Report

Teaser, summary, work performed and final results

Periodic Reporting for period 2 - EUROPIUM (The origin of heavy elements: a nuclear physics and astrophysics challenge)

Teaser

The EUROPIUM project addresses the exciting question about the origin of heavy elements in the universe. This is a fundamental question, and major advances towards its answer are important for society for a general understanding of nature. The objectives of EUROPIUM are to...

Summary

The EUROPIUM project addresses the exciting question about the origin of heavy elements in the universe. This is a fundamental question, and major advances towards its answer are important for society for a general understanding of nature. The objectives of EUROPIUM are to simulate extreme astrophysical environments, such as core-collapse supernovae and neutron star mergers, and explore which nuclear reactions take place and which heavy elements are synthesized.

We simulate neutron star mergers and core-collapse supernovae following the evolution of the ejected matter. We estimate the variability in the conditions of this ejected material by taking into account: 1) astrophysical variations due to different initial conditions (e.g., mass of the stars, rotation, magnetic fields, …), 2) uncertainties in the simulation due to unknown microphysics (neutrinos and high-density equation of state) and to numerical methods (e.g., neutrino transport scheme). Based on these simulations, we calculate which nuclear reactions occur and which elements are produced. In addition, we estimate the uncertainties due to the unknown nuclear physics input used in our reaction network. Finally, we compare our calculations with uncertainties to the elemental abundances observed in the atmosphere of the oldest stars. We also calculate the kilonova light curve that is triggered by the radioactive decay of the freshly synthesized heavy, neutron-rich nuclei, which has been observed for the first time in detail after the neutron star merger GW170818 detected by gravitational waves.

Combining hydrodynamical simulations, nucleosynthesis, and observations, we will understand the role of core-collapse supernovae and neutron star mergers in the chemical history of the universe and advance our understanding of the origin of heavy elements.

Work performed

The project is divided in three work packages (WP1-3):

WP1: Long-time evolution of neutron star mergers and their ejecta. We have achieved results about the impact of neutrinos on the neutron-richness of the ejecta [Classical and Quantum Gravity 35, 034001 (2018)]. This clearly indicates that more effort is necessary in future simulations to consider neutrinos in the most appropriate way. Moreover, we have investigated the role of the equation of state (EOS) on the ejecta amount and composition [Phys. Rev. D 96, 124005 (2017)]. This was a timely publication (including kilonova light curves) just after the observation of the neutron star merger GW170817 and subsequent kilonova observation. Presently, we are starting the first tests to run our simulations for 100ms after the merger.

WP2: Multidimensional simulations of neutrino-driven winds. We have performed a unique and critical comparison of different neutrino transport schemes based on the same hydrodynamic code [J. Phys. G 46, 014001 (2019)]. This has guided us towards using an efficient neutrino treatment for a large set of simulations, combined with an accurate but slow treatment for few cases. We have achieved results for the first systematic study of the late supernova dynamical evolution and neutrino-driven wind with rotation. Moreover, the EOS effects have been explored in the explosion phase [submitted to Phys. Rev. Lett, arxiv:1812.02002] and are being studying for the neutrino-driven wind. Finally, in this WP we are also investigating explosions with fast rotation and magnetic fields.

WP3: Nuclear physics and astrophysics uncertainties on nucleosynthesis of heavy elements. The analysis of lighter heavy elements (from iron to silver) is almost completed with the first survey of astrophysical conditions in neutrino-driven supernova ejecta nucleosynthesis [Astrophys. J. 855, 135 (2018)] and a systematic Monte Carlo study to identify key reactions. Moreover, we have compared to observations within a collaboration with Heidelberg observers [Astron. & Astrophys. in press, arxiv:1812.07574]. For the heavy elements, we have investigated astrophysical and nuclear physics uncertainties in the production of thorium and uranium and we are comparing to observations.

Final results

Until the end of the project, the milestones of the three WP will be realized. Especially in WP2, most of the results or necessary code development are underway. In WP1, we expect several new and unexpected results, as now we can compare to the detailed kilonova observation and maybe even more observations become available until the end of the project. We will focus now on reproducing in detail the observed light curve(s) in order to learn more about neutron star mergers and the extreme physics in these environments. Combining the outcome of these two WP, we will have the most complete nucleosynthesis calculations for heavy elements within WP3. This will put EUROPIUM in a unique position to answer the long-standing question about the origin of heavy elements.