various scenarios by looking for a transient electromagnetic signal associated 10.1103/Physics.10.89 physics.aps.org 2017 American Physical Society 07 August 2017 Physics 10, 89
Physics – American Physical Society (APS)
Published: Aug 7, 2017
Access the full text.
Sign up today, get DeepDyve free for 14 days.