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Star clusters as cosmic laboratories for Astrophysics, Dynamics and Fundamental Physics

Final Report Summary - COSMIC-LAB (Star clusters as cosmic laboratories for Astrophysics, Dynamics and Fundamental Physics)

Galactic Globular Clusters (GCs) are the most populous and dense stellar aggregates in the Milky Way. They are dynamically active systems, where phenomena like stellar collisions, mass exchanges within binary systems, in/outward migration of different classes of stars are very affective. In turn, such a vigorous dynamical activity can substantially modify the (otherwise normal) evolution of stars, thus creating a variety of exotic objects (like Blue Stragglers stars, millisecond pulsars, etc.) that cannot be interpreted in the context of the passive evolution of single stars.
Cosmic-Lab was a five-year project with the precise aim of finally clarifying the complex interplay between dynamics and stellar evolution, by using Galactic GCs as cosmic laboratories, and three classes of exotica (Blue Straggler Stars, Millisecond pulsars and intermediate-mass black holes) as probe particles. The wide variety of results obtained within the project provided both new knowledge about the internal evolution of Galactic GCs, and new methodologies to properly investigate these systems and their stellar populations. A schematic summary of the major results obtained in the three research areas is the following.
Two innovative tools have been defined in the context of the Blue Straggler Star research: (1) a “clock” to measure the dynamical age of GCs, i.e. level of dynamical evolution suffered by a stellar system since its formation, and (2) a “scale” to measure the mass of stars, allowing to pinpoint anomalously heavy objects in a multitude of otherwise indistinguishable low-mass sisters. These empirical tools promise quantum jumps in the description of GC internal dynamics and in the understanding of the evolutionary processes of Blue Straggler Stars.
The identifications of 8 new optical companions to Millisecond pulsars (MSPs) in GCs obtained within Cosmic-Lab doubled the number of previously known objects and it is now helping to delineate a coherent evolutionary paths of these exotica. Particularly intriguing are the case of
IGR J1824−24525 in the GC M28, which is the first empirical confirmation of the predicted transition between the accretion-powered and the rotation-powered stages of MSP evolution, and the case of the neutron star burster EXO 1745-248 in Terzan 5, which is a system catch during the pre-natal stage of a MSP, an evolutionary phase during which heavy mass accretion on the compact object occurs, thus producing X-ray outbursts and the re-acceleration of the neutron star.
An innovative methodology (based on high spatial resolution spectroscopy with adaptive optics-assisted instruments) has been proposed to determine the velocity dispersion profile in the innermost regions of even the densest GCs. This has been proven to be the safest approach to the search for the possible signature of intermediate-mass black holes in Galactic GCs and it is now adopted in dedicated observing campaigns at the ESO-VLT.
A total of 64 papers has been published in high-impact factor, peer reviewed, international journals, at the impressive rate of more than one paper per month. The results have been also presented in 70 invited/contributed talks at international conferences and/or at the major science institutes worldwide. Both the papers and the presentations can be freely downloaded from the project website: where the entire activity (in terms of scientific results, products and tools, amount of awarded telescope time, press releases, freely downloadable images and videos) is reported. The project also consisted in an invaluable breeding ground for the formation of young scientists with a broad view in Astrophysics.