Dynamical and radiative properties of astrophysical supersonic jets I. Cocoon morphologies

We present the results of a numerical analysis of the propagation and interaction of a supersonic jet with the external medium. We discuss the motion of the head of the jet into the ambient in different physical conditions, carrying out calculations with different Mach numbers and density ratios of the jet to the exteriors. Performing the calculation in a reference frame in motion with the jet head, we can follow in detail its long term dynamics. This numerical scheme allows us also to study the morphology of the cocoon for different physical parameters. We find that the propagation velocity of the jet head into the ambient medium strongly influences the morphology of the cocoon, and this result can be relevant in connection to the origin and structure of lobes in extragalactic radiosources.Comment: 14 pages, TeX. Accepted for A&

Active Galactic Nuclei, Radio Jets and Acceleration of UHECRs

We present the general properties of the Active Galactic Nuclei (AGNs) and discuss the origin and structure of jets that are associated to a fraction of these objects. We then we address the problems of particle acceleration at highly relativistic energies and set limits on the luminosity of AGN jets for being origin of UHECRs.Comment: Proceedings of the Cosmic ray International Seminar CRIS 2008 - Origin, Mass Composition and Acceleration Mechanisms of UHECRs - Malfa (Italy), September 15-19, 200

Astrophysical fluid simulations of thermally ideal gases with non-constant adiabatic index: numerical implementation

An Equation of State (\textit{EoS}) closes the set of fluid equations. Although an ideal EoS with a constant \textit{adiabatic index} $\Gamma$ is the preferred choice due to its simplistic implementation, many astrophysical fluid simulations may benefit from a more sophisticated treatment that can account for diverse chemical processes. Here, we first review the basic thermodynamic principles of a gas mixture in terms of its thermal and caloric EoS by including effects like ionization, dissociation as well as temperature dependent degrees of freedom such as molecular vibrations and rotations. The formulation is revisited in the context of plasmas that are either in equilibrium conditions (local thermodynamic- or collisional excitation- equilibria) or described by non-equilibrium chemistry coupled to optically thin radiative cooling. We then present a numerical implementation of thermally ideal gases obeying a more general caloric EoS with non-constant adiabatic index in Godunov-type numerical schemes.We discuss the necessary modifications to the Riemann solver and to the conversion between total energy and pressure (or vice-versa) routinely invoked in Godunov-type schemes. We then present two different approaches for computing the EoS.The first one employs root-finder methods and it is best suited for EoS in analytical form. The second one leans on lookup table and interpolation and results in a more computationally efficient approach although care must be taken to ensure thermodynamic consistency. A number of selected benchmarks demonstrate that the employment of a non-ideal EoS can lead to important differences in the solution when the temperature range is $500-10^4$ K where dissociation and ionization occur. The implementation of selected EoS introduces additional computational costs although using lookup table methods can significantly reduce the overhead by a factor $3\sim 4$.Comment: 17 pages, 10 figures, Accepted for publication in A&

Development of new nanostructured electrodes in Microbial Fuel Cells (MFCs)

The aim of my thesis work is to investigate new nanostructured materials, obtained by the electrospinning technique, in order to design 3D arrangement of the electrodes, leading thus to improve the energy efficiency of energy production devices, such as microbial fuel cells (MFCs). The carbon nanofibers reveal to be the most promising material in the field of bio electrochemistry; in fact, up to now the best performing microbial fuel cells are fabricated using carbon and carbon based material electrodes. To further enhance the performances of bio anodes and bio cathodes, a set of properties are then required to be overcome, such as a proper surface morphology and chemistry, good biofilm adhesion and electron transfer, and a good electrical conductivity. This work aims to demonstrate that the electrospun nanofibers own all the necessary properties, revealing themselves as the most innovative and promising structures for anodes and cathodes for microbial fuel cells. The nanofibers ensure all the properties listed above; in particular, during my Ph.D. I have investigated and studied the carbon based nanofibers to be applied as cathode and as anode in these kind of the devices. In this thesis, it will be demonstrated that the nanostructured electrodes improve the efficiency devices thanks both to the low impedance and to the interaction with the microorganisms. The high micrometric porosity characteristics of the realized anodic material create the ideal habitat for the microorganism’s proliferation. Moreover, different solution for the cathode material have been developed using ceramic nanofibers, such as MnxOy nanofibers and carbon nanofibers, in order to improve the performance of the devices. The layer made of these nanofibers, in fact, catalyzes the oxygen reduction reaction if the oxygen is used as terminal electron acceptor in the devices; thus these catalysts can substitute the platinum layer, which is the most used today, granting a cheaper and eco friendlier material

Selective large-eddy simulation of hypersonic flows. Procedure to activate the filtering in unresolved regions only (arXiv:1211.1305, submitted to Computer Physics Communications)

A new method for the localization of the regions where the turbulent fluctuations are unresolved is applied to the large-eddy simulation (LES) of a compressible turbulent jet with an initial Mach number equal to 5. The localization method used is called selective LES and is based on the exploitation of a scalar probe function f which represents the magnitude of the stretching-tilting term of the vorticity equation normalized with the enstrophy (Tordella et al. 2007). For a fully developed turbulent field of fluctuations, statistical analysis shows that the probability that f is larger than 2 is almost zero, and, for any given threshold, it is larger if the flow is under-resolved. By computing the spatial field of f in each instantaneous realization of the simulation it is possible to locate the regions where the magnitude of the normalized vortical stretching-tilting is anomalously high. The sub-grid model is then introduced into the governing equations in such regions only. The results of the selective LES simulation are compared with those of a standard LES, where the sub-grid terms are used in the whole domain. The comparison is carried out by assuming as reference field a higher resolution Euler simulation of the same jet. It is shown that the selective LES modifies the dynamic properties of the flow to a lesser extent with respect to the classical LES. In particular, the prediction of the enstrophy distribution and of the energy and density spectra are substantially improve

Making Fanaroff-Riley I radio sources. Numerical Hydrodynamic 3D Simulations of Low Power Jets

Extragalactic radio sources have been classified into two classes, Fanaroff-Riley I and II, which differ in morphology and radio power. Strongly emitting sources belong to the edge-brightened FR II class, and weakly emitting sources to the edge-darkened FR I class. The origin of this dichotomy is not yet fully understood. Numerical simulations are successful in generating FR II morphologies, but they fail to reproduce the diffuse structure of FR Is. By means of hydro-dynamical 3D simulations of supersonic jets, we investigate how the displayed morphologies depend on the jet parameters. Bow shocks and Mach disks at the jet head, which are probably responsible for the hot spots in the FR II sources, disappear for a jet kinetic power L_kin < 10^43 erg/s. This threshold compares favorably with the luminosity at which the FR I/FR II transition is observed. The problem is addressed by numerical means carrying out 3D HD simulations of supersonic jets that propagate in a non-homogeneous medium with the ambient temperature that increases with distance from the jet origin, which maintains constant pressure. The jet energy in the lower power sources, instead of being deposited at the terminal shock, is gradually dissipated by the turbulence. The jets spread out while propagating, and they smoothly decelerate while mixing with the ambient medium and produce the plumes characteristic of FR I objects. Three-dimensionality is an essential ingredient to explore the FR I evolution because the properties of turbulence in two and three dimensions are very different, since there is no energy cascade to small scales in two dimensions, and two-dimensional simulations with the same parameters lead to FRII-like behavior.Comment: 11 pages, 12 figures, to appear on A&

Diffusive shock acceleration in extragalactic jets

We calculate the temporal evolution of distributions of relativistic electrons subject to synchrotron and adiabatic processes and Fermi-like acceleration in shocks. The shocks result from Kelvin-Helmholtz instabilities in the jet. Shock formation and particle acceleration are treated in a self-consistent way by means of a numerical hydrocode. We show that in our model the number of relativistic particles is conserved during the evolution, with no need of further injections of supra-thermal particles after the initial one. From our calculations, we derive predictions for values and trends of quantities like the spectral index and the cutoff frequency that can be compared with observations.Comment: 12 pages containing 7 postscript figures; uses A&A macros. Accepted for publication in Astronomy and Astrophysic

A Particle Module for the PLUTO code: II - Hybrid Framework for Modeling Non-thermal emission from Relativistic Magnetized flows

We describe a new hybrid framework to model non-thermal spectral signatures from highly energetic particles embedded in a large-scale classical or relativistic MHD flow. Our method makes use of \textit{Lagrangian} particles moving through an Eulerian grid where the (relativistic) MHD equations are solved concurrently. Lagrangian particles follow fluid streamlines and represent ensembles of (real) relativistic particles with a finite energy distribution. The spectral distribution of each particle is updated in time by solving the relativistic cosmic ray transport equation based on local fluid conditions. This enables us to account for a number of physical processes, such as adiabatic expansion, synchrotron and inverse Compton emission. An accurate semi-analytically numerical scheme that combines the method of characteristics with a Lagrangian discretization in the energy coordinate is described. In presence of (relativistic) magnetized shocks, a novel approach to consistently model particle energization due to diffusive shock acceleration has been presented. Our approach relies on a refined shock-detection algorithm and updates the particle energy distribution based on the shock compression ratio, magnetic field orientation and amount of (parameterized) turbulence. The evolved distribution from each \textit{Lagrangian} particle is further used to produce observational signatures like emission maps and polarization signals accounting for proper relativistic corrections. We further demonstrate the validity of this hybrid framework using standard numerical benchmarks and evaluate the applicability of such a tool to study high energy emission from extra-galactic jets.Comment: 23 pages, 14 figures, Accepted for publication in The Astrophysical Journa

BeppoSAX observations of low power radio galaxies: possible detection of obscured nuclei

We present the first results of BeppoSAX observations of a small sample of low brightness FRI radio galaxies. The flux of all the targets is consistent with a thermal spectrum, as due to the presence of hot intracluster gas or galactic corona. Moreover in three sources a non thermal absorbed spectrum can be present in the MECS spectrum at energies larger than 7 keV, while for a fourth object a high energy flux has been detected in the PDS instrument at energies larger than 15 keV. This component could be related to the inner AGN surrounded by an obscuring torus.Comment: 4 pages, LateX, 3 figures (included). Uses espcrc2.sty (included). To appear in: "The Active X-ray Sky: Results from BeppoSAX and Rossi-XTE", Rome, Italy, 21-24 October, 1997, Eds.: L. Scarsi, H. Bradt, P. Giommi and F. Fior