Cosmic Impact of Massive Stars: Mass Loss and Mixing

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The aim of this Consortium proposal is to improve numerical calculations of the most massive stars in the Universe. Massive stars play a key role through the light they shine and the chemical elements they produce. At the end of their lives, their cores collapse into black holes which, because their gravities are so huge, even light cannot escape from. The lives of massive stars and the related chemistry relate to several key STFC roadmap questions. ``Question A: How did the universe begin and how is it evolving?'': Stars can be used to probe the Universe and its evolution from its infancy, and indeed, the first stars formed only 400 million years after the Big Bang. Even if most of those stars are long dead, their chemical fingerprints are stored in long-lived low-mass extremely metal-poor stars that survive today. Comparing our models to observations of these metal-poor stars provides information about the properties of the first stars and galaxies (question A4) and, more generally, this proposal addresses key questions A5 and A6: ``How do galaxies and stars evolve?''. Stellar models are also used as a theoretical framework for the interpretation of large observational surveys, such as the VLT-Flames survey of massive stars, and to study as-yet unexplained observations. For example, our models weighed the most massive stars discovered to date, and the masses determined (up to 320 solar masses at birth) drastically upsetthe previous upper mass limit of stars. Furthermore, our stellar models will be the main input for supernova simulations. Thus this project also addresses the question: ``D: How can we explore and understand the extremes of the universe?''. Stellar models also provide input for the recentlyopened window of gravitational waves (``D2: gravitational waves properties'') by constraining the masses of gravitational-wave emitting "heavy" black holes. Mass loss is a key process that determines the final mass of massive stars as they lose more than half of their initial constituents via powerful stellar winds.Stellar models provide a crucial theoretical framework for interpretation of data from experimental and observational facilities representing billions of pounds of investments. Theory, however, is lagging behind progress on the observational/experimental side, limiting scientific progress. The stellar models developed in our proposal will provide a crucially needed improved theoretical framework for interpretation of STFC-funded facilities, will make use of STFC's computing facilities, and connect them to STFC-supported nuclearphysics facilities. Our work will thus maximise returns on STFC investment.In this proposal we focus on the two key processes, the uncertainty of which is crippling the predictive power of stellar models, for which great progress can be made in the next three years and for which we have world-leading expertise: mixing (Project A: PI Hirschi) and mass loss (Project B: PI Vink). We request 2 PDRAs distributed across the two projects, each improving the treatment of a key process in massive-star models. With the improved treatment of mixing and mass loss, we willimprove predictions of the cosmic impact of massive stars. In particular we will make predictions concerning hot topics such as final masses of very massive stars linked to gravitational wave emission, compactness of supernova progenitors and the possibility of pre-explosive activity. The improved treatments will be implemented in an open-source stellar modelling code and are likely to become the new standards for mixing and mass loss inmassive stars, with wide ranging applications and impact in astrophysics.