Mapping Stellar Helium

Project Information

CartographY

Grant agreement ID: 804752

DOI

10.3030/804752

Project closed

EC signature date 3 October 2018

Start date 1 April 2019

End date 30 June 2026

Funded under

• EXCELLENT SCIENCE - European Research Council (ERC)

Total cost

€ 1 496 203,00

EU contribution

€ 1 496 203,00

Coordinated by

THE UNIVERSITY OF BIRMINGHAM
United Kingdom

Periodic Reporting for period 4 - CartographY (Mapping Stellar Helium)

Reporting period: 2023-10-01 to 2025-03-31

Summary of the context and overall objectives of the project

Much of modern-day astrophysics is underpinned by the assumption that we have reliable models for stellar evolution in order to accurately ‘model’ stars. Unfortunately we have very little to prove this is the case. A well-known problem is that we rarely have a complete set of observations of any given star - from a small number of observations we must piece together the true picture. But, whatever observations we have, we do not have a direct measure of the initial stellar helium content. Initial He has a dramatic impact on inferred masses and ages of stars - e.g. an increase in initial He fraction of 0.05 for a Sun-like star will lead to a reduction of stellar MS lifetime of ~50%. Hence any quoted age is only valid for the adopted treatment of initial He and uncertainties quoted are a measure of precision not accuracy. This is not good enough for the many fields of astrophysics that rely on accurate stellar masses and ages.

In most real world cases progress has been made by tying initial He to initial metallicity but these relations are at best poorly calibrated, or at worst completely inappropriate. This systematic failure limits our understanding of our universe when we try to understand stars, the demographics of exoplanets, and the structure and evolution of our own Milky Way.

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

To face this challenge we have used a data-driven approach, using Bayesian Hierarchical Models (BHM’s), to leverage the exceptionally diverse and numerous observations of stars to solve the problem of initial He. BHM’s allow us to beat the curse of dimensionality that would otherwise render this task computationally impossible. By using multi-level BHM’s we will use the statistics of the population to map out the initial He of stars and then generate precise and accurate estimates of fundamental stellar parameters.

We have addressed a number of challenges of applying BHM's to the problem of stellar evolution. By far the most challenging aspect of the problem, is that using, models of stellar evolution is slow - very slow. We have used machine learning to speed up this step by producing a neural network emulator of stellar evolution. This step has reduced out computational time scale by 6 orders of magnitude and given us access to mathematical properties of our models that would otherwise be impossible to calculate.

By combining our neural network emulator with state-of-the-art probabilistic programming languages we have been able to simultaneously model the population and the individual properties of 80 stars. And this method we have scales well. Tests of artificial data suggest we could now deploy our method to over tern thousand stars with a run time of less than a single day.

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)

Our application has already led to much better constraint on stellar fundamental parameters and out constraint of the enrichment of Helium in our stellar neighbourhood. This constraint will only improve as we increase the number of stars we can process into the hundreds of thousands.

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Artwork from one of our press releases (A star spinning and opened up using asteroseismology)