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Research Fellows Directory

Alice Zocchi

Dr Alice Zocchi

Research Fellow


University of Surrey

Research summary

Globular clusters (GCs) are compact groups of about a million stars, held together by their mutual gravitational attraction, and characterised by high density in their central regions. They are among the oldest stellar systems we know, and as such they retain information on the properties of the early Universe. Studies of their structure and dynamics provide invaluable insights to understand not only their evolution, but also the formation and evolution of their host systems. GCs are nearly gas-free systems, characterised by an apparently simple geometry, with finite size probably determined by tidal truncation: these unique properties make them excellent laboratories for studies of stellar dynamics, and ideal systems to analyse with N-body simulations. For a long time, they have been approximated as spherically symmetric, non-rotating, isotropic systems. The spherical King models (King 1966), constructed to match this physical picture, are usually considered as the zeroth-order dynamical reference model, and are sometimes successful in representing the observed characteristics of these systems.

In reality, this simple physical picture fails to capture the complexity of GCs and suffers from a number of limitations, which become more evident now that much improved observations are available, pointing out the need for more realistic models. For example, we now know that GCs harbour multiple stellar populations (Carretta et al. 2009; Gratton et al. 2012), and in some cases there are dynamical signatures for the presence of intermediate-mass black holes in their centres (Luetzgendorf et al. 2013, Zocchi et al. 2017). New data also confirm that for some GCs internal rotation plays a major role (Bellazzini et al. 2012), and that they may exhibit significant deviations from spherical symmetry (Chen & Chen 2010).

A proper identification of the main physical ingredients (rotation, pressure anisotropy, tides, ...) that shape the internal dynamics of GCs and that determine their current complex properties will allow us to draw conclusions on their origin, and on the origin of their host systems. This is indeed the purpose of my current research activity.

The European Space Agency's cornerstone mission Gaia is now measuring the distances, positions and velocities of thousands of stars in the GCs of our Galaxy, with unprecedented accuracy. This new generation of data, coupled with astrometric measurements by the Hubble Space Telescope (e.g. in the high-density cluster cores) and other state-of-the-art photometric and spectroscopic ground-based surveys (e.g. Gaia-ESO, Gilmore et al. 2012), will enable us to unlock, for the first time, the full six-dimensional position and velocity "phase-space" properties of individual stars in GCs. We are therefore truly about to enter a new "golden age" for the study of the internal dynamics of these stellar systems. The purpose of this proposal is precisely to leverage the richness of such new-generation astrometric data in order to gain a fundamental understanding of this emerging phase-space complexity and ultimately to reveal the dynamical evolution of the clusters, their elusive stellar populations and putative black holes, and their role as tracers of the history of our Galaxy.

Interests and expertise (Subject groups)

Grants awarded

Unique signatures of intermediate-mass black holes in globular clusters

Scheme: Newton International Fellowships

Dates: May 2014 - Feb 2016

Value: £88,500

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