Supernova matter in the Lab
A team of scientists from the Physics Centre of the Faculty of Sciences and Technology of the University of Coimbra (FCTUC) and the University of Caen, in Normandy, France, has identified the properties of matter artificially created "in the laboratory", with characteristics similar to those of matter arising from supernovae or the fusion of neutron stars.
The experiment was carried out at the GANIL (Grand Accélérateur National d'Ions Lourds) laboratory as part of the INDRA collaboration. It made it possible to produce matter similar to that produced in this kind of highly explosive event by the collision of a tin nucleus with a xenon nucleus. This experiment helps to better understand the conditions under which supernovae are born and evolve, and the fusion of neutron stars.
The results of the study have already been published in Physical Review Letters, a journal of the American Physical Society. According to Constança Providência and Helana Pais, from the FCTUC Physics Centre, "it allows us to know how the medium is formed in events such as supernovae or the fusion of neutron stars, and to determine how the energy is transferred between the different components, namely the energy deposited in the star by the neutrinos before they escape into the universe. In the case of neutron star fusion, this knowledge can indicate how much material is ejected and observed in the form of a kilonova".
Helena Pais has been in charge of analysing the experimental data that determine the interactions that take place in the matter resulting from this type of event, and the conditions under which there are still small aggregates before the matter becomes homogeneous due to the increase in density, because "at low densities, matter is not homogeneous and its properties determine the evolution of a supernova or the fusion of two stars", explains the researcher.
For a correct interpretation of the results, the theoretical model previously developed by Constança Providência and Helena Pais was also essential.
Neutron stars, along with black holes, are among the most compact objects in the Universe. Although they have a mass comparable to that of the Sun, between one and two solar masses, their radius does not exceed 15 km, much smaller than the Sun's radius, which is around 700,000 km. We can think of these stars as a giant atomic nucleus.
Neutron stars are born in very explosive events - supernovae. "This type of event releases more energy in a few days than the Sun does in an entire lifetime! It is now believed that the heaviest elements we know of, including precious metals such as gold and platinum, could be formed when two neutron stars collide," the researchers explain. They add that "describing either of these events requires knowledge of how stellar matter behaves, from very low densities to densities many times that of matter at the centre of an atomic nucleus".
These stars, which are essentially made up of neutrons, also contain another type of particle inside. "In addition to protons and electrons, which together with neutrons make up atoms, which are nothing more than the building blocks of terrestrial matter. It is also believed that several other types of particles and possibly new states of matter, some of which can be created and studied in particle accelerators, may exist inside these compact objects," explain Helena Pais and Constança Providência.
"Hyperons (particles similar to nucleons but containing strange quarks), Bose-Einstein condensates of pions or kaons (a special type of bosonic matter) and quark matter are some examples. Cold quarks, which are inaccessible in the laboratory, can also exist in different phases inside these stars, each phase with unique properties. This is why nuclear and particle physicists are so interested in studying neutron stars. Moreover, since these objects are very compact, they are also excellent laboratories for testing the theory of general relativity," they note.
The study was funded by the Portuguese Foundation for Science and Technology (FCT) and the COST PHAROS Action. Researcher Helena Pais received funding from the LPC - University of Caen, where she went on a mission to analyse the data.
Scientific Paper: “Low Density In-Medium Effects on Light Clusters from Heavy-Ion Data”, Helena Pais, Rémi Bougault, Francesca Gulminelli, Constança Providência, et al, Phys. Rev. Lett. 125, 012701 (2020), https://journals.aps.org/prl/pdf/10.1103/PhysRevLett.125.012701. Also at: https://arxiv.org/abs/1911.10849.
Translation by Diana Taborda