Andrea Mignone - Academia.edu (original) (raw)

Papers by Andrea Mignone

Astronomy & Astrophysics

Context.Nonthermal emission from active galactic nucleus (AGN) jets extends up to large scales, e... more Context.Nonthermal emission from active galactic nucleus (AGN) jets extends up to large scales, even though they are prone to many magnetohydrodynamic instabilities.Aims.The main focus of this study is to understand the effect of magnetohydrodynamic instabilities on the nonthermal emission from large-scale AGN jets.Methods.We performed high-resolution three-dimensional numerical magnetohydrodynamic simulations of a plasma column to investigate the dynamical and emission properties of jet configurations at kiloparsec scales with different magnetic field profiles, jet speeds, and density contrast. We also obtained synthetic nonthermal emission signatures for different viewing angles using an approach that assumes static particle spectra and that is obtained by evolving the particle spectra using Lagrangian macroparticles incorporating the effects of shock acceleration and radiative losses.Results.We find that shocks due to the Kelvin-Helmholtz (KH) instability in the axial magnetic fi...

arXiv: High Energy Astrophysical Phenomena, 2016

Extragalactic radiosources have been classified in two classes, Fanaroff-Riley I and II, which di... more Extragalactic radiosources have been classified in 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 while the weak ones 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's head, likely responsible for the presence of hot spots in the FR II sources, disappear for a jet kinetic power less than 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 three-dimensional HD simulations of supersonic jets that propagate in a non homog...

Nature Communications, 2021

The shaping of astrophysical outflows into bright, dense, and collimated jets due to magnetic pre... more The shaping of astrophysical outflows into bright, dense, and collimated jets due to magnetic pressure is here investigated using laboratory experiments. Here we look at the impact on jet collimation of a misalignment between the outflow, as it stems from the source, and the magnetic field. For small misalignments, a magnetic nozzle forms and redirects the outflow in a collimated jet. For growing misalignments, this nozzle becomes increasingly asymmetric, disrupting jet formation. Our results thus suggest outflow/magnetic field misalignment to be a plausible key process regulating jet collimation in a variety of objects from our Sun’s outflows to extragalatic jets. Furthermore, they provide a possible interpretation for the observed structuring of astrophysical jets. Jet modulation could be interpreted as the signature of changes over time in the outflow/ambient field angle, and the change in the direction of the jet could be the signature of changes in the direction of the ambient ...

Monthly Notices of the Royal Astronomical Society, 2021

Cosmic rays (CRs) are frequently modelled as an additional fluid in hydrodynamic (HD) and magneto... more Cosmic rays (CRs) are frequently modelled as an additional fluid in hydrodynamic (HD) and magnetohydrodynamic (MHD) simulations of astrophysical flows. The standard CR two-fluid model is described in terms of three conservation laws (expressing conservation of mass, momentum, and total energy) and one additional equation (for the CR pressure) that cannot be cast in a satisfactory conservative form. The presence of non-conservative terms with spatial derivatives in the model equations prevents a unique weak solution behind a shock. We investigate a number of methods for the numerical solution of the two-fluid equations and find that, in the presence of shock waves, the results generally depend on the numerical details (spatial reconstruction, time stepping, the CFL number, and the adopted discretization). All methods converge to a unique result if the energy partition between the thermal and non-thermal fluids at the shock is prescribed using a subgrid prescription. This highlights t...

The Astrophysical Journal Supplement Series, 2020

Astrophysical fluid flow studies often encompass a wide range of physical processes to account fo... more Astrophysical fluid flow studies often encompass a wide range of physical processes to account for the complexity of the system under consideration. In addition to gravity, a proper treatment of thermodynamic processes via continuum radiation transport and/or photoionization is becoming the state of the art. We present a major update of our continuum radiation transport module, Makemake, and a newly developed module for photoionization, Sedna, coupled to the magnetohydrodynamics code PLUTO. These extensions are currently not publicly available; access can be granted on a case-by-case basis. We explain the theoretical background of the equations solved, elaborate on the numerical layout, and present a comprehensive test suite for radiation-ionization hydrodynamics. The grid based radiation and ionization modules support static one-dimensional, two-dimensional, and three-dimensional grids in Cartesian, cylindrical, and spherical coordinates. Each module splits the radiation field into two components, one originating directly from a point source-solved using a ray-tracing scheme-and a diffuse component-solved with a three-dimensional flux-limited diffusion (FLD) solver. The FLD solver for the continuum radiation transport makes use of either the equilibrium one-temperature approach or the linearization two-temperature approach. The FLD solver for the photoionization module enables accounting for the temporal evolution of the radiation field from direct recombination of free electrons into hydrogen's ground state as an alternative to on-the-spot approximation. A brief overview of completed and ongoing scientific studies is given to explicitly illustrate the multipurpose nature of the numerical framework presented.

The Astrophysical Journal, 2018

We describe a new hybrid framework to model non-thermal spectral signatures from highly energetic... more We describe a new hybrid framework to model non-thermal spectral signatures from highly energetic particles embedded in a large-scale classical or relativistic magnetohydrodynamic (MHD) flow. Our method makes use of 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 semianalytically numerical scheme that combines the method of characteristics with a Lagrangian discretization in the energy coordinate is described. In the presence of (relativistic) magnetized shocks, a novel approach to consistently model particle energization due to diffusive shock acceleration is 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 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 extragalactic jets.

Astronomy & Astrophysics, 2016

Context. Extragalactic radio sources have been classified into two classes, Fanaroff-Riley I and ... more Context. 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. Aims. 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 −1. This threshold compares favorably with the luminosity at which the FR I/FR II transition is observed. Methods. 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. Results. 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. Conclusions. 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.

Monthly Notices of the Royal Astronomical Society, 2017

An important ingredient in numerical modelling of high temperature magnetized astrophysical plasm... more An important ingredient in numerical modelling of high temperature magnetized astrophysical plasmas is the anisotropic transport of heat along magnetic field lines from higher to lower temperatures. Magnetohydrodynamics typically involves solving the hyperbolic set of conservation equations along with the induction equation. Incorporating anisotropic thermal conduction requires to also treat parabolic terms arising from the diffusion operator. An explicit treatment of parabolic terms will considerably reduce the simulation time step due to its dependence on the square of the grid resolution (x) for stability. Although an implicit scheme relaxes the constraint on stability, it is difficult to distribute efficiently on a parallel architecture. Treating parabolic terms with accelerated super-time-stepping (STS) methods has been discussed in literature, but these methods suffer from poor accuracy (first order in time) and also have difficult-to-choose tuneable stability parameters. In this work, we highlight a second-order (in time) Runge-Kutta-Legendre (RKL) scheme (first described by Meyer, Balsara & Aslam 2012) that is robust, fast and accurate in treating parabolic terms alongside the hyperbolic conversation laws. We demonstrate its superiority over the first-order STS schemes with standard tests and astrophysical applications. We also show that explicit conduction is particularly robust in handling saturated thermal conduction. Parallel scaling of explicit conduction using RKL scheme is demonstrated up to more than 10 4 processors.

Acta Astronautica, 2017

Pulsed fusion propulsion might finally revolutionise manned space exploration by providing an aff... more Pulsed fusion propulsion might finally revolutionise manned space exploration by providing an affordable and relatively fast access to interplanetary destinations. However, such systems are still in an early development phase and one of the key areas requiring further investigations is the operation of the magnetic nozzle, the device meant to exploit the fusion energy and generate thrust. One of the last pulsed fusion magnetic nozzle design is the so called multi-coil parabolic reaction chamber: the reaction is thereby ignited at the focus of an open parabolic chamber, enclosed by a series of coaxial superconducting coils that apply a magnetic field. The field, beside confining the reaction and preventing any contact between hot fusion plasma and chamber structure, is also meant to reflect the explosion and push plasma out of the rocket. Reflection is attained thanks to electric currents induced in conductive skin layers that cover each of the coils, the change of plasma axial momentum generates thrust in reaction. This working principle has yet to be extensively verified and computational Magneto-Hydro Dynamics (MHD) is a viable option to achieve that. This work is one of the first detailed ideal-MHD analysis of a a multi-coil parabolic reaction chamber of this kind and has been completed employing PLUTO, a freely distributed computational code developed at the Physics Department of the University of Turin. The results are thus a preliminary verification of the chamber's performance. Nonetheless, plasma leakage through the chamber structure has been highlighted. Therefore, further investigations are required to validate the chamber design. Implementing a more accurate physical model (e.g. Hall-MHD or relativistic-MHD) is thus mandatory, and PLUTO shows the capabilities to achieve that.

Astrophysics and Space Science Proceedings, 2009

ABSTRACT With the recent improvements in available observational data, simulating the radiative p... more ABSTRACT With the recent improvements in available observational data, simulating the radiative processes in YSO jets will provide a valuable tool for model discrimination. We have added a radiative cooling module and time-dependent ionization state computation for 29 ion species to our MHD code PLUTO. Also, post-processing routines for the realistic computation of emission lines are now available. From 2D simulations, synthetic surface brightness maps and position-velocity diagrams for the line emissions can be directly computed, to be compared with observations.

Proceedings of the International Astronomical Union, 2010

The MHD simulations of stellar jets recently included complex models of radiative emission comput... more The MHD simulations of stellar jets recently included complex models of radiative emission computation, allowing for better predictions in terms of emission line ratios. Employing also Adaptive Mesh Refinement, the large-scale propagation of jets could be followed. The simulation of multiple shockwaves originating in perturbations close to the jet origin and travelling along the jet beam allows for the construction of synthetic emission maps at various wavelengths, to be directly compared to observations. We apply this procedure for the jets originating from RW Aurigae.

Lecture Notes in Physics, 2008

The series Lecture Notes in Physics (LNP), founded in 1969, reports new developments in physics r... more The series Lecture Notes in Physics (LNP), founded in 1969, reports new developments in physics research and teaching-quickly and informally, but with a high quality and the explicit aim to summarize and communicate current knowledge in an accessible way. Books published in this series are conceived as bridging material between advanced graduate textbooks and the forefront of research and to serve three purposes: • to be a compact and modern up-to-date source of reference on a well-defined topic • to serve as an accessible introduction to the field to postgraduate students and nonspecialist researchers from related areas • to be a source of advanced teaching material for specialized seminars, courses and schools Both monographs and multi-author volumes will be considered for publication. Edited volumes should, however, consist of a very limited number of contributions only. Proceedings will not be considered for LNP. Volumes published in LNP are disseminated both in print and in electronic formats, the electronic archive being available at springerlink.com. The series content is indexed, abstracted and referenced by many abstracting and information services, bibliographic networks, subscription agencies, library networks, and consortia.

ABSTRACT In the present work we discuss how the strength of magnetic fields determines the charac... more ABSTRACT In the present work we discuss how the strength of magnetic fields determines the characteristics of solutions in models where the collimated outflow and the accretion disk are treated consistently. We perform a complete analysis of the range of magnetic field by non-relativistic 2.5 dimension numerical simulations using the PLUTO code. The main results are that magnetic fields around equipartition with plasma pressure allow steady super-fast-magnetosonic collimated jet solutions; magnetic fields below equipartition correspond to intermittent collimated outflows, while magnetic fields above equipartition lead to sub-alfvenic winds. These results allow to conclude that the configuration proposed by Blandford and Payne to interpret supersonic jets is viable both for equipartition and weaker magnetic fields.

Astrophysics and Space Science Proceedings, 2009

ABSTRACT

The Astrophysical Journal, 2011

We consider the problem of convergence in homogeneous shearing box simulations of magneto-rotatio... more We consider the problem of convergence in homogeneous shearing box simulations of magneto-rotationally driven turbulence. When there is no mean magnetic flux, if the equations are non dimensionalized with respect to the diffusive scale, the only free parameter in the problem is the size of the computational domain. The problem of convergence then relates to the asymptotic form of the solutions as the computational box size becomes large. By using a numerical code with a high order of accuracy we show that the solutions become asymptotically independent of domain size. We also show that cases with weak magnetic flux join smoothly to the zero flux cases as the flux vanishes. These results are consistent with the operation of a subcritical small-scale dynamo driving the turbulence. We conclude that for this type of turbulence the angular momentum transport is a proportional to the diffusive flux and therefore has limited relevance in astrophysical situations.

The Astrophysical Journal, 2008

We study the evolution of an accelerating hyperrelativistic shock under the presence of upstream ... more We study the evolution of an accelerating hyperrelativistic shock under the presence of upstream inhomogeneities wrinkling the discontinuity surface. The investigation is conducted by means of numerical simulations using the PLUTO code for astrophysical fluid dynamics. The reliability and robustness of the code are demonstrated against well known results coming from the linear perturbation theory. We then follow the nonlinear evolution of two classes of perturbing upstream atmospheres and conclude that no lasting wrinkle can be preserved indefinitely by the flow. Finally we derive analytically a description of the geometrical effects of a turbulent upstream ambient on the discontinuity surface.

The Astrophysical Journal, 2012

We investigate supersonic, axisymmetric magnetohydrodynamic (MHD) jets with a time-dependent inje... more We investigate supersonic, axisymmetric magnetohydrodynamic (MHD) jets with a time-dependent injection velocity by numerical simulations with the PLUTO code. Using a comprehensive set of parameters, we explore different jet configurations in the attempt to construct models that can be directly compared to observational data of microjets. In particular, we focus our attention on the emitting properties of traveling knots and construct, at the same time, accurate line intensity ratios and surface brightness maps. Direct comparison of the resulting brightness and line intensity ratios distributions with observational data of microjets shows that a closer match can be obtained only when the jet material is pre-ionized to some degree. A very likely source for a pre-ionized medium is photoionization by X-ray flux coming from the central object.

Monthly Notices of the Royal Astronomical Society, 2011

Young low-mass stars are characterized by ejection of collimated outflows and by circumstellar di... more Young low-mass stars are characterized by ejection of collimated outflows and by circumstellar discs which they interact with through accretion of mass. The accretion builds up the star to its final mass and is also believed to power the mass outflows, which may in turn remove the excess angular momentum from the star-disc system. However, although the process of mass accretion is a critical aspect of star formation, some of its mechanisms are still to be fully understood. A point not considered to date and relevant for the accretion process is the evidence of very energetic and frequent flaring events in these stars. Flares may easily perturb the stability of the discs, thus influencing the transport of mass and angular momentum. Here we report on 3D magnetohydrodynamic modelling of the evolution of a flare with an idealized non-equilibrium initial condition occurring near the disc around a rotating magnetized star. The model takes into account the stellar magnetic field, the gravitational force, the viscosity of the disc, the magnetic-field-oriented thermal conduction (including the effects of heat flux saturation), the radiative losses from optically thin plasma and the coronal heating. We show that, during its first stage of evolution, the flare gives rise to a hot magnetic loop linking the disc to the star. The disc is strongly perturbed by the flare: disc material evaporates under the effect of the thermal conduction and an overpressure wave propagates through the disc. When the overpressure reaches the opposite side of the disc, a funnel flow starts to develop there, accreting substantial disc material on to the young star from the side of the disc opposite to the flare.

Monthly Notices of the Royal Astronomical Society, 2013

We investigate the onset of pressure-driven toroidal-mode instabilities in accretion mounds on ne... more We investigate the onset of pressure-driven toroidal-mode instabilities in accretion mounds on neutron stars by 3D magnetohydrodynamic (MHD) simulations using the PLUTO MHD code. Our results confirm that for mounds beyond a threshold mass, instabilities form fingerlike channels at the periphery, resulting in mass-loss from the magnetically confined mound. Ring-like mounds with hollow interior show the instabilities at the inner edge as well. We perform the simulations for mounds of different sizes to investigate the effect of the mound mass on the growth rate of the instabilities. We also investigate the effect of such instabilities on observables such as cyclotron resonant scattering features and timing properties of such systems.

Astronomy & Astrophysics

Context.Nonthermal emission from active galactic nucleus (AGN) jets extends up to large scales, e... more Context.Nonthermal emission from active galactic nucleus (AGN) jets extends up to large scales, even though they are prone to many magnetohydrodynamic instabilities.Aims.The main focus of this study is to understand the effect of magnetohydrodynamic instabilities on the nonthermal emission from large-scale AGN jets.Methods.We performed high-resolution three-dimensional numerical magnetohydrodynamic simulations of a plasma column to investigate the dynamical and emission properties of jet configurations at kiloparsec scales with different magnetic field profiles, jet speeds, and density contrast. We also obtained synthetic nonthermal emission signatures for different viewing angles using an approach that assumes static particle spectra and that is obtained by evolving the particle spectra using Lagrangian macroparticles incorporating the effects of shock acceleration and radiative losses.Results.We find that shocks due to the Kelvin-Helmholtz (KH) instability in the axial magnetic fi...

arXiv: High Energy Astrophysical Phenomena, 2016

Extragalactic radiosources have been classified in two classes, Fanaroff-Riley I and II, which di... more Extragalactic radiosources have been classified in 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 while the weak ones 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's head, likely responsible for the presence of hot spots in the FR II sources, disappear for a jet kinetic power less than 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 three-dimensional HD simulations of supersonic jets that propagate in a non homog...

Nature Communications, 2021

The shaping of astrophysical outflows into bright, dense, and collimated jets due to magnetic pre... more The shaping of astrophysical outflows into bright, dense, and collimated jets due to magnetic pressure is here investigated using laboratory experiments. Here we look at the impact on jet collimation of a misalignment between the outflow, as it stems from the source, and the magnetic field. For small misalignments, a magnetic nozzle forms and redirects the outflow in a collimated jet. For growing misalignments, this nozzle becomes increasingly asymmetric, disrupting jet formation. Our results thus suggest outflow/magnetic field misalignment to be a plausible key process regulating jet collimation in a variety of objects from our Sun’s outflows to extragalatic jets. Furthermore, they provide a possible interpretation for the observed structuring of astrophysical jets. Jet modulation could be interpreted as the signature of changes over time in the outflow/ambient field angle, and the change in the direction of the jet could be the signature of changes in the direction of the ambient ...

Monthly Notices of the Royal Astronomical Society, 2021

Cosmic rays (CRs) are frequently modelled as an additional fluid in hydrodynamic (HD) and magneto... more Cosmic rays (CRs) are frequently modelled as an additional fluid in hydrodynamic (HD) and magnetohydrodynamic (MHD) simulations of astrophysical flows. The standard CR two-fluid model is described in terms of three conservation laws (expressing conservation of mass, momentum, and total energy) and one additional equation (for the CR pressure) that cannot be cast in a satisfactory conservative form. The presence of non-conservative terms with spatial derivatives in the model equations prevents a unique weak solution behind a shock. We investigate a number of methods for the numerical solution of the two-fluid equations and find that, in the presence of shock waves, the results generally depend on the numerical details (spatial reconstruction, time stepping, the CFL number, and the adopted discretization). All methods converge to a unique result if the energy partition between the thermal and non-thermal fluids at the shock is prescribed using a subgrid prescription. This highlights t...

The Astrophysical Journal Supplement Series, 2020

Astrophysical fluid flow studies often encompass a wide range of physical processes to account fo... more Astrophysical fluid flow studies often encompass a wide range of physical processes to account for the complexity of the system under consideration. In addition to gravity, a proper treatment of thermodynamic processes via continuum radiation transport and/or photoionization is becoming the state of the art. We present a major update of our continuum radiation transport module, Makemake, and a newly developed module for photoionization, Sedna, coupled to the magnetohydrodynamics code PLUTO. These extensions are currently not publicly available; access can be granted on a case-by-case basis. We explain the theoretical background of the equations solved, elaborate on the numerical layout, and present a comprehensive test suite for radiation-ionization hydrodynamics. The grid based radiation and ionization modules support static one-dimensional, two-dimensional, and three-dimensional grids in Cartesian, cylindrical, and spherical coordinates. Each module splits the radiation field into two components, one originating directly from a point source-solved using a ray-tracing scheme-and a diffuse component-solved with a three-dimensional flux-limited diffusion (FLD) solver. The FLD solver for the continuum radiation transport makes use of either the equilibrium one-temperature approach or the linearization two-temperature approach. The FLD solver for the photoionization module enables accounting for the temporal evolution of the radiation field from direct recombination of free electrons into hydrogen's ground state as an alternative to on-the-spot approximation. A brief overview of completed and ongoing scientific studies is given to explicitly illustrate the multipurpose nature of the numerical framework presented.

The Astrophysical Journal, 2018

We describe a new hybrid framework to model non-thermal spectral signatures from highly energetic... more We describe a new hybrid framework to model non-thermal spectral signatures from highly energetic particles embedded in a large-scale classical or relativistic magnetohydrodynamic (MHD) flow. Our method makes use of 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 semianalytically numerical scheme that combines the method of characteristics with a Lagrangian discretization in the energy coordinate is described. In the presence of (relativistic) magnetized shocks, a novel approach to consistently model particle energization due to diffusive shock acceleration is 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 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 extragalactic jets.

Astronomy & Astrophysics, 2016

Context. Extragalactic radio sources have been classified into two classes, Fanaroff-Riley I and ... more Context. 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. Aims. 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 −1. This threshold compares favorably with the luminosity at which the FR I/FR II transition is observed. Methods. 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. Results. 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. Conclusions. 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.

Monthly Notices of the Royal Astronomical Society, 2017

An important ingredient in numerical modelling of high temperature magnetized astrophysical plasm... more An important ingredient in numerical modelling of high temperature magnetized astrophysical plasmas is the anisotropic transport of heat along magnetic field lines from higher to lower temperatures. Magnetohydrodynamics typically involves solving the hyperbolic set of conservation equations along with the induction equation. Incorporating anisotropic thermal conduction requires to also treat parabolic terms arising from the diffusion operator. An explicit treatment of parabolic terms will considerably reduce the simulation time step due to its dependence on the square of the grid resolution (x) for stability. Although an implicit scheme relaxes the constraint on stability, it is difficult to distribute efficiently on a parallel architecture. Treating parabolic terms with accelerated super-time-stepping (STS) methods has been discussed in literature, but these methods suffer from poor accuracy (first order in time) and also have difficult-to-choose tuneable stability parameters. In this work, we highlight a second-order (in time) Runge-Kutta-Legendre (RKL) scheme (first described by Meyer, Balsara & Aslam 2012) that is robust, fast and accurate in treating parabolic terms alongside the hyperbolic conversation laws. We demonstrate its superiority over the first-order STS schemes with standard tests and astrophysical applications. We also show that explicit conduction is particularly robust in handling saturated thermal conduction. Parallel scaling of explicit conduction using RKL scheme is demonstrated up to more than 10 4 processors.

Acta Astronautica, 2017

Pulsed fusion propulsion might finally revolutionise manned space exploration by providing an aff... more Pulsed fusion propulsion might finally revolutionise manned space exploration by providing an affordable and relatively fast access to interplanetary destinations. However, such systems are still in an early development phase and one of the key areas requiring further investigations is the operation of the magnetic nozzle, the device meant to exploit the fusion energy and generate thrust. One of the last pulsed fusion magnetic nozzle design is the so called multi-coil parabolic reaction chamber: the reaction is thereby ignited at the focus of an open parabolic chamber, enclosed by a series of coaxial superconducting coils that apply a magnetic field. The field, beside confining the reaction and preventing any contact between hot fusion plasma and chamber structure, is also meant to reflect the explosion and push plasma out of the rocket. Reflection is attained thanks to electric currents induced in conductive skin layers that cover each of the coils, the change of plasma axial momentum generates thrust in reaction. This working principle has yet to be extensively verified and computational Magneto-Hydro Dynamics (MHD) is a viable option to achieve that. This work is one of the first detailed ideal-MHD analysis of a a multi-coil parabolic reaction chamber of this kind and has been completed employing PLUTO, a freely distributed computational code developed at the Physics Department of the University of Turin. The results are thus a preliminary verification of the chamber's performance. Nonetheless, plasma leakage through the chamber structure has been highlighted. Therefore, further investigations are required to validate the chamber design. Implementing a more accurate physical model (e.g. Hall-MHD or relativistic-MHD) is thus mandatory, and PLUTO shows the capabilities to achieve that.

Astrophysics and Space Science Proceedings, 2009

ABSTRACT With the recent improvements in available observational data, simulating the radiative p... more ABSTRACT With the recent improvements in available observational data, simulating the radiative processes in YSO jets will provide a valuable tool for model discrimination. We have added a radiative cooling module and time-dependent ionization state computation for 29 ion species to our MHD code PLUTO. Also, post-processing routines for the realistic computation of emission lines are now available. From 2D simulations, synthetic surface brightness maps and position-velocity diagrams for the line emissions can be directly computed, to be compared with observations.

Proceedings of the International Astronomical Union, 2010

The MHD simulations of stellar jets recently included complex models of radiative emission comput... more The MHD simulations of stellar jets recently included complex models of radiative emission computation, allowing for better predictions in terms of emission line ratios. Employing also Adaptive Mesh Refinement, the large-scale propagation of jets could be followed. The simulation of multiple shockwaves originating in perturbations close to the jet origin and travelling along the jet beam allows for the construction of synthetic emission maps at various wavelengths, to be directly compared to observations. We apply this procedure for the jets originating from RW Aurigae.

Lecture Notes in Physics, 2008

The series Lecture Notes in Physics (LNP), founded in 1969, reports new developments in physics r... more The series Lecture Notes in Physics (LNP), founded in 1969, reports new developments in physics research and teaching-quickly and informally, but with a high quality and the explicit aim to summarize and communicate current knowledge in an accessible way. Books published in this series are conceived as bridging material between advanced graduate textbooks and the forefront of research and to serve three purposes: • to be a compact and modern up-to-date source of reference on a well-defined topic • to serve as an accessible introduction to the field to postgraduate students and nonspecialist researchers from related areas • to be a source of advanced teaching material for specialized seminars, courses and schools Both monographs and multi-author volumes will be considered for publication. Edited volumes should, however, consist of a very limited number of contributions only. Proceedings will not be considered for LNP. Volumes published in LNP are disseminated both in print and in electronic formats, the electronic archive being available at springerlink.com. The series content is indexed, abstracted and referenced by many abstracting and information services, bibliographic networks, subscription agencies, library networks, and consortia.

ABSTRACT In the present work we discuss how the strength of magnetic fields determines the charac... more ABSTRACT In the present work we discuss how the strength of magnetic fields determines the characteristics of solutions in models where the collimated outflow and the accretion disk are treated consistently. We perform a complete analysis of the range of magnetic field by non-relativistic 2.5 dimension numerical simulations using the PLUTO code. The main results are that magnetic fields around equipartition with plasma pressure allow steady super-fast-magnetosonic collimated jet solutions; magnetic fields below equipartition correspond to intermittent collimated outflows, while magnetic fields above equipartition lead to sub-alfvenic winds. These results allow to conclude that the configuration proposed by Blandford and Payne to interpret supersonic jets is viable both for equipartition and weaker magnetic fields.

Astrophysics and Space Science Proceedings, 2009

ABSTRACT

The Astrophysical Journal, 2011

We consider the problem of convergence in homogeneous shearing box simulations of magneto-rotatio... more We consider the problem of convergence in homogeneous shearing box simulations of magneto-rotationally driven turbulence. When there is no mean magnetic flux, if the equations are non dimensionalized with respect to the diffusive scale, the only free parameter in the problem is the size of the computational domain. The problem of convergence then relates to the asymptotic form of the solutions as the computational box size becomes large. By using a numerical code with a high order of accuracy we show that the solutions become asymptotically independent of domain size. We also show that cases with weak magnetic flux join smoothly to the zero flux cases as the flux vanishes. These results are consistent with the operation of a subcritical small-scale dynamo driving the turbulence. We conclude that for this type of turbulence the angular momentum transport is a proportional to the diffusive flux and therefore has limited relevance in astrophysical situations.

The Astrophysical Journal, 2008

We study the evolution of an accelerating hyperrelativistic shock under the presence of upstream ... more We study the evolution of an accelerating hyperrelativistic shock under the presence of upstream inhomogeneities wrinkling the discontinuity surface. The investigation is conducted by means of numerical simulations using the PLUTO code for astrophysical fluid dynamics. The reliability and robustness of the code are demonstrated against well known results coming from the linear perturbation theory. We then follow the nonlinear evolution of two classes of perturbing upstream atmospheres and conclude that no lasting wrinkle can be preserved indefinitely by the flow. Finally we derive analytically a description of the geometrical effects of a turbulent upstream ambient on the discontinuity surface.

The Astrophysical Journal, 2012

We investigate supersonic, axisymmetric magnetohydrodynamic (MHD) jets with a time-dependent inje... more We investigate supersonic, axisymmetric magnetohydrodynamic (MHD) jets with a time-dependent injection velocity by numerical simulations with the PLUTO code. Using a comprehensive set of parameters, we explore different jet configurations in the attempt to construct models that can be directly compared to observational data of microjets. In particular, we focus our attention on the emitting properties of traveling knots and construct, at the same time, accurate line intensity ratios and surface brightness maps. Direct comparison of the resulting brightness and line intensity ratios distributions with observational data of microjets shows that a closer match can be obtained only when the jet material is pre-ionized to some degree. A very likely source for a pre-ionized medium is photoionization by X-ray flux coming from the central object.

Monthly Notices of the Royal Astronomical Society, 2011

Young low-mass stars are characterized by ejection of collimated outflows and by circumstellar di... more Young low-mass stars are characterized by ejection of collimated outflows and by circumstellar discs which they interact with through accretion of mass. The accretion builds up the star to its final mass and is also believed to power the mass outflows, which may in turn remove the excess angular momentum from the star-disc system. However, although the process of mass accretion is a critical aspect of star formation, some of its mechanisms are still to be fully understood. A point not considered to date and relevant for the accretion process is the evidence of very energetic and frequent flaring events in these stars. Flares may easily perturb the stability of the discs, thus influencing the transport of mass and angular momentum. Here we report on 3D magnetohydrodynamic modelling of the evolution of a flare with an idealized non-equilibrium initial condition occurring near the disc around a rotating magnetized star. The model takes into account the stellar magnetic field, the gravitational force, the viscosity of the disc, the magnetic-field-oriented thermal conduction (including the effects of heat flux saturation), the radiative losses from optically thin plasma and the coronal heating. We show that, during its first stage of evolution, the flare gives rise to a hot magnetic loop linking the disc to the star. The disc is strongly perturbed by the flare: disc material evaporates under the effect of the thermal conduction and an overpressure wave propagates through the disc. When the overpressure reaches the opposite side of the disc, a funnel flow starts to develop there, accreting substantial disc material on to the young star from the side of the disc opposite to the flare.

Monthly Notices of the Royal Astronomical Society, 2013

We investigate the onset of pressure-driven toroidal-mode instabilities in accretion mounds on ne... more We investigate the onset of pressure-driven toroidal-mode instabilities in accretion mounds on neutron stars by 3D magnetohydrodynamic (MHD) simulations using the PLUTO MHD code. Our results confirm that for mounds beyond a threshold mass, instabilities form fingerlike channels at the periphery, resulting in mass-loss from the magnetically confined mound. Ring-like mounds with hollow interior show the instabilities at the inner edge as well. We perform the simulations for mounds of different sizes to investigate the effect of the mound mass on the growth rate of the instabilities. We also investigate the effect of such instabilities on observables such as cyclotron resonant scattering features and timing properties of such systems.