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Tesi etd-12062010-083455


Thesis type
Tesi di dottorato di ricerca
Author
PASQUATO, MARIO
URN
etd-12062010-083455
Title
Globular clusters and intermediate-mass black holes
Settore scientifico disciplinare
FIS/05
Corso di studi
FISICA
Commissione
tutor Prof. Bertin, Giuseppe
commissario Capuzzo Dolcetta, Roberto
commissario Nipoti, Carlo
commissario Romaniello, Martino
commissario Prof.ssa Degl'Innocenti, Scilla
commissario Prof. Shore, Steven N.
Parole chiave
  • Buchi neri di massa intermedia
  • Dinamica collisionale
  • Ammassi globulari
Data inizio appello
07/12/2010;
Consultabilità
completa
Riassunto analitico
Globular Clusters (GCs) are among the most studied objects in astronomy. They historically were regarded as a single-burst stellar population, as opposed to galaxies, which show <br>evidence of a more complex star formation history. Such apparent simplicity led astronomers <br>to regard them as the ideal stellar evolutionary laboratory, while in the field of dynamics, the <br>truncated-Maxwellian King (1966) models were generally accepted as good fits to the surface <br>brightness profiles of most Galactic GCs. In the last decade, much of this long-standing confidence in GC simplicity was challenged by improved observations. The Hubble Space Telescope <br>produced accurate HR diagrams, which for some GCs can be explained only by multiple stellar <br>populations (Gratton et al. 2004; Bedin et al. 2004; Piotto et al. 2005, 2007). Exotic objects <br>such as blue stragglers, X-ray sources, and pulsars proved ubiquitous, likely the result of an <br>interplay between cluster dynamics and stellar evolution (e.g. Belczynski et al. 2006; Shara &amp; <br>Hurley 2006; Hut 2006). High-resolution imaging of GC cores revealed central density cusps <br>at odds with King-model expectations of a flat core (Noyola &amp; Gebhardt 2006, 2007). On the <br>other hand, direct N-body numerical simulations allowed to simulate the dynamics of GCs with <br>an almost realistic number of stars and dynamical ingredients such as binaries, tidal mass-loss <br>and a spectrum of stellar masses. <br>In this context, this Thesis is focused on a particular new ingredient in GC dynamics: Intermediate Mass Black Holes (IMBHs). IMBHs are elusive objects the existence of which is an <br>intriguing issue in its own right, for the consequences it would have on the seeding of super-massive black holes, on explaining Ultra Luminous X-ray Sources (ULXs), and on modeling <br>potential astrophysical sources of gravitational radiation. <br>An approach stressing model-independence, non-parametric statistical tools and extensive <br>data visualization is followed throughout, and is a distinctive feature of this Thesis. A catalogue <br>of GC structural parameters (luminosity, fraction-of-light radius and average surface brightness) <br>is obtained from a model-independent spline-smoothing algorithm applied to GC surface brightness profiles. The parameters thus obtained, together with other properties from the literature, are <br>extensively explored using data-visualization techniques appropriate for multivariate data-sets <br>(Pasquato &amp; Bertin 2008, 2010). Tools such as cluster analysis, quantile-quantile plots, kernel <br>density estimation, and conditioning plots can lead to the discovery of a number of interesting <br>features, usually hidden to previous research. A relation between deviations from the GC fundamental plane and the slope of central cusps in the surface brightness profile is found (Pasquato &amp; <br>Bertin 2008). If such cusps originate from IMBHs, this would point to a global effect of IMBHs <br>on the GC fundamental plane. On the other hand, cuspy profiles appear naturally in simulated <br>GCs evolved beyond core-collapse even without an IMBH (Trenti et al. 2010). <br>In this Thesis we contribute to the development of a new method to look for IMBHs in GCs, <br>based on the effects on mass segregation predicted from N-body simulations with a realistic number of stars. An IMBH is expected to reduce the amount of mass segregation <br>observed in relaxed GCs. The method is applied to two GCs using HST archival data. NGC <br>2298 is shown to be an unlikely host to an IMBH (Pasquato et al. 2009), while M10 is more <br>promising but requires a quantitative determination of the stellar binary fraction to allow a conclusion (Beccari et al. 2010). The model-independent calculation of GC structural parameters <br>presented in this Thesis is an integral part of the framework I devised to compare simulations <br>and observations on an equal footing. <br>N-body simulations of GCs with binaries and a realistic mass spectrum are run to core-collapse and beyond and analyzed as if they were observed GC data-sets (Trenti et al. 2010). The <br>surface brightness profile of main-sequence stars does not undergo deep core collapse, because <br>the collapse of dark remnants and/or binaries provide energy to the system. King model fits <br>to simulated post-core collapse GCs are shown to produce unstable results with respect to GC <br>structural parameters, lending further support to the non-parametric approach introduced here.
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