The nuclear pasta phase

Neutron stars are what is left behind from core-collapse supernovae. These violent explosions are very energetic, and occur when the iron core of very massive stars, with at least eight solar masses, becomes unstable against gravity. Under a possible thin atmosphere, neutron stars have a crust. The outer crust is constituted by a lattice of nuclei in a gas of moving electrons. Towards the interior of the star, the nuclei become more and more neutron rich, and eventually neutrons start to drip out of the nuclei. The onset of neutron drip defines the lower boarder of the inner crust. A further increase of density towards the interior of the star may favour, at some point, the formation of clusters, which aren’t spherical as normal nuclei are, but instead have exotic shapes, due to a competition between the nuclear and Coulomb interactions. These clusters lie just below the homogeneous core of the star. They were termed “nuclear pasta” [Ravenhall-83] because they resemble the italian pasta. Ravenhall, Pethick, and Wilson were the first ones to study the basic properties of these exotic structures. These clusters also exist briefly in a collapsing stellar core during the formation of a core-collapse supernova, where a neutron star is born.

 Neutron star structure. a, Schematic slice through a neutron star. Letters N, n, p, e, μ refer to the presence of nuclei, fluid neutrons and protons, electrons and muons, respectively. The inner core composition is still uncertain and various exotic possibilities exist, including hyperons and deconfined quark matter. b, Detailed picture of the composition of the inner crust. At lower densities, a lattice of superheavy, neutron-rich nuclei is immersed in a fluid of neutrons (which are likely to be superfluid) and a relativistic electron gas. At high enough densities the nuclei might deform and connect along certain directions to form extended tubes, sheets and bubbles of nuclear matter. These nuclear pasta phases might form a layer at the base of the neutron star crust, sometimes referred to as the mantle; searching for observational signatures of such phases is of great interest. Ranges of density and thickness given for each layer represent current uncertainties in the physics of neutron star crusts.

Neutron star structure. a, Schematic slice through a neutron star. Letters N, n, p, e, μ refer to the presence of nuclei, fluid neutrons and protons, electrons and muons, respectively. The inner core composition is still uncertain and various exotic possibilities exist, including hyperons and deconfined quark matter. b, Detailed picture of the composition of the inner crust. At lower densities, a lattice of superheavy, neutron-rich nuclei is immersed in a fluid of neutrons
(which are likely to be superfluid) and a relativistic electron gas. At high enough densities the nuclei might deform and connect along certain directions to form extended tubes, sheets and bubbles of nuclear matter. These nuclear pasta phases might form a layer at the base of the neutron star crust, sometimes referred to as the mantle; searching for observational signatures of such phases is of great interest. Ranges of density and thickness given for each layer represent current uncertainties in the physics of neutron star crusts.
Courtesy of W.G. Newton

What are the observational evidences for the existence of these exotic structures? That is a question still under investigation, but recent studies, see e.g. [Pons13, Newton13], show some hints towards the existence of the pasta phases. Middle-aged pulsars, which are highly magnetized rotating neutron stars, with spins of up to about 10 seconds, should spin down rapidly to periods of up to 100 seconds, due to magneto-dipole radiation losses. Well, the only problem is that we haven’t observed these long-period pulsars. Pons, Viganò and Rea, have shown that this may be due to the fact that the pasta phase should affect the spin period of these pulsars, by limiting its maximum value, and thus giving us the first observational evidence of these geometrical structures, located in the inner crust of these objects.
There is currently considerable interest in searching for a better description of nuclear matter under exotic conditions, and more studies of this pasta phase of matter are still needed to fully understand this state of matter, and to get a better description of these exotic objects called neutron stars. For a review, see e.g, [Chamel08].

Evolution of the neutron density distribution with the density, for a fixed temperature and proton fraction. From left to right: droplets (gnocchi), rods (spaguetti), cross-rods, slabs (lasagna), tubes (penne), bubbles (swiss cheese). Each shape is shown at the correspondent onset density. Blue (red) color indicates the bottom (top) of the density scale.

Evolution of the neutron density distribution with the density, for a fixed temperature and proton fraction. From left to right: droplets (gnocchi), rods (spaguetti), cross-rods, slabs (lasagna), tubes (penne), bubbles (swiss cheese). Each shape is shown at the correspondent onset density. Blue (red) color indicates the bottom (top) of the density scale.

Author: Helena Pais

 

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