Turbulent and Explosive Lives of Massive Stars

Project Information

TEL-STARS

Grant agreement ID: 945806

DOI

10.3030/945806

EC signature date 20 August 2020

Start date 1 January 2021

End date 31 October 2026

Funded under

• EXCELLENT SCIENCE - European Research Council (ERC)

Total cost

€ 1 499 696,00

EU contribution

€ 1 499 696,00

Coordinated by

HITS GGMBH
Germany

Periodic Reporting for period 3 - TEL-STARS (Turbulent and Explosive Lives of Massive Stars)

Reporting period: 2024-01-01 to 2025-06-30

Summary of the context and overall objectives of the project

Stars are the basic building blocks of the visible Universe and have produced almost all chemical elements heavier than helium. Massive stars with luminosities of up to several million times that of our Sun are cosmic powerhouses. Their immense radiation, strong stellar winds and powerful supernova (SN) explosions helped to re-ionise the Universe after the Dark Ages, drive the evolution of galaxies and laid the foundation for life as we know it. Massive stars end their lives in spectacular SNe and leave behind neutron stars (NSs) and black holes (BHs) that open the door to physics inaccessible otherwise on Earth. NSs are made of ultra-compact matter and the so-called magnetars are supposed to host the strongest magnetic fields in the entire Cosmos. Thanks to large transient surveys and the detection of gravitational waves from merging NSs and BHs, massive star research shifted into the focus of modern astrophysics. An accurate understanding of the lives and final fates of massive stars is essential, yet large gaps remain.
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.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

The project progresses on all of the four main objectives. (i) We explore various stellar-merger situations with 3D MHD simulations. Simulations are ongoing and important insights could already be made. (ii) We have successfully developed the physical model and numerical setup to follow the evolution of grids of binary stars. We identified which initial binary configurations likely lead to mergers and observations support our findings. (iii) We have developed a semi-analytic model of neutrino-driven core-collapse SNe and successfully applied it to a wide range of modelled stars. (iv) A first emulator for stellar-evolution models employing a feed-forward neural network is ready and will allow for better comparisons of our models to observations.
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.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

The main results mentioned above are all state-of-the-art and partly beyond that. In particular, our prediction of characteristic BH masses from binary-stripped stars may have far-reaching implications. In the next years, gravitational-wave observatories will find many more binary BH mergers that can probe our theory. If further supported by the data, it may help constrain uncertain stellar and nuclear physics and could be used in measuring the accelerated expansion of the Universe.
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.

Birth of a magnetic massive star in a stellar merger. Colour indicates the magnetic field strength.