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STARs at the EXtreme

Periodic Reporting for period 1 - STAREX (STARs at the EXtreme)

Reporting period: 2020-01-01 to 2021-06-30

The project STAREX investigates the properties of the first generations of stars. The first generations of stars are expected to have been massive and even extremely massive stars. Due to their short lifetimes (a few million years), they have now disappeared from the present-day universe. It is not impossible however, that with new facilities as for instance the James Webb Space telescope, and new very large ground facilities as the European Extremely Large Telescope, or the Square Kilometer Array, we may be able to see some observable signature of these very first generations of stars. Our aim is to obtain new predictions for these potentially observable features by making significant advances in the physics driving the evolution of stars in general and of the first stellar populations in particular. The strategy to improve this physics is simple (although challenging): to build stellar models with a physics that allows to reproduce key observable features shown by present-day stars. Then to use this physics for computing models with an initial composition that corresponds to Pop III or very metal poor stars and see the consequences. Of course, we cannot be sure that the same physics operating now in stars was the one operating in the first generations. But we think that this is certainly the first reasonable assumption to be explored. Indeed, how to model stars that we cannot see if the physics used cannot reproduce the stars that we can observe? From the point of view of the improvement of the physics of the models, the STAREX project is focused on the transport processes of the angular momentum and of the chemical species in stars. These transport processes change all the outputs of the stellar models. These transport processes affect single star and stars whose evolution is impacted by their interactions with nearby companions. Nowadays thanks to technics like asteroseismology and the large spectroscopic surveys we have access to an incredibly rich amount of data that not only provide information on what does happen at the surface of stars but also in their interiors. STAREX takes profits from these observations to constrain the physics of the models. This is the topic of the Work Package (WP) 1. A present enigma that we address in STAREX is the origin of the extremely massive black holes, up to 1 billion of solar masses, that can be observed at very high redshift at a time when the Universe was hardly 1 Gyr old. In STAREX we want to study the formation of these supermassive black holes through the formation of supermassive stars. This is the topic of the WP 2. STAREX wants to investigate the possible impact of various dark matter particles on the evolution of the first generations of massive and supermassive stars. This is the topic of the WP 3. In the WP 4, STAREX wants to study through dynamical models the evolution of the first stellar clusters and make predictions for the binary statistics, nucleosynthesis, and produce integrated spectra. Finally, in the WP 5, STAREX wants to study the different types of energetic transients (supernovae or gravitational wave events) that result from Pop III and very metal poor populations. The STAREX team is now constituted of four PhD students, of 3 post docs (one working at 20%, one at 50% and one at 100%) and three senior permanent staff. This next October we shall welcome in the team a fourth post doc for helping us to explore the dynamics of stellar clusters and the impact of close binary evolution, stellar collisions and mergings.
During the first 18 months, 30 papers in referee journals have been published or are in press. In WP 1 we aimed at improving the physics of the transport processes in stars beyond the present state of the art. This has produced 15 papers. This WP is the heart of the STAREX project since the quality of all the other WPs depends on the quality of the physics of the stellar models. In the WP 2, we studied the formation of supermassive black holes by direct collapse. We published 4 papers aiming at obtaining more reliable maximum mass of the SMSs and studying the physics of accretion. The aim of the WP 3 is to study the potential impact of different candidates for dark matter (axions, weakly interactive massive particles). In one paper we investigated different solar models (without any dark matter included in this study) to compare their predictions with respect to gravity modes and neutrino fluxes. This is a prerequisite for deducing constraints on possible dark matter candidates from solar models. The WP 4 focuses on the investigations of populations of first generations stars. Four papers have been so far published along this line of research studying binary statistics, ionizing power, nucleosynthesis coming out from populations of Pop III stars. In the WP 5 our aim is to use the end point of the evolution of massive stars to shed some light on the first stellar generations. As indicated above, when a Pop III star explodes as a core collapse supernova there is some chance that we can observe it. Five papers have been published so far along this line of research. We studied the maximum mass of black holes that can be formed by single stars, we also proposed a new method that hopefully will allow us to produce a unique library of supernovae progenitors.
We list the progresses made so far beyond the present state of the art:
- First models including the impact of the convective boundary mixing have been obtained in collaboration with the Keele team.
- Some recent published formalism for improving the transport of angular momentum in radiative zones has been tested. We showed that a detailed comparison with asteroseismic data still shows significant discrepancies that need to be addresses
- 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 method has been proposed to determine the maximum mass of supermassive stars obtained 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, at the moment still speculative, stellar winds of 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. This method will be used to produce a pre-supernova library that will be unique and exhaustive in term of the different internal structures that it will provide associated with light curve evolution and spectral evolution.

The main aims that we want achieve until the end of the project are: To Identify the physics that is still missing for reliably describing the transport of angular momentum, to predict observational signature of supermassive stars. to explore the impact of candidate dark matter particles on the evolution of Pop III massive and supermassive stars, to predict observational signature of the first stellar populations, to predict the frequency and properties of the energetic transient events from the first stellar generations.
STAREX offers a global, systematic, innovative, and consistent approach to the study of the first st