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NEw Science from the phase space of old stellar SYstems

Periodic Reporting for period 1 - NESSY (NEw Science from the phase space of old stellar SYstems)

Reporting period: 2015-10-15 to 2017-10-14

In the last few years our traditional interpretative paradigm of the internal dynamics of globular star clusters has been revolutionized by a series of discoveries about their chemical, structural, and kinematic properties. The existence of multiple stellar populations is now regarded as a ubiquitous phenomenon, while for decades globular clusters have been viewed as the epitome of a “simple stellar population”. Empirical scaling relations between super massive black holes masses and the velocity dispersion of their host galaxy encourage to consider globular clusters as the host systems of intermediate-mass black holes. Finally, little attention has been traditionally paid to the role played by angular momentum in the dynamical evolution of these systems, yet an increasing number of young and old star clusters are now being observed to have evidence of rotation.

The astrometric mission Gaia, by allowing the acquisition of the proper motion of thousands of stars in Galactic clusters with exquisite detail, is now unlocking the full phase space of these stellar systems. Such a tremendous observational progress, coupled with recent improvements on the side of numerical simulations, calls for a renewed effort on dynamical modelling.

This action, by means of a unique combination of analytical models, numerical simulations, and the exploitation of new astrometric data from Hubble Space Telescope and other state-of-the-art spectroscopic datasets, has made a significant contribution towards the definition of a more realistic dynamical paradigm for this class of stellar systems, at beginning of the era of precision astrometry for Galactic studies.
For this action, extensive direct N-body simulations have been made, together with the development of analytical arguments and interpretative studies. Special emphasis has been given to the study of the kinematical evolution of collisional stellar systems and to the characterization of (i) the role of angular momentum and pressure anisotropy and (ii) the interplay between internal rotation and the external tidal field in the internal dynamics of star clusters.

Specific highlights include the development of a new theoretical understanding of the role of phase space complexity in the long-term dynamical evolution of collisional systems (Breen, Varri, Heggie, 2017), the realization that star clusters evolve towards a condition of only partial synchronization (Tiongco, Vesperini, Varri 2016), and the development of a global dynamical model of Galactic cluster 47 Tucanae, which fully describes its rich structure in velocity space, unveiling an unexpected degree of internal rotation (Bellini, Bianchini, Varri et al. 2017).

In addition, innovative results have been obtained on the construction of an analytical model which includes a population of ‘potential escapers’ (Daniel, Heggie, Varri 2017) and the investigation of the physical origin of stellar envelopes around collisional systems (Penarrubia, Varri et al. 2017).
The fundamental understanding of the emerging phase space complexity of globular clusters developed in this action will allow us to address many open questions about their rich dynamical evolution, elusive stellar populations, putative black holes, tantalizing dark matter content, and, ultimately, to formulate a modern view of their role within the history of our Galaxy, in the era of Gaia. The investigation of the equilibrium, stability, and evolution of anisotropic, differentially rotating stellar systems conducted in this action has rarely been explored. This study is therefore particularly innovative and offers a significant contribution towards the definition of a new chapter in fundamental stellar dynamics.

Direct socio-economic impact is not expected as the immediate results of this project concerns fundamental research. However, indirect impact has been achieved through public outreach and educational activities.
The Galactic globular cluster 47 Tucanae (NGC 104). Credit: NASA, ESA, the Hubble Heritage (STScI).