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

Papers by A. Mignone

Monthly Notices of the Royal Astronomical Society, 2014

At least 5 per cent of the massive stars are moving supersonically through the interstellar mediu... more At least 5 per cent of the massive stars are moving supersonically through the interstellar medium (ISM) and are expected to produce a stellar wind bow shock. We explore how the mass loss and space velocity of massive runaway stars affect the morphology of their bow shocks. We run two-dimensional axisymmetric hydrodynamical simulations following the evolution of the circumstellar medium of these stars in the Galactic plane from the main sequence to the red supergiant phase. We find that thermal conduction is an important process governing the shape, size and structure of the bow shocks around hot stars, and that they have an optical luminosity mainly produced by forbidden lines, e.g. [O III]. The Hα emission of the bow shocks around hot stars originates from near their contact discontinuity. The Hα emission of bow shocks around cool stars originates from their forward shock, and is too faint to be observed for the bow shocks that we simulate. The emission of optically-thin radiation mainly comes from the shocked ISM material. All bow shock models are brighter in the infrared, i.e. the infrared is the most appropriate waveband to search for bow shocks. Our study suggests that the infrared emission comes from near the contact discontinuity for bow shocks of hot stars and from the inner region of shocked wind for bow shocks around cool stars. We predict that, in the Galactic plane, the brightest, i.e. the most easily detectable bow shocks are produced by high-mass stars moving with small space velocities.

ERCOFTAC Series, 2010

A method for the localization of the regions where the turbulent fluctuations are unresolved is a... more A method for the localization of the regions where the turbulent fluctuations are unresolved is applied to the selective large-eddy simulation (LES) of a compressible turbulent jet of Mach number equal to 5. This method is based on the introduction of a scalar probe function f which represents the magnitude of the twisting-stretching term normalized with the enstrophy [1]. The statistical analysis shows that, for a fully developed turbulent field of fluctuations, the probability that f is larger than 2 is zero, while, for an unresolved field, is finite. By computing f in each instantaneous realization of the simulation it is possible to locate the regions where the magnitude of the normalized stretching-twisting is anomalously high. This allows the identification of the regions where the subgrid model should be introduced into the governing equations (selective filtering).

Astrophysics and Space Science Proceedings, 2009

Aims. The transport of energy through radiation is very important in many astrophysical phenomena... more Aims. The transport of energy through radiation is very important in many astrophysical phenomena. In dynamical problems the timedependent equations of radiation hydrodynamics have to be solved. We present a newly developed radiation-hydrodynamics module specifically designed for the versatile MHD code PLUTO . Methods. The solver is based on the flux-limited diffusion approximation in the two-temperature approach. All equations are solved in the co-moving frame in the frequency independent (grey) approximation. The hydrodynamics is solved by the different Godunov schemes implemented in PLUTO , and for the radiation transport we use a fully implicit scheme. The resulting system of linear equations is solved either using the successive over-relaxation (SOR) method (for testing purposes), or matrix solvers that are available in the PETSc library. We state in detail the methodology and describe several test cases in order to verify the correctness of our implementation. The solver works in standard coordinate systems, such as Cartesian, cylindrical and spherical, and also for nonequidistant grids. Results. We have presented a new radiation-hydrodynamics solver coupled to the MHD-code PLUTO that is a modern, versatile and efficient new module for treating complex radiation hydrodynamical problems in astrophysics. As test cases, either purely radiative situations, or full radiation-hydrodynamical setups (including radiative shocks and convection in accretion discs) have been studied successfully. The new module scales very well on parallel computers using MPI. For problems in star or planet formation, we have added the possibility of irradiation by a central source. et al.: Radiation hydrodynamics integrated in the code PLUTO to show the correctness of the implemented equations: four test cases with an analytical solution (section 4.1 to 4.4) and two others in which our results are compared with those from other codes (section 4.5 and 4.6). We end with a summary and conclusions.

ABSTRACT A plethora of analytical studies have addressed the physical mechanisms of jet launching... more ABSTRACT A plethora of analytical studies have addressed the physical mechanisms of jet launching and propagation in young stellar objects. However, their link to observations is still missing due to the complexity of the emission processes involved. In this work we address this issue, by presenting MHD simulations of two-component YSO jet models that are based on analytical disk and stellar outflow solutions. We include ionization and optically thin radiation losses during the temporal evolution of the flow and we post process the output files to generate synthetic emission maps. Our results are confronted to observational data and we find that our models predict the correct range of values for the density, temperature and velocity of YSO jets. Moreover, the synthetic emission maps of the - 39 - doublets [OI], [N II] and [S II] outline a well collimated and knot-structured jet, which is surrounded by a less dense and slower wind, not observable in these lines. The jet is found to have a small opening angle and a radius that is also comparable to observations.

A common approach for the interpretation of such astrophysical objects as active galactic nuclei ... more A common approach for the interpretation of such astrophysical objects as active galactic nuclei (AGN) and young stellar objects (YSOs) are the MHD models of collimated outflows, driven by accretion disks in the proximity of a central attractor. In this study we present a series of time dependent axisymmetric numerical simulations of a jet-launching accretion disk, where both the outflow

Herbig-Haro jets cool radiatively in a very effective way. Nevertheless, high resolution observat... more Herbig-Haro jets cool radiatively in a very effective way. Nevertheless, high resolution observations show that temperature along the jet remains above 5,000 K and ionization above 10%, showing a tendency to increase in some regions. Therefore one needs to examine whether there are mechanisms that can successfully balance radiative losses. In this paper we assume that the fraction of the

Nuovo Cimento della Societa Italiana di Fisica C, 2009

ABSTRACT The new High Power Computing facilities available for the scientific community allowed t... more ABSTRACT The new High Power Computing facilities available for the scientific community allowed the use of increasingly complex codes for the numerical simulation of physical processes. Recent magnetohydrodynamic (MHD) simulations of astrophysical jets could finally include non-ideal effects, such as the radiative cooling we will discuss in this work in the context of protostellar jets. This makes the simulations more reliable and, with the recent improvements in available observational data, will provide a valuable tool for model discrimination. From 2D adaptive refinement (AMR) simulations, synthetic surface brightness maps for the line emissions are computed, to be compared with observations.

Nuovo Cimento Della Societa Italiana Di Fisica C-Colloquia on Physics, 2009

ABSTRACT In this study we discuss the impact of the magnetic field's strength onto the ch... more ABSTRACT In this study we discuss the impact of the magnetic field's strength onto the characteristics of solutions in models where both the collimated outflow and the accretion disk are treated consistently. We perform an analysis on 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 for steady super-fast-magnetosonic collimated jet solutions; magnetic fields below equipartition correspond to intermittent collimated outflows, whereas above equipartition cases lead to sub-alfvenic wind. This allows to conclude that the configuration proposed by Blandford and Payne to interpret supersonic jets is viable both for equipartition and weaker magnetic fields.

EAS Publications Series, 2012

Hypersonic, collimated jets are being lately intensively studied in Earth laboratories, trying to... more Hypersonic, collimated jets are being lately intensively studied in Earth laboratories, trying to reproduce some of the physical properties of a subclass of astrophysical jets that are the Herbig-Haro (HH) jets. These jets are produced in the regions around Young Stellar Objects (YSOs), that are proto-stars located inside galactic Giant Molecular Clouds. In addition to the novel experimental approach, HH or YSO jets have been object of interest by the astrophysical community since a few decades and studied by means of observations at different wavelengths and analytical and numerical modeling. We present laboratory experiments and 2D numerical simulations of hypersonic jets, comparing the results of experiments and simulations that reproduce the evolution of the above mentioned jets. The experimental flows match two main scaling parameter requirements for proto-stellar jets, i.e. the ejection Mach number M and the jet/ambient density ratio η. In particular, η goes from slightly underdense to overdense values. Furthermore, as a development of previous works, we consider here the dependence of the jet structure and morphology on the Mach number, in the range 10 to 15.

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.

Physical Review E, 2004

We reexamine the stability of an interface separating two nonmagnetized relativistic fluids in re... more We reexamine the stability of an interface separating two nonmagnetized relativistic fluids in relative motion, showing that, in an appropriate reference frame, it is possible to find analytic solutions to the dispersion relation. Moreover, we show that the critical value of the Mach number, introduced by compressibility, is unchanged from the nonrelativistic case if we redefine the Mach number as M= [beta/ (1- beta(2) )(1/2) ] [ beta(s) / (1- beta(2)(s) )(1/2) ](-1) , where beta and beta(s) are, respectively, the speed of the fluid and the speed of sound (in units of the speed of light).

New Journal of Physics, 2011

Astrophysical jets are ubiquitous throughout the universe. They can be observed to emerge from pr... more Astrophysical jets are ubiquitous throughout the universe. They can be observed to emerge from protostellar objects, stellar x-ray binaries and supermassive black holes located at the center of active galaxies, and they are believed to originate from a central object that is surrounded by a magnetized accretion disc. With the motivations to understand whether hypersonic Newtonian jets produce any similarity to the morphologies observed in jets from young stellar objects (YSOs) and whether numerical codes, based on Godunov-type schemes, capture the basic physics of shocked flows, we have conceived a laboratory experiment and performed three-dimensional (3D) numerical simulations that reproduce the mid-to-long-term evolution of hypersonic jets. Here we show that these jets propagate, maintaining their collimation over long distances, in units of the jet initial radius. The jets studied are quasi-isentropic, are both lighter and heavier than the ambient and meet the two main scaling parameter requirements for proto-stellar jets: the ejection Mach number and the ambient/jet density ratio.

New Astronomy Reviews, 2003

ABSTRACT

New Astronomy Reviews, 2003

Monthly Notices of the Royal Astronomical Society, 2014

At least 5 per cent of the massive stars are moving supersonically through the interstellar mediu... more At least 5 per cent of the massive stars are moving supersonically through the interstellar medium (ISM) and are expected to produce a stellar wind bow shock. We explore how the mass loss and space velocity of massive runaway stars affect the morphology of their bow shocks. We run two-dimensional axisymmetric hydrodynamical simulations following the evolution of the circumstellar medium of these stars in the Galactic plane from the main sequence to the red supergiant phase. We find that thermal conduction is an important process governing the shape, size and structure of the bow shocks around hot stars, and that they have an optical luminosity mainly produced by forbidden lines, e.g. [O III]. The Hα emission of the bow shocks around hot stars originates from near their contact discontinuity. The Hα emission of bow shocks around cool stars originates from their forward shock, and is too faint to be observed for the bow shocks that we simulate. The emission of optically-thin radiation mainly comes from the shocked ISM material. All bow shock models are brighter in the infrared, i.e. the infrared is the most appropriate waveband to search for bow shocks. Our study suggests that the infrared emission comes from near the contact discontinuity for bow shocks of hot stars and from the inner region of shocked wind for bow shocks around cool stars. We predict that, in the Galactic plane, the brightest, i.e. the most easily detectable bow shocks are produced by high-mass stars moving with small space velocities.

ERCOFTAC Series, 2010

A method for the localization of the regions where the turbulent fluctuations are unresolved is a... more A method for the localization of the regions where the turbulent fluctuations are unresolved is applied to the selective large-eddy simulation (LES) of a compressible turbulent jet of Mach number equal to 5. This method is based on the introduction of a scalar probe function f which represents the magnitude of the twisting-stretching term normalized with the enstrophy [1]. The statistical analysis shows that, for a fully developed turbulent field of fluctuations, the probability that f is larger than 2 is zero, while, for an unresolved field, is finite. By computing f in each instantaneous realization of the simulation it is possible to locate the regions where the magnitude of the normalized stretching-twisting is anomalously high. This allows the identification of the regions where the subgrid model should be introduced into the governing equations (selective filtering).

Astrophysics and Space Science Proceedings, 2009

Aims. The transport of energy through radiation is very important in many astrophysical phenomena... more Aims. The transport of energy through radiation is very important in many astrophysical phenomena. In dynamical problems the timedependent equations of radiation hydrodynamics have to be solved. We present a newly developed radiation-hydrodynamics module specifically designed for the versatile MHD code PLUTO . Methods. The solver is based on the flux-limited diffusion approximation in the two-temperature approach. All equations are solved in the co-moving frame in the frequency independent (grey) approximation. The hydrodynamics is solved by the different Godunov schemes implemented in PLUTO , and for the radiation transport we use a fully implicit scheme. The resulting system of linear equations is solved either using the successive over-relaxation (SOR) method (for testing purposes), or matrix solvers that are available in the PETSc library. We state in detail the methodology and describe several test cases in order to verify the correctness of our implementation. The solver works in standard coordinate systems, such as Cartesian, cylindrical and spherical, and also for nonequidistant grids. Results. We have presented a new radiation-hydrodynamics solver coupled to the MHD-code PLUTO that is a modern, versatile and efficient new module for treating complex radiation hydrodynamical problems in astrophysics. As test cases, either purely radiative situations, or full radiation-hydrodynamical setups (including radiative shocks and convection in accretion discs) have been studied successfully. The new module scales very well on parallel computers using MPI. For problems in star or planet formation, we have added the possibility of irradiation by a central source. et al.: Radiation hydrodynamics integrated in the code PLUTO to show the correctness of the implemented equations: four test cases with an analytical solution (section 4.1 to 4.4) and two others in which our results are compared with those from other codes (section 4.5 and 4.6). We end with a summary and conclusions.

ABSTRACT A plethora of analytical studies have addressed the physical mechanisms of jet launching... more ABSTRACT A plethora of analytical studies have addressed the physical mechanisms of jet launching and propagation in young stellar objects. However, their link to observations is still missing due to the complexity of the emission processes involved. In this work we address this issue, by presenting MHD simulations of two-component YSO jet models that are based on analytical disk and stellar outflow solutions. We include ionization and optically thin radiation losses during the temporal evolution of the flow and we post process the output files to generate synthetic emission maps. Our results are confronted to observational data and we find that our models predict the correct range of values for the density, temperature and velocity of YSO jets. Moreover, the synthetic emission maps of the - 39 - doublets [OI], [N II] and [S II] outline a well collimated and knot-structured jet, which is surrounded by a less dense and slower wind, not observable in these lines. The jet is found to have a small opening angle and a radius that is also comparable to observations.

A common approach for the interpretation of such astrophysical objects as active galactic nuclei ... more A common approach for the interpretation of such astrophysical objects as active galactic nuclei (AGN) and young stellar objects (YSOs) are the MHD models of collimated outflows, driven by accretion disks in the proximity of a central attractor. In this study we present a series of time dependent axisymmetric numerical simulations of a jet-launching accretion disk, where both the outflow

Herbig-Haro jets cool radiatively in a very effective way. Nevertheless, high resolution observat... more Herbig-Haro jets cool radiatively in a very effective way. Nevertheless, high resolution observations show that temperature along the jet remains above 5,000 K and ionization above 10%, showing a tendency to increase in some regions. Therefore one needs to examine whether there are mechanisms that can successfully balance radiative losses. In this paper we assume that the fraction of the

Nuovo Cimento della Societa Italiana di Fisica C, 2009

ABSTRACT The new High Power Computing facilities available for the scientific community allowed t... more ABSTRACT The new High Power Computing facilities available for the scientific community allowed the use of increasingly complex codes for the numerical simulation of physical processes. Recent magnetohydrodynamic (MHD) simulations of astrophysical jets could finally include non-ideal effects, such as the radiative cooling we will discuss in this work in the context of protostellar jets. This makes the simulations more reliable and, with the recent improvements in available observational data, will provide a valuable tool for model discrimination. From 2D adaptive refinement (AMR) simulations, synthetic surface brightness maps for the line emissions are computed, to be compared with observations.

Nuovo Cimento Della Societa Italiana Di Fisica C-Colloquia on Physics, 2009

ABSTRACT In this study we discuss the impact of the magnetic field's strength onto the ch... more ABSTRACT In this study we discuss the impact of the magnetic field's strength onto the characteristics of solutions in models where both the collimated outflow and the accretion disk are treated consistently. We perform an analysis on 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 for steady super-fast-magnetosonic collimated jet solutions; magnetic fields below equipartition correspond to intermittent collimated outflows, whereas above equipartition cases lead to sub-alfvenic wind. This allows to conclude that the configuration proposed by Blandford and Payne to interpret supersonic jets is viable both for equipartition and weaker magnetic fields.

EAS Publications Series, 2012

Hypersonic, collimated jets are being lately intensively studied in Earth laboratories, trying to... more Hypersonic, collimated jets are being lately intensively studied in Earth laboratories, trying to reproduce some of the physical properties of a subclass of astrophysical jets that are the Herbig-Haro (HH) jets. These jets are produced in the regions around Young Stellar Objects (YSOs), that are proto-stars located inside galactic Giant Molecular Clouds. In addition to the novel experimental approach, HH or YSO jets have been object of interest by the astrophysical community since a few decades and studied by means of observations at different wavelengths and analytical and numerical modeling. We present laboratory experiments and 2D numerical simulations of hypersonic jets, comparing the results of experiments and simulations that reproduce the evolution of the above mentioned jets. The experimental flows match two main scaling parameter requirements for proto-stellar jets, i.e. the ejection Mach number M and the jet/ambient density ratio η. In particular, η goes from slightly underdense to overdense values. Furthermore, as a development of previous works, we consider here the dependence of the jet structure and morphology on the Mach number, in the range 10 to 15.

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.

Physical Review E, 2004

We reexamine the stability of an interface separating two nonmagnetized relativistic fluids in re... more We reexamine the stability of an interface separating two nonmagnetized relativistic fluids in relative motion, showing that, in an appropriate reference frame, it is possible to find analytic solutions to the dispersion relation. Moreover, we show that the critical value of the Mach number, introduced by compressibility, is unchanged from the nonrelativistic case if we redefine the Mach number as M= [beta/ (1- beta(2) )(1/2) ] [ beta(s) / (1- beta(2)(s) )(1/2) ](-1) , where beta and beta(s) are, respectively, the speed of the fluid and the speed of sound (in units of the speed of light).

New Journal of Physics, 2011

Astrophysical jets are ubiquitous throughout the universe. They can be observed to emerge from pr... more Astrophysical jets are ubiquitous throughout the universe. They can be observed to emerge from protostellar objects, stellar x-ray binaries and supermassive black holes located at the center of active galaxies, and they are believed to originate from a central object that is surrounded by a magnetized accretion disc. With the motivations to understand whether hypersonic Newtonian jets produce any similarity to the morphologies observed in jets from young stellar objects (YSOs) and whether numerical codes, based on Godunov-type schemes, capture the basic physics of shocked flows, we have conceived a laboratory experiment and performed three-dimensional (3D) numerical simulations that reproduce the mid-to-long-term evolution of hypersonic jets. Here we show that these jets propagate, maintaining their collimation over long distances, in units of the jet initial radius. The jets studied are quasi-isentropic, are both lighter and heavier than the ambient and meet the two main scaling parameter requirements for proto-stellar jets: the ejection Mach number and the ambient/jet density ratio.

New Astronomy Reviews, 2003

ABSTRACT

New Astronomy Reviews, 2003