The fate of intermediate-mass stars: Investigating thermonuclear electron capture supernovae and their observational properties

• Applicant: Professor Dr. Friedrich Röpke
• Subject Area: Astrophysics and Astronomy
• Term: since 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 537700965

Stars in the intermediate mass range (~7 to 11 solar masses) fuse carbon non-explosively and produce high-density oxygen-neon (ONe) cores. The final fate of these stars is equivocal. Do they produce a large fraction of the neutron stars in the Universe? Are they important for the formation of objects of great astrophysical interest, such as binary millisecond pulsars or sources of gravitational waves? How do they contribute to the enrichment of galaxies with heavy elements? A well-studied scenario involves strong electron-capture reactions that destabilize the stellar cores and lead to gravitational collapse forming neutron stars. The heat released due to the electron captures, however, ignites explosive thermonuclear oxygen burning. The associated energy production counteracts the gravitational collapse and may lead to thermonuclear explosions instead. These "thermonuclear electron capture supernovae" have received very little attention, although stellar evolution calculations show that the central densities reached in the ONe cores likely favor a thermonuclear explosion. In preliminary work, we presented the first three-dimensional simulations of this scenario and analyzed its implications for galactic chemical evolution. While we found no fundamental reason to abandon the scenario, it remains a theoretical concept that awaits direct observational verification. This is the goal of the proposed project. Comparing single simulations to single observations is insufficient to prove the validity of astrophysical models. Therefore, in a first step, we aim at improving our three-dimensional hydrodynamic explosion simulations with updating the input physics. With this tool we will explore the parameter space of initial conditions such as the central density at the onset of the thermonuclear explosion and the geometry of the igniting thermonuclear flame inside the stellar core. This produces a comprehensive set of models. In a nucleosynthetic postprocessing step, we will determine the nucleosynthetic yields of the explosion and the exact chemical composition of the ejecta. In a second step we will perform radiative transfer simulations with two different codes that allow us to predict for the first time observables from the thermonuclear explosion scenario. These will be compared to astronomical data. The combination of both steps will enable us to assess the validity of the thermonuclear electron capture supernova scenario. The existence of such events would have tremendous impact on formation channels for systems involving neutron stars and on galactic chemical evolution.

• DFG Programme: Research Grants