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Neutron Stars' Hidden Nuclear Pasta UvA-DARE (Digital Academic Repository) Neutron stars' hidden nuclear pasta Rea, N. DOI 10.1063/PT.3.2957 Publication date 2015 Document Version Final published version Published in Physics Today Link to publication Citation for published version (APA): Rea, N. (2015). Neutron stars' hidden nuclear pasta. Physics Today, 68(10), 62-63. https://doi.org/10.1063/PT.3.2957 General rights It is not permitted to download or to forward/distribute the text or part of it without the consent of the author(s) and/or copyright holder(s), other than for strictly personal, individual use, unless the work is under an open content license (like Creative Commons). Disclaimer/Complaints regulations If you believe that digital publication of certain material infringes any of your rights or (privacy) interests, please let the Library know, stating your reasons. In case of a legitimate complaint, the Library will make the material inaccessible and/or remove it from the website. Please Ask the Library: https://uba.uva.nl/en/contact, or a letter to: Library of the University of Amsterdam, Secretariat, Singel 425, 1012 WP Amsterdam, The Netherlands. You will be contacted as soon as possible. UvA-DARE is a service provided by the library of the University of Amsterdam (https://dare.uva.nl) Download date:26 Sep 2021 quick study Neutron stars’ hidden nuclear pasta Nanda Rea The novel, amorphous state of matter that exists in the crust of neutron stars leaves a trace in the spins of magnetized, rapidly rotating pulsars. Nanda Rea is a staff scientist at the Spanish National Research charged particles. The currents generated by the electrons Council in Barcelona, Spain, and a research group leader at the partially dissipate the star’s magnetic field and heat the crust. The density of the outer crust spans about three orders, University of Amsterdam in Amsterdam, the Netherlands. from 109 g/cm3 to 1011 g/cm3. As the density increases, electron- capture reactions become more favorable; the resulting ithin just a few years of the neutron’s discov- Coulomb lattice is thus made from progressively more exotic ery in 1932, the idea of a neutron star had been and neutron-rich nuclei. At the so-called neutron drip point, articulated. Specifically, many physicists sug- where the density hits about 4 × 1011 g/cm3, nuclei are unable W gested that the supernova explosion of a star to hold more neutrons. Beyond that critical density, neutrons of 10 or so solar masses might signal a transi- start to behave as a superfluid. tion from a normal star to a compact, highly magnetic, rap- As the density of the inner crust increases beyond that idly rotating star composed mainly of neutrons. It took some of the drip point, the neutron-rich nuclei at first remain in a 30 years to confirm the notion, but in 1967 Jocelyn Bell and Coulomb lattice, but one that is now embedded in a fluid of Antony Hewish discovered the first neutron star by spotting electrons and neutrons. However, at a density of something its beamed radio emission. That phenomenon arises because like 8 × 1013 g/cm3, astrophysicists expect the spherical nuclei particles radiate along the star’s magnetic axis as they rapidly to begin to reorganize into exotic and complex pasta-like accelerate in its magnetic field; as the star rotates, it carries shapes as they try to balance short-range nuclear attraction the radiation beam along, creating a lighthouse effect com- and long-range Coulomb repulsion. About 300 m deeper into monly called pulsar emission. the neutron star lies the boundary of the neutron-star core; Neutron stars are extraordinary objects (see the article the density has increased to 1.8 × 1014 g/cm3 and supercon- by Lars Bildsten and Tod Strohmayer, PHYSICS TODAY, Febru- ductivity starts to become important. ary 1999, page 40). A typical one might have a radius of 12 km, The 300-m-thick region terminating at the core, often a density exceeding that of nuclear matter, a magnetic field called the mantle, separates the outer crust’s uniform crystal of 1012 gauss—two trillion times Earth’s field—and a rotation lattice from the core’s uniform liquid of nucleons, electrons, period of 0.1 s. Astrophysicists have spotted almost 2500 neu- and possibly more exotic particles. It is where nuclear pasta is tron stars and have learned that they come in many different cooked and served. Several research groups have carried out types, that their magnetic fields collectively span eight orders molecular dynamics simulations to ascertain the shapes of of magnitude, and that their spin periods span about six pasta matter at different densities. The exact shapes and tran- orders. sitions vary from model to model, and the astrophysics com- Despite decades of theoretical and observational efforts, munity has yet to reach consensus on the details. The models however, we still have a great deal to learn about neutron do agree that the pasta shapes become more complex as the stars. We don’t know, for example, the equation of state that density increases deep within the neutron star. Panel b of the relates the pressure and density in their interior; intuitively, figure shows the results of a representative simulation, which that equation tells how squeezable a neutron star is. Nor do finds gnocchi at relatively modest density and a complex, we know just what makes up such objects. In particular, we fresh strozzapreti kind of pasta at the base of the inner crust. don’t know what anomalous forms of matter exist at the high More than just pulsars densities and strong magnetic fields of neutron stars. The question of neutron-star composition has inspired research Presumably, all neutron stars are governed by the same equa- by physicists in many subdisciplines, including astrophysics, tion of state. In light of that presumption, the neutron-star nuclear physics, and particle physics. population has displayed a bewildering variety of physics— More than just neutrons so much that astrophysicists have established several classes of neutron stars. About 2000 stars are radio pulsars. The ra- Given their name, you might think that neutron stars are diation emitted by such pulsars ranges from radio to gamma- giant balls packed with neutrons. In fact, they are much more ray energies—the “radio” in their name notwithstanding— complicated. Outside the star is a large region—the magne- and it is powered by the pulsar’s rotational energy. tosphere—dominated by the star’s magnetic field. The mag- Magnetars, the strongest magnets in the universe, are ex- netic field lines permeating the region are anchored in a stiff tremely bright x-ray pulsars with fields typically of about crust, and they determine the motion of the charged particles 1014–1015 G. They exhibit peculiar flaring that generally lasts that fill the magnetosphere. Neutron stars might even have for 0.01–100 s but sometimes endures for years. Current an atmosphere, just a few centimeters thick, of hydrogen, he- theory posits that magnetar emissions are powered by decay lium, carbon, and possibly heavier elements. and instabilities of the star’s magnetic field rather than by Panel a of the figure shows the interior of a neutron star. rotational energy. The outer crust of the star comprises rapidly moving elec- X-ray dim isolated neutron stars (XDINSs) are x-ray pul- trons and mainly stationary nuclei—for example, iron-56— sars, probably old magnetars, that are relatively nearby— arranged in a Coulomb lattice of essentially point-like, within about 1000 light-years of Earth. They emit thermal 62 October 2015 Physics Today www.physicstoday.org Reuse of AIP Publishing content is subject to the terms at: https://publishing.aip.org/authors/rights-and-permissions. Download to IP: 145.18.108.179 On: Mon, 04 Apr 2016 09:54:59 a b ine sol rystall id, fre in c e el Neutron lei ect uc ro drip point N ns lei in cr h nuc ystall -ric ine on ns and n so tr ectro eutro lid eu e el ns , N fre DENSITY Anatomy of a neutron star. As the cross section Core (a) shows, the outer crust of a neutron star is made Density (g/cm3 ) 1.8×1014 8×1013 4×1011 109 up of nuclei in a crystalline lattice that is surrounded by free electrons. According to current theory, c 10−9 deeper into the crust, the nuclei get ever more neutron rich until, at the neutron drip point, neutrons start to travel unbound. Well beyond 10−10 the drip point, nuclei come together in a state of matter called nuclear pasta. The interior of the star is a superconducting fluid core. (b) This molecular 10−11 dynamics simulation shows that with increasing density, gnocchi-shaped nuclei arrange into a more complex pasta. (c) Neutron stars spin down as they −12 VATIVE 10 radiate. The data points here (from various sources) show the rate of spin down versus the period for three classes of neutron stars. The colored curves 10−13 give simulation results. One of the simulation variables is the extent to which crustal material is PERIOD DERI crystalline or disordered. Crystalline material 10−14 (yellow curve) allows for long periods, in conflict with observations. But if the crustal material is 10−15 highly disordered, the spin period is bounded. Magnetars The spin-period data thus represent the first X-ray dim isolated neutron stars Normal pulsars observational evidence for nuclear pasta. (Panel b Phys. Rev. C 70 10−16 adapted from C. J. Horowitz et al., , 0.1 1.0 10.0 065806, 2004; panel c adapted from J.
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