Rethinking the dynamical paradigm of low-mass stellar systems

All large galaxies, including our own Milky Way, contain hundreds of 'globular clusters'. Each is a dense group of about a million stars held together by their mutual gravitational attraction. Through a mathematician's eye, they correspond to the deep-rooted problem of understanding the motion of N bodies interacting by long-range forces. Through an astronomer's telescope, they appear as local relics of the ancient universe, as they were among the first stellar structures to emerge at the dawn of the formation of galaxies.

Observational astronomers have studied star clusters for decades, but in recent years the quality and range of these observations has leapt beyond the range of existing theory. In particular, the European space observatory Gaia is now measuring the positions and the velocities of thousands of stars in globular clusters of our Galaxy, with unprecedented accuracy. This new generation of data, coupled with measurements by the Hubble Space Telescope and other state-of-the-art and forthcoming astronomical facilities, enable theorists to explore, for the first time, the full 'phase space' of star clusters by studying both the position and velocity of their individual stars.

Theorists' models are based on simplifying assumptions, and at present almost always include the ideas that the clusters are spherical, do not rotate, and are composed only of stars - all born at the same time. Three recent developments are undermining these assumptions and requiring us to treat the clusters as they are, and not as we would wish them to be: (i) the realisation that the motions of their stars is much more complex than we expected and that their internal rotation is the rule rather than the exception; (ii) the empirical evidence that the stars in clusters were not all born at once in a single population (an assumption which was once part of their very definition); and (iii) the recent discovery of a numerous 'hybrid' stellar systems which now makes arduous to trace the distinction between 'star clusters' and 'dwarf galaxies'.

In addition, star clusters have recently been recognised as prolific cradles for small, 'stellar-mass', black holes and they have been long speculated to be the ideal birth site of larger, 'intermediate-mass', ones. Currently undetected, this class of astrophysical objects is a crucial missing link in the population of cosmic black holes. But new facilities such as the Laser Interferometer Gravitational-Wave Observatory and the Kamioka Gravitational Wave Detector will soon help us tackle this open problem from a new perspective.

With a combination of mathematical techniques and numerical simulations, I aim to propose a more realistic description of the dynamics of star clusters, to interpret such new-generation astronomical data. As a result, this research programme will allow me to address three open problems in modern astrophysics: the origin of stars which are among the oldest in the universe, the possible existence a 'missing link' in the population of cosmic black holes, and the limits of the presence of invisible 'dark' matter in small stellar systems.

A fundamental understanding of the history and evolution of these intriguing stellar systems will, therefore, provide a new perspective on the first building blocks of galaxies and on the origin of our Milky Way.

The programme of research that is the subject of this application will have an impact on audiences well beyond my immediate academic community. Two primary channels are envisaged for this purpose: knowledge transfer and public societal engagement, with immediate benefits for the civil society and wider public and longer-term advantages for the private sector.

The proposed research will provide a rich source of scientific projects well-suited for PhD theses, with an emphasis on data-intensive astronomy and mathematics. During the course of the programme, I intend to offer doctoral projects within two new Centres for Doctoral Training (CDT) associated with my host institution - in 'big data' astronomy (STFC ScotDIST) and data-intensive mathematics (EPSRC MAC-MIGS), respectively.

The University of Edinburgh is currently undergoing a phase of rapid expansion of its teaching and training programmes in data-intensive science in view of its contribution to the 'Data-Driven Innovation' segment within 'Accelerating Growth' - the 'City Region Deal' for Edinburgh and South East Scotland, secured in Summer 2017. The aim of this initiative funded by the UK and Scottish Governments is to establish the region as the 'data capital' of Europe over the next 15 years, attracting investment, fuelling entrepreneurship, and delivering inclusive growth while reducing inequalities and deprivation. The Higgs Centre for Innovation, which was inaugurated at the Royal Observatory of Edinburgh in May 2018, will play a major role in this initiative, both as a 'business incubator' for start-ups and as a student placement environment.

The PhD projects I foresee will therefore directly contribute to several dimensions of the 'City Region Deal' agenda. Both CDTs envisage 6-month student placements with industry as part of their 4-year training, which will naturally create, on the medium and long-term, a vehicle for exploitation of the results of the proposed research and for collaboration with the private sector.

More generally, I strongly subscribe to a culture of scientific openness and sharing. I have defined a management plan so that the data and the tools developed in this programme will be accessible to the broader astronomical research community and beyond, and I will make provisions to facilitate their re-use by external users.

In addition, several activities to increase the understanding of science among the general public and a broad range of stakeholders are proposed in this application. In the design of the dissemination strategy, special attention has been paid to the integration with initiatives and networks already existing within the host institution and the City of Edinburgh. This will allow maximising the audience that will be engaged through the proposed activities.

Among other actions I have detailed in a public societal engagement plan, one event will particularly target young female students in secondary schools, with the goal of (i) promoting careers in science, by presenting positive role models (ii) spreading the awareness, among teachers and students, about a number of social and psychological factors that might affect the performance of students in scientific subjects (e.g., unconscious bias), and (iii) suggesting effective techniques to minimise their effects in classroom settings.

I also intend to prepare a masterclass presenting an overview of the background and the new results obtained in this programme, in a form accessible to pupils at the National 4/5 exam level. This event will be developed with the help of an outreach team at the University of Edinburgh which is proficient in the design and delivery of innovative science experiences for 10-14-year-olds, complementing their school curricula. Both activities will be offered in targeted schools in South East Scotland and it will directly contribute to fulfiling the vision of inclusive growth of local authorities outlined by the 'City Region Deal'.