Lifting stellar structure and evolution to higher dimensions in the era of space asteroseismology

Project Information

4D-STAR

Grant agreement ID: 101071505

DOI

10.3030/101071505

EC signature date 20 December 2022

Start date 1 September 2023

End date 31 August 2029

Funded under

• European Research Council (ERC)

Total cost

€ 9 938 446,00

EU contribution

€ 9 938 446,00

Coordinated by

KATHOLIEKE UNIVERSITEIT LEUVEN
Belgium

Periodic Reporting for period 1 - 4D-STAR (Lifting stellar structure and evolution to higher dimensions in the era of space asteroseismology)

Reporting period: 2023-09-01 to 2025-02-28

Summary of the context and overall objectives of the project

Stars are the source of radiation, chemistry, and life in the Universe. Models of how stars live are key ingredients in planetary, astrophysical, and cosmological research. A star is a hot plasma rotating around an axis. Small stars like the Sun rotate slowly but bigger ones with more mass rotate faster, shaping them as flattened spheroids. Yet, current stellar models simplify the flattening or treat stars as spheres during their lives, using 1 spatial and 1 time dimension (1+1D). Rotation and magnetism induce transport processes in 3 spatial dimensions, which change over time, requiring a 3+1D treatment. Current age-dating of stars is done from 1+1D models, with uncertainties up to 1000%. Accurate ages of stars are the dominant missing ingredient to understand stellar and planetary evolution, the emergence of life, and the chemistry in our Universe.

4D-STAR will answer the fundamental question of how rotating spheroids evolve in time and build up their chemistry during their lives. We will develop a new 3+1D theory of stellar rotation for flattened spheroids evolving over millions to billions of years, from birth to death. Lifting stellar models to 3+1D can only be done now, using asteroseismic data of thousands of stars in all life phases. Such data reveal nonradial oscillations, or starquakes, allowing us to infer internal stellar rotation, magnetism, chemistry, and the ages of stars with 10% accuracy.

4D-STAR will provide open-source modules to compute the evolution of rotating magnetic stars in 3+1D, calibrated to asteroseismic observables of single stars and stars in binaries and clusters. 4D-STAR brings a paradigm shift based on mathematical modelling, astrophysics, and computational science. Its breadth, challenges, and goals require a transdisciplinary integration of four teams led by an asteroseismologist, a theoretician specialised in transport, a hydrodynamicist, and a stellar evolution software developer, each with proven track records.

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

For work packages 1 and 4, there has been significant progress in the development of new theories of transport processes for three dimensional stellar evolution and their implementation into simulations. A new mixing-length theory including convection and rotation has been developed and is being implemented into current simulation and stellar evolution codes. Additionally, theoretical studies to determine 3D transport processes in single magnetic rotating low-mass stars, treating the Coriolis acceleration and Lorentz force have been carried out. The results of these studies will be implemented into current stellar simulations and stellar evolution codes in order to compare the results with current observational constraints. 3D numerical simulations have also been developed in order to perform studies into the influence of many effects on the processes in the stellar interior. Results from these studies will continue to inform the development of new theories and be compared to current observational results.

For work packages 2 and 3, good progress has been made to gather the sample for the asteroseismic calibration of the new 3+1D stellar evolution code. Large data sets of pulsating stars are being gathered from existing space data and characterised using current asteroseismic modelling methods. These samples will provide the minimum performance requirements for the new 3+1D stellar evolution code, and a basis from which to improve beyond the current possibilities with stellar modelling. This work is being carried out on both individual stars, and in clusters, providing multiple methods by which the new 3+1D stellar evolution code can be calibrated.

For work package 5, a two pronged approach to building a new stellar evolution code has been taken. On the one hand, the existing ESTER code is being improved and expanded from 2+1D to 3+1D. Major achievements here include the implementation of new physics in the ESTER model to aid the modelling of two dimensional stars, the development of a software development kit for ESTER to allow ease of installation and use by new users, and the calibration of ESTER with current observables. On the other hand, there is the ground-up development of a new 3+1D stellar evolution code, to provide in independent model of stellar evolution from the pre-existing ESTER. This software is in a good state, with an installation framework set up, and new solvers and physics modules being developed for this new code. Both of these software development paths will aid and guide each other to reach the goal of successfully modelling stars in 3+1D.

For work package 6, there has been good progress in the development of a STEM module for high school students on the topic of asteroseismology. The module is currently being tested by high school teachers in the classroom. This will enable us to gain feedback and improve the module before public release.

Overall the project is in a good position to conduct further research to achieve the ultimate goal of a fully fledged 3+1D stellar evolution code.

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 results achieved in the first 18 months of the project are providing a solid framework, from which the future research of the project can grow. The necessary physics for the new stellar evolution model is developing well, and is already being implemented into codes that will be available for testing. The first 18 months of the project has also allowed all of the nodes of the SyG to form a cohesive plan for tackling the issues that will be faced in the project. As this SyG is still in its early stages, much more further research is necessary in order to reach the goals of the project. However, the progress so far is good.