Cosmic Impact of Massive Stars: Convective Mixing and Mass Loss

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 upset
the 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 recently
opened 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 nuclear
physics 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 will
improve 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 in
massive stars, with wide ranging applications and impact in astrophysics.

The members of our cosmic impact of massive stars (CIMS) consortium and their groups have strong track records in delivering impact through industrial engagement, education and outreach.

CIMS members and their groups have highly productive collaborations with industrial partners in a variety of fields, ranging from computer technology, through medical imaging to homeland security and nuclear energy. Moreover, the multi-disciplinary nature of our CIMS consortium affords our students and PDRAs broadened training opportunities. They will benefit from our industrial links, particularly in software optimisation alongside experts at Numascale. Designing and optimising advanced simulation codes, and Big Data mining as required by our massive datasets, are aligned with UK economic needs, so this training ensures the marketability of our students in diverse career paths.

Our CIMS Consortium is cognisant of its role in inspiring and training the next generation of scientists responsible for ensuring the UK's international competitiveness in both the academic and industrial sectors, so our outreach and public engagement activities are a key aspect of our mission. Our investigators and their institutes undertake a spectrum of
activities. With CIMS science spanning scales from the very small (nuclei) to the very large (stellar scale and beyond), and utilising state-of-the-art technology, from large scale high-performance computing facilities to generate evolving simulations of astronomical objects like stars (Project A), to international accelerator facilities such as CERN to study nuclear reaction rates, our consortium has a unique opportunity to capture the imagination of non-scientists.

CIMS will bring a greater breadth and depth to these activities through its interdisciplinary composition, and will add value through dedicated "Impact" sessions which will be scheduled as part of our regular consortium meetings, promoting best-practice in our efforts to maximise impact and facilitate knowledge exchange between the institutes and, in a wider context, with stakeholders and the general public.

A more detailed explanation of our impact capacity and plans is outlined in the attached "pathways to impact" document.