Strange particles in the heart of magnetic stars

Magnetars, neutron stars with strong magnetic fields, are among the most extreme things in the universe. Its magnetic field is so strong that it will affect its composition and, in so doing, its radius or evolution. Due to the intense density at the core of the magnetar, the material exists in a particularly strange shape. But the models you describe are perfect. And so Kawan Marques, of the Federal University of Santa Catarina, in Florianópolis, Brazil, and his colleagues studied the effect of the presence of rare particles, delta baryons, at the cores of magnetars.

The extremely massive star has a short life during which it burns the hydrogen and helium it contains at breakneck speed. The energy released by thermonuclear fusion reactions creates enough pressure to balance the mass of the star. But when the fuel runs out, the star collapses in on itself and explodes in a supernova: the outer layers are violently expelled as the core collapses and forms a neutron star (or sometimes a black hole).

During the formation of a neutron star, the velocity of the primary star’s rotation and its magnetic field increase by several orders of magnitude. If the final rotation period is on the order of a second or less, then we are talking about a pulsar. But if the initial star already has a large magnetic field, then the magnetic field of the final object can reach 1011 Tesla on the surface: it is then called a magnetar. In comparison, a typical neutron star has a magnetic field on the order of 10 .8 The sun’s average tesla is about 10 – 4 Tesla, and those of Earth are only 50 microtesla.

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Under the influence of extreme density, matter takes an atypical shape in neutron stars. A large part of the electrons fuse with the protons of an atom’s nucleus to form neutrons. Then matter consists of neutron-rich nuclei, even from a fluid of neutrons, protons, and electrons. But in the center of the star, the pressure is so strong that it allows more exotic particles to emerge, which we only encounter on Earth in high-energy collisions inside particle accelerators. These particles, like the proton and neutron, belong to the baryon family, that is, they are made up of three quarks (a proton is made of two quarks). a quark ; neutron, two and one a).

Until now, specialists in their models of neutron stars considered baryons related to the proton and neutron (which have a spin equal to 1/2 and are grouped into a group called the “octet”). One question that arises is the effect of the magnetic field on this exotic material. In 2015, Deparati Chatterjee, of the IUCAA Institute, in Pune, India, co-author of this new study, with Jerome Novak and Michaela Ortel, of the Laboratory of Universe and Theories (Loth), in Paris, studied the impact of a new study. Intense magnetic field on a liquid of neutrons or detached quarks. This effect was found to be weak.

Kauan Marquez and colleagues took into account in their model a new particle, delta baryons, which have a rotation of 3/2 and which belong to a “decuplet.” According to the researchers, these particles should have a low probability of appearing inside the exotic matter of neutron and magnetic stars, because they are barely 30% heavier than the proton and neutron, and are even lighter than some spin 1/2 baryon. . Another interest for delta baryons is their high “anomalous magnetic moment,” an intrinsic property that makes them more sensitive to the magnetic field. Thus, matter containing delta baryons must interact more strongly than a simple liquid interaction of neutrons with a magnetic field, particularly in a magnetar.

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To include delta baryons, the Cowan Marquis team used two models to describe the formation of a neutron star on a microscopic scale. In some cases, the delta baryons exceed 10% of the content of the star’s core (with or without a magnetic field). But the researchers had difficulty studying the effects of the presence of these baryons on the properties of the magnetar: the magnetic field distorts the star so much that the classical equations derived from general relativity (the Tolman-Oppenheimer-Volkoff equations), which assume a spherically symmetrical geometry, cannot be used. So the researchers used the Einstein-Maxwell equations, which are difficult to solve.

With what result? According to these new calculations, magnetars enriched with delta particles have, for a given mass, a smaller radius than magnetars without these particles. “The team showed that magnets can indeed contain delta baryons, Jerome Novak points out. But most interestingly, their magnetic simulations are compatible with the constraints of observation. Moreover, the mass of such enriched magnets can be up to two solar masses,” he insists. Jerome Novak said, “This result is not easy to get. In general, when we add the probability of the particle’s appearance, it “dilutes” the equation of state and reduces the maximum accessible mass, which is often below the observed threshold for two solar masses. Another step to understand these strange things.

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About the Author: Irene Alves

"Bacon ninja. Guru do álcool. Explorador orgulhoso. Ávido entusiasta da cultura pop."

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