STARs at the EXtreme

Project Information

STAREX

Grant agreement ID: 833925

DOI

10.3030/833925

Project closed

EC signature date 29 October 2019

Start date 1 January 2020

End date 31 December 2024

Funded under

• EXCELLENT SCIENCE - European Research Council (ERC)

Total cost

€ 2 500 000,00

EU contribution

€ 2 500 000,00

Coordinated by

UNIVERSITE DE GENEVE
Switzerland

Periodic Reporting for period 4 - STAREX (STARs at the EXtreme)

Reporting period: 2024-07-01 to 2024-12-31

Summary of the context and overall objectives of the project

The project STAREX has investigated the properties of stars showing extreme properties. We mean here stars showing the lowest initial metal content (down to zero metallicity), and/or the highest initial masses (up to one million solar masses), and or the highest initial rotations (up to rotations rates driving chemically homogeneous evolution), and/or by their initial content in the dark matter under the form of Weakly Interactive Massive Particles, and/or showing very high accretion rates during their formation process (up to 1 million solar masses per year). These properties are very relevant for describing the evolution of the first stellar generations in the Universe and, their radiative, chemical, and mechanical feedback in galaxies. Due to their short lifetimes (a few million years), the first massive, very massive, and supermassive stars have now disappeared from the present-day universe. Indirect observational constraints are however available as the observations of long-lived extremely iron-poor low-mass stars whose surface composition shows signs of the composition of the ejecta of these first stars, the constraints on reionization in the high redshift universe, the apparition of supermassive black holes (up to 1 billion solar masses) while the Universe was still very young (less than 1 billion years old), the observation of regions of high redshift galaxies with very high nitrogen-to-oxygen ratios indicating a rapid enrichment in a material having been processed at least by CNO burning. The project STAREX has addressed all these questions exploring the impact of normal and extreme stars. We have also improved the physics of our stellar models, exploring the consequences of new mass loss rate recipes, new nuclear reaction rates, new ways of implementing convection in stellar models based on 3D hydrodynamical modeling, new approaches for implementing the transport of the chemical species and of the angular momentum in massive stars based on the most recent constraints coming from asteroseismology. STAREX has produced new predictions for the impact of extreme stars on reionization, on the chemical evolution of galaxies, and on the formation of supermassive black holes at high-redshift. It has allowed 4 students to get their PhD. The STAREX project has produced more than 130 publications during the five years of its duration.

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

During the 60 months of the STAREX project, 134 papers in refereed journals have been published or are in press. Among the most significant results, we can cite: A new approach for implementing results of 3D hydodynamical simulations in 1D stellar evolution models has been found (see e.g. papers 120,121,130). We have explored the impacts of a more consistent approach for accounting line-driven stellar winds in hot massive star models (e.g. 97,127) . We have improved our stellar evolution code to allow the code to describe the ultimate phases of the evolution before the core collapse (135) . A great part of our effort was also devoted in finding the best approach to implement the physics of the transport of chemical species and of angular momentum in stars using the guidelines given by the asteroseismic constraints (e.g. 77,85,92,94). A new criterion indicating when the General Relativity instability triggers the collapse of supermassive stars has been found (14). Also new upper mass limit of stars formed by accretion has been determined using pulsation analysis (106). We studied the potential impact of Weakly Interactive Massive Particles (WIMPS), a candidate for explaining dark matter in the Universe (105). We also investigated how the energy produced by wimp disintegration may affect the evolution of supermassive stars. We also combine the physics of WIMPS with that of rotation to show that stars with WIMPS may actually reach complete homogenization due to the fact that the excess energy coming from wimp disintegrations may prolonge sufficiently the star lifetimes to allow even a moderately efficient mixing process to have a dramatic impact on the evolution of the chemical composition of the star (not yet published). We computed the statistics of close binaries born in the first stellar clusters formed at very high redshifts in mini haloes (6). We also estimated the impact of the first stars on the reionization accounting for the effects of fast rotating stars evolving chemically homogeneously (68). We investigated also the impact of the first stellar generations in producing material allowing to explain the carbon-enhanced metal-poor stars (27), the regions with a high nitrogen-to-oxygen ratio in high redshift galaxies (88). We also made predictions of the impact of chemically homogenouslThe results were disseminated in participations at conferences. Each member of the STAREX team could participate to 3-4 conferences, maiking a total participation to about 40 international conferences. We organized one international conference in Geneva on supermassive stars and a Summer School on the AMUSE library of Astrophysical Codes. Two press releases were published.

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)

We list the main progresses reached through the STAREX project:
- First models including the impact of the convective boundary mixing have been obtained in collaboration with the Keele team.
- First consistent comparisons have been performed between models and observations to deduce empirically the size of the convective cores in initial masses between 9 and 25 solar masses.
- A new approach for implementing the effects of the Tayler-Spruit dynamo has been obtained that can provide a reasonable fit to the most recent asteroseismic constraints on the internal rotation of the Sun, sub-giants and red giants.
- We showed the impact of rotation and mass loss on the evolution of very massive stars at very Low Metallicity (stars with masses between 100 and 300 solar masses).
- A new method has been proposed to determine the maximum mass of supermassive stars formed by accretion as well as the impact of rotation on these maximum masses.
- We suggested a new scenario for the formation of the CEMP-no stars invoking an enrichment due to possible stellar mass loss processes affecting Pop III stars.
- We studied the binary statistics in the first stellar clusters in the Universe with a new approach.
- We proposed a new method for studying the link between the surface properties of a star and its internal structure based on what we called Snapshot models.
- We showed the impact of stars evolving homogeneously on the ionizing power of the first stars, on the UV luminosity of high redshift galaxies and on the 21 cm emission.
- We suidied the potential impact of WIMPS in normal and in supermassive stars.
- We made new estimates for the production of primary nitrogen in Pop III and in very metal-poor stars. We also did a study focussing on the origin of fluorine in the Universe
- Our stellar evolution code GENEC has been significantly improved thanks to that project allowing it to study very different implementations for the physics of the transport processes in stars. Thus the end of STAREX is also the beginning of a new chapter.

STAREX offers a global, systematic, innovative, and consistent approach to the study of the first st