145 research outputs found
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&
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} 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 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 .Comment: 17 pages, 10 figures, Accepted for publication in A&
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&
Dynamical and radiative properties of astrophysical supersonic jets
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
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
Aerosol mediated localized dissolution to enhance the electrical behavior and sensitivity of piezoresistive nanofiber-based flexible sensors
This work proposes the use of solvents in the form of small size droplets to improve the connections among nanofibers (NFs) in electrospun composite nanofibers with carbon nanotube multiwalled (MWCNT) by improving the electrical and piezoresistive behavior of such electrically conductive polymer composites. The here proposed Aerosol Mediated Localized Dissolution (AMLD) process has been shown to be effective in improving the 3D microporous NF mat by inducing local dissolution that is effective in improving the connections among fibers within the mat. The AMLD process is demonstrated here for polyethylene oxide (PEO) / MWCNTs composite nanofibers, showing that the electrical conductivity is particularly improved in those samples with low content of MWCNTs, even below the original percolation threshold. The improved electrical conductivity is coupled with exceptional sensitivity of the flex sensor for low MWCNTs contents, this is particularly due to the ability of the AMLD process to preserve the high surface area of the 3D mat by inducing better fiber-to-fiber contacts in few regions only. In addition, this work demonstrates the effectiveness of applying an electrical potential difference during the AMLD process to improve the alignment of MWCNTs within the 3D microporous NF mat. The combination of voltage and AMLD allow to obtain a gauge factor as high as 571.9 with a MWCNTs loading of 1 wt%
Time-Dependent MHD Shocks and Line Emission: The Case of the DG Tau Jet
The line emission from a growing number of Herbig-Haro jets can be observed
and resolved at angular distances smaller than a few arcseconds from the
central source. The interpretation of this emission is problematic, since the
simplest model of a cooling jet cannot sustain it. It has been suggested that
what one actually observes are shocked regions with a filling factor of . In this framework, up to now, comparisons with observations have been
based on stationary shock models. Here we introduce for the first time the
self-consistent dynamics of such shocks and we show that considering their
properties at different times, i.e. locations, we can reproduce observational
data of the DG Tau microjet. In particular, we can interpret the spatial
behavior of the [SII]6716/6731 and [NII]/[OI]6583/6300 line intensity ratios
adopting a set of physical parameters that yield values of mass loss rates and
magnetic fields consistent with previous estimates. We also obtain the values
of the mean ionization fraction and electron density along the jet, compare
these values with the ones derived from observations using the sulfur doublet
to constrain the electron density (e.g. Bacciotti et al. 1995).Comment: 6 pages, 3 figure
3D relativistic MHD numerical simulations of X-shaped radio sources
Context. A significant fraction of extended radio sources presents a peculiar X-shaped radio morphology: in addition to the classical double lobed structure, radio emission is also observed along a second axis of symmetry in the form of diffuse wings or tails. In a previous investigation we showed the existence of a connection between the radio morphology and the properties of the host galaxies. Motivated by this connection we performed two-dimensional numerical simulations showing that X-shaped radio sources may naturally form as a jet propagates along the major axis a highly elliptical density distribution, because of the fast expansion of the cocoon along the minor axis of the distribution.
Aims. We intend to extend our analysis by performing three-dimensional numerical simulations and investigating the role of different parameters in determining the formation of the X-shaped morphology.
Methods. The problem is addressed by numerical means, carrying out three-dimensional relativistic magnetohydrodynamic simulations of bidirectional jets propagating in a triaxial density distribution.
Results. We show that only jets with power ≲ 1044 erg s-1 can give origin to an X-shaped morphology and that a misalignment of 30° between the jet axis and the major axis of the density distribution is still favourable to the formation of this kind of morphology. In addition we compute synthetic radio emission maps and polarization maps.
Conclusions. In our scenario for the formation of X-shaped radio sources only low power FRII can give origin to such kind of morphology. Our synthetic emission maps show that the different observed morphologies of X-shaped sources can be the result of similar structures viewed under different perspectives
The different flavors of extragalactic jets: the role of relativistic flow deceleration
We perform three-dimensional numerical simulations of relativistic (with a
Lorentz factor of 10), non magnetized jets propagating in a uniform density
environment, in order to study the effect of the entrainment and the consequent
deceleration. Our simulations investigate the jet propagation inside the galaxy
core, where, most likely, the deceleration occurs more efficiently. We compare
cases with different density and pressure ratios with respect to the ambient
medium finding that low density jets are efficiently decelerated and reach a
quasi steady state in which, over a length of 600 jet radii, slow down from
highly to sub-relativistic velocities. At the opposite, denser jets keep highly
relativistic velocity over the same length. We discuss these results in
relation to the Faranoff Riley (FR) radio-sources classification. We infer that
lower density jets can give rise to FR 0 and FR I radio-sources, while higher
density jets may be connected to FR II radio-sources.Comment: 11 pages, 16 figures, A&A accepte
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