The team, led by ELEMENTS’ Principal Investigator Achim Schwenk, focused on the interactions between protons, neutrons, and nucleons in the star’s crust. Their calculations demonstrated that protons, mixed with neutrons, help maintain the strange shapes of nuclear matter, refining the understanding of neutron stars’ composition.

This discovery helps improve theoretical models and allows for better comparisons with astrophysical observations, offering new insights into the mysterious nature of neutron stars.

Read more about the findings here.

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For the first time, scientists from GSI Helmholtzzentrum für Schwerionenforschung, TU Darmstadt and the Max-Planck-Institute for Astrophysics – among them ELEMENTS-PI Gabriel Martínez-Pinedo and Postdoc Oliver Just – have reported on a new possible process solving for the mystery of the p-nuclei’s existence. Their latest publication in Physical Review Letters describes the so-called vr-process: After the number of free neutrons to be captured drops, the absorption of neutrinos by heavy nuclei leads to the conversion of their bound neutrons to protons, causing a series of decay and capture reactions which results in the formation of the specific p-nuclei. There has to be clarified yet which type of stellar explosion could provide the suitable environment where such a process could happen. So far, one further publication claims that environments with strong magnetic fields could serve the purpose.

Image: NASA

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The aim of the School is to provide an overview of the CBM project, review of experimental data from RHIC, LHC and GSI experiments, to present current understanding of the physical phenomena relevant to the physics of nuclear matter at extreme conditions, as well as the theoretical expectations from the future experiments.

Click here for more information on this year’s School. Registration closes on 31 July.

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Read more about the Research Training Group here.

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At the S-DALINAC, Norbert Pietralla’s team has succeeded for the first time in measuring directly the reduction of energy consumption by a particle accelerator due to the reuse of previously applied energy in a two-fold energy recovery mode: Electrons were sent twice through the main accelerator for double acceleration by using recirculation beamlines. Following the double acceleration, a phase shift was implemented, so that the electrons were decelerated again during the following two main accelerator passes. The recovery of energy during deceleration allows higher beam currents and thus higher beam powers to be achieved. This development enables future large-scale projects, such as LHeC at CERN, which would not be technically and economically feasible without the energy recovery technology.

Further details on the realisation and the results achieved can be found in the publication in Nature Physics and this feature.

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Now physicists at Goethe University Frankfurt have succeeded in adding further crucial pieces to the puzzle. The working group led by Prof. Luciano Rezzolla at the Institute of Theoretical Physics developed more than a million different equations of state that satisfy the constraints set by data obtained from theoretical nuclear physics on the one hand, and by astronomical observations on the other. When evaluating the equations of state, the working group made a surprising discovery: “Light” neutron stars (with masses smaller than about 1.7 solar masses) seem to have a soft mantle and a stiff core, whereas “heavy” neutron stars (with masses larger than 1.7 solar masses) instead have a stiff mantle and a soft core. “This result is very interesting because it gives us a direct measure of how compressible the centre of neutron stars can be,” says Prof. Luciano Rezzolla, “Neutron stars apparently behave a bit like chocolate pralines: light stars resemble those chocolates that have a hazelnut in their centre surrounded by soft chocolate, whereas heavy stars can be considered more like those chocolates where a hard layer contains a soft filling.”

Crucial to this insight was the speed of sound, a study focus of Bachelor’s student Sinan Altiparmak. This quantity measure describes how fast sound waves propagate within an object and depends on how stiff or soft matter is. Here on Earth, the speed of sound is used to explore the interior of the planet and discover oil deposits.

By modelling the equations of state, the physicists were also able to uncover other previously unexplained properties of neutron stars. For example, regardless of their mass, they very probably have a radius of only 12 km. Thus, they are just as large in diameter as Goethe University’s hometown Frankfurt. Author Dr. Christian Ecker explains: “Our extensive numerical study not only allows us to make predictions for the radii and maximum masses of neutron stars, but also to set new limits on their deformability in binary systems, that is, how strongly they distort each other through their gravitational fields. These insights will become particularly important to pinpoint the unknown equation of state with future astronomical observations and detections of gravitational waves from merging stars.”

So, while the exact structure and composition of matter inside neutron stars continues to remain a mystery, the wait until its discovery can certainly be sweetened with a chocolate or two.

Publications:

Sinan Altiparmak, Christian Ecker, Luciano Rezzolla: On the Sound Speed in Neutron Stars. The Astrophysical Journal Letters (2022) https://iopscience.iop.org/article/10.3847/2041-8213/ac9b2a

Christian Ecker & Luciano Rezzolla: A general, scale-independent description of the sound speed in neutron stars. The Astrophysical Journal Letters (2022) https://iopscience.iop.org/article/10.3847/2041-8213/ac8674

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“Kilonova science is emerging as a new field in astrophysics, offering an enormous discovery potential for understanding neutron stars, the origin of the heavy elements in particular, the physics of exotic heavy nuclei, and the phases of hot, ultra-dense, and exotic matter”, Bauswein explains his research focus. “The increasing sensitivity of gravitational wave detectors, also providing improved sky localisations for follow-up observations, and the next generation of telescopes, means that we expect an abundance of new kilonovae observations in the coming years. I look forward to exploring the research field in the best possible way together with my colleagues within the framework of the ERC Synergy Grant.”

The research project HEAVYMETAL (How Neutron Star Mergers make Heavy Elements) aims to make a big step in explaining kilonova explosions by spectroscopically dissecting their emissions and connecting them quantitatively to the physical properties of the neutron star merger. In doing so, HEAVYMETAL will probe the origin of the heavy elements, and delineate the nuclear and astrophysical pathways that created them — the so-called “r-process”. The research team will try to decipher the details of the observed spectra and use that information to gain unprecedented insight into the physical processes of the neutron star merger.

HEAVYMETAL brings together experts from different fields related to kilonova research who, by working together, can exploit synergies in the ambitious goal of explaining element synthesis: Andreas Bauswein and his team at GSI/FAIR have a long and high impact track record in connecting advanced hydrodynamical simulations to r-process nucleosynthesis, kilonova modelling and the properties of high-density matter. Already in 2017, Bauswein succeeded in securing an ERC Starting Grant of 1.5 million euros with his project GreatMoves on the simulation of neutron star mergers. In addition to Bauswein, Professor Darach Watson, University of Copenhagen, Denmark, Professor Padraig Dunne, University College Dublin, Ireland, and Dr. Stuart Sim, Queen’s University, Belfast, UK, are also members of the research team funded by the ERC Synergy Grant.

Click here for further information.

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So far, only neutron stars were known to produce chargeless nuclear systems. Now, for the first time, Duer and colleagues were able to create the isolated four-neutron system at the Radioactive Ion Beam Factory at RIKEN (Japan). The key success factor was the employment of a knockout reaction at large momentum transfer with a radioactive high-energy ⁸He beam.

You can find out more about the experiment here in English or German.

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Congratulations to everyone involved on this outstanding achievement!!

If you would like to read more about the process, please click here. If you would like to know which role Black Holes play in ELEMENTS, you might find this video helpful.

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