Where in the universe are heavy elements synthesized? How are these elements produced?
These are two exciting and interdisciplinary questions in nuclear astrophysics today. Elements heavier than iron are synthesized by the slow (s-) and rapid (r-) neutron capture processes. The r-process produces half of the heavy elements up to bismuth and all the uranium and thorium observed in our solar system. Moreover, the r-process was the only contribution to heavy elements in the early galaxy. In the r-process, nuclei capture neutrons on time scales faster than beta decays. The necessary extreme neutron density is exactly what makes this process a big challenge. From the nuclear physics perspective, the nucleosynthesis path evolves along extreme neutron-rich nuclei far from stability. Therefore, few experimental information is available and theoretical models can be very uncertain. From the astrophysics perspective, the challenge is to find the environment(s) with high neutron density and where matter is ejected on short time scales. This points to explosive events involving the most neutron-rich objects in the universe: neutron stars. The favourite candidates are thus core-collapse supernovae, that mark the explosive end of the life of massive stars and the birth of neutron stars, and the merger of two neutron stars or of a neutron star and a black hole.
We work along two lines to understand the origin of heavy elements: astrophysics simulations of supernova explosions and compact binary mergers that probe nuclear matter at the extremes combined with nucleosynthesis calculations, which involve the most neutron-rich nuclei. The nuclear physics input and the evolution of the astrophysical environment are the most important ingredients of the nucleosynthesis calculations. The impact of the nuclear physics input is studied in our group in close collaboration with theory and experimental groups at GSI, TU Darmstadt, and other international institutions.