Weekly Seminars for February 2017
Tuesday, February 21, 2017 - h 15:30
ICRANet Pescara 
Piazza della Repubblica 10, 65122 - Pescara (Italy) 

ICRANet Seminars

Speaker: Prof. Marco Merafina
Institution: Department of Physics, University of Rome La Sapienza, Italy
In the analysis of the evolution of globular clusters, stellar encounters strongly contribute in phase
space mixing of stellar orbits. In this scenario, thermodynamics plays a central role in the gravitational
equilibrium and stability of the clusters, being binary relaxation time shorter than the age of such systems.
On the other hand, the observations of luminosity profiles of globular clusters, at different values of the
central gravitational potential, show self similar curves that suggested a unique sequence of models with
the same distribution function and different parameters during the dynamical evolution [1].
This feature was also confirmed by several numerical simulations of globular clusters
existing in literature. This means that the evolution of globular clusters can be studied by considering
small thermodynamic transformations which keep constant the functional form of the velocity
distribution of stars like in the framework of the Boltzmann statistical mechanics. Here we construct
models with a different approach by applying thermodynamic principles to a Boltzmann distribution
function with an Hamiltonian function which contains an effective potential depending on the kinetic
energy of the stars, due to the effect of tidal interactions induced by the hosting galaxy. This particular
form of the Hamiltonian function derives from solving the Fokker-Planck equation in a different way
with respect to the King approach. The distribution obtained also corresponds to a maximum of the
entropy, and therefore implies a thermodynamical equilibrium based on collisions among stars in the
Fokker-Planck approximation. This result is a real breakthrough point in the analysis of dynamical
evolution of globular clusters. We obtain new relations for the thermodynamical equilibrium in presence
of a gravitational potential, a different form of the virial theorem, and introduce the concept of
thermodynamic and kinetic temperature, pressure and chemical potential (the intensive quantities).
In addition, we can show that a globular cluster can be described as a model containing regions with
positive and negative specific heat producing thermodynamic instabilities which drive the systems towards
the gravothermal catastrophe, without the necessity of an external thermal bath. The influence
of the effective potential on the virial theorem will be described, together with the related consequences
on the gravothermal stability limit for star clusters through the analysis of the caloric curve
and its critical points. This new limit is different from one obtained by Katz in 1978 [2] and now is
in complete accordance with the value related to the maximum of cluster distribution constructed by
data from Harris Catalogue for globular clusters [3], solving the unexplained discrepancy evidenced
since forty years. This agreement between theory and observations is obtained also for small clusters,
giving the conditions for the disruption of clusters, never obtained from the theoretical point of view,
but observed in several clusters. Finally, new developments on N-body simulations for glubular clusters
are presented. The results confirm the presence and the predicted form of the effective potential
arising from tidal interactions between the cluster and the hosting galaxy, as well the possibility to
verify this effect by complete observations of transverse and radial velocities of stars of a globular
cluster even in one single shell at given radius.

1. I.R. King, AJ, 71, 64 (1966).
2. J. Katz, MNRAS, 183, 765 (1978).
3. W.E. Harris, AJ, 112, 1487 (1996).

Wednesday March 1st, 2017 - 2,30 pm
Aula Rasetti (Physics Dept., Old Building, 2nd Floor) 
ICRA Seminar
Speaker: Dr. Aleesandro Nagar

Institution: IHES (Institute d’Hautes Studes Scientifiques) - France

Title: Gravitational waves from coalescing binary neutron stars: interfacing analytical with numerical results
It is discussed the interfacing between analytic and numerical treatments of gravitational waves from coalescing binary neutron stars.
N.B. - In the following link, you can find the official announcement on the Physics Department website: http://www.phys.uniroma1.it/fisica/sites/default/files/bollettini/bollettino27022017.pdf
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