Periodic Reporting for period 2 - TEL-STARS (Turbulent and Explosive Lives of Massive Stars)
Berichtszeitraum: 2022-07-01 bis 2023-12-31
Most stars are not alone but have a companion with whom they orbit in a binary-star system. While stars evolve, they grow in size and mass transfer from one star to the other may begin. In about a quarter of cases, such a phase leads to the coalescence of both stars. Magnetic fields are generated in mergers, possibly explaining the more than 70-year-old mystery of the origin of strong, surface magnetic fields in some massive stars. When magnetic massive stars explode in SNe, they may form highly-magnetic NSs, thereby offering a solution to the origin of the strongest magnets in the Universe. It is unclear whether and how SN explosions are affected by previous binary mass exchanges. For example, the famous SN 1987A is best explained by a merger product, and also Eta Carinae, a star that became the second brightest star in the night sky during its great eruption in ~1840, might be the result of merging stars. For these reasons, we conduct the first three-dimensional (3D), magnetohydrodynamic (MHD) simulations of stellar mergers. They will allow us to better understand these complex physical phases and emerging phenomena such as strong magnetism. Furthermore, we make detailed models of various binary mass-transfer phases, map out the binary-star parameter space that leads to stellar mergers and develop computationally cheaper, simplified merger models. We can then follow the evolution of a large set of models up to the SN stage to better understand the diversity of SNe and other transients. Every model must be checked against observations for validation. To this end, we will develop machine-learning techniques to compare our models to observations of stars and be able to use the observations to inform the modelling. The project will open new possibilities for massive star research, solve long-standing questions and offer plentiful extensions.
A central question is to understand which stars explode in SNe and which collapse into BHs. To this end, we have systematically studied the evolution of massive single and binary stars from birth to the pre-SN stage and the outcome of the ensuing core collapse. A picture emerges in which stars of specific core masses explode in SNe while they collapse into BHs for others. The emerging explosion landscape suggests a bimodal BH mass distribution and that stars collapsing into BHs as red supergiants have a narrow range of luminosities. The latter agrees with observations of a red supergiant that collapsed into a BH. We find significant differences in the core structures of single and binary stars. For example, envelope stripping in binary stars suppresses BH formation and leads to more energetic SN explosions. It further implies that BHs of universal masses form regardless of the initial chemical composition of stars. Gravitational-wave merger events of binary BHs show evidence for the existence of such universal BH masses. In mass-accreting binary stars and stellar mergers, long-lived blue supergiant phases occur. Such stars can explode in SNe with extremely high ejecta masses and others do not explode but collapse into some of the most massive BHs.
In collaboration with another group, we study the intricate common-envelope phase of binary stars. In this phase, a giant engulfs a smaller star, and a large fraction of the giant's envelope might be ejected. For the first time, we reached full envelope ejection in a simulation and observed magnetically-driven bipolar outflows. The simulation explains how some iconic planetary nebulae formed and why their shapes are so asymmetric. We further expanded our work to common-envelope phases with massive stars, which is key in the evolution towards gravitational-wave merger events. We utilised these simulations to develop a simplified model for use in stellar evolution codes. The first results give hope that a predictive theory for common envelope events in stellar evolution codes might be within reach.
We could successfully apply our gained knowledge to problems beyond our planned goals. In Hirai et al. 2021, we found a way by which an initially stable triple system becomes unstable and induces a stellar merger. The merger outcome closely resembles the Homunculus nebula of Eta Carinae. Again involving stellar mergers in triple-star systems (Stegmann et al. 2022), we showed how one can explain the small mass ratios observed in some binary BH mergers. Thanks to our expertise, we were able to help interpret the first discovered stellar-mass BH around a massive star outside the Milky Way Galaxy (Shenar et al. 2022). Theory predicts that a few per cent of all massive stars should have such a dark companion–searches are ongoing!
We expect to finish our exploration of 3D MHD simulations of stellar mergers. The simulations help us understand this complex phase and can later be used to make simplifying models of stellar mergers. These can then be used to explore the properties of a large set of merged stars. Moreover, we started the planned exploration of seismic fingerprints of merged stars and found some promising first results. The work on stellar emulators and a Bayesian framework to compare our models to observations has also begun.