2,555 research outputs found
Galactic Outflows and the pollution of the Galactic Environment by Supernovae
We here explore the effects of the SN explosions into the environment of
star-forming galaxies like the Milky Way. Successive randomly distributed and
clustered SNe explosions cause the formation of hot superbubbles that drive
either fountains or galactic winds above the galactic disk, depending on the
amount and concentration of energy that is injected by the SNe. In a galactic
fountain, the ejected gas is re-captured by the gravitational potential and
falls back onto the disk. From 3D nonequilibrium radiative cooling
hydrodynamical simulations of these fountains, we find that they may reach
altitudes up to about 5 kpc in the halo and thus allow for the formation of the
so called intermediate-velocity-clouds (IVCs) which are often observed in the
halos of disk galaxies. The high-velocity-clouds that are also observed but at
higher altitudes (of up to 12 kpc) require another mechanism to explain their
production. We argue that they could be formed either by the capture of gas
from the intergalactic medium and/or by the action of magnetic fields that are
carried to the halo with the gas in the fountains. Due to angular momentum
losses to the halo, we find that the fountain material falls back to smaller
radii and is not largely spread over the galactic disk. Instead, the SNe ejecta
fall nearby the region where the fountain was produced, a result which is
consistent with recent chemical models of the galaxy. The fall back material
leads to the formation of new generations of molecular clouds and to supersonic
turbulence feedback in the disk.Comment: 10 pages, 5 figures; paper of invited talk for the Procs. of the 2007
WISER Workshop (World Space Environment Forum), Alexandria, Egypt, October
2007, Spa. Sci. Rev
Magnetic Field Effects on the Head Structure of Protostellar Jets
We present the results of 3-D SPMHD numerical simulations of
supermagnetosonic, overdense, radiatively cooling jets. Two initial magnetic
configurations are considered: (i) a helical and (ii) a longitudinal field. We
find that magnetic fields have important effects on the dynamics and structure
of radiative cooling jets, especially at the head. The presence of a helical
field suppresses the formation of the clumpy structure which is found to
develop at the head of purely hydrodynamical jets. On the other hand, a cooling
jet embedded in a longitudinal magnetic field retains clumpy morphology at its
head. This fragmented structure resembles the knotty pattern commonly observed
in HH objects behind the bow shocks of HH jets. This suggests that a strong
(equipartition) helical magnetic field configuration is ruled out at the jet
head. Therefore, if strong magnetic fields are present, they are probably
predominantly longitudinal in those regions. In both magnetic configurations,
we find that the confining pressure of the cocoon is able to excite
short-wavelength MHD K-H pinch modes that drive low-amplitude internal shocks
along the beam. These shocks are not strong however, and it likely that they
could only play a secondary role in the formation of the bright knots observed
in HH jets.Comment: 14 pages, 2 Gif figures, uses aasms4.sty. Also available on the web
page http://www.iagusp.usp.br/preprints/preprint.html. To appear in The
Astrophysical Journal Letter
Dynamo in the Intra-Cluster Medium: Simulation of CGL-MHD Turbulent Dynamo
The standard magnetohydrodynamic (MHD) description of the plasma in the hot,
magnetized gas of the intra-cluster (ICM) medium is not adequate because it is
weakly collisional. In such collisionless magnetized gas, the microscopic
velocity distribution of the particles is not isotropic, giving rise to kinetic
effects on the dynamical scales. These kinetic effects could be important in
understanding the turbulence, as so as the amplification and maintenance of the
magnetic fields in the ICM. It is possible to formulate fluid models for
collisonless or weakly collisional gas by introducing modifications in the MHD
equations. These models are often referred as kinetic MHD (KMHD). Using a KMHD
model based on the CGL-closure, which allows the adiabatic evolution of the two
components of the pressure tensor (the parallel and perpendicular components
with respect to the local magnetic field), we performed 3D numerical
simulations of forced turbulence in order to study the amplification of an
initially weak seed magnetic field. We found that the growth rate of the
magnetic energy is comparable to that of the ordinary MHD turbulent dynamo, but
the magnetic energy saturates in a level smaller than of the MHD case. We also
found that a necessary condition for the dynamo works is to impose limits to
the anisotropy of the pressure.Comment: 3 pages, 1 figure, 274 IAU Symposium: Advances in Plasma Astrophysic
Right Cardiac Intracavitary Metastases from a Primary Intracranial Myxofibrosarcoma
Primary intracranial myxofibrosarcoma is exceedingly rare, with less than 10 cases published. We present a case of a 23-year-old man with previous history of a primary low grade myxofibrosarcoma of the left parietal-occipital convexity resected in March 1999. He subsequently underwent several interventions for multiple local recurrent disease until March 2004. At that time, complete remission was documented. About 8 years later, in February 2012, the patient was admitted to the emergency room with refractory acute pulmonary oedema. On work up, sustained monomorphic ventricular tachycardia and hyperechoic myocardial mass with invasion of the right ventricular cavity were detected. Electrical cardioversion was unsuccessful and irreversible cardiac arrest followed. The autopsy confirmed multiple bilateral lung metastases, malignant pulmonary embolism and myocardial invasion by the primary tumour, with intracavitary cardiac thrombosis and absence of intracranial disease. To the best of our knowledge, this is the first report of extracranial metastases of this neoplasm.info:eu-repo/semantics/publishedVersio
Features of collisionless turbulence in the intracluster medium from simulated Faraday Rotation maps
Observations of the intracluster medium (ICM) in galaxy clusters suggest for
the presence of turbulence and the magnetic fields existence has been proved
through observations of Faraday Rotation and synchrotron emission. The ICM is
also known to be filled by a rarefied weakly collisional plasma. In this work
we study the possible signatures left on Faraday Rotation maps by collisionless
instabilities. For this purpose we use a numerical approach to investigate the
dynamics of the turbulence in collisionless plasmas based on an
magnetohydrodynamical (MHD) formalism taking into account different levels of
pressure anisotropy. We consider models covering the sub/super-Alfv\'enic and
trans/supersonic regimes, one of them representing the fiducial conditions
corresponding to the ICM. From the simulated models we compute Faraday Rotation
maps and analyze several statistical indicators in order to characterize the
magnetic field structure and compare the results obtained with the
collisionless model to those obtained using standard collisional MHD framework.
We find that important imprints of the pressure anisotropy prevails in the
magnetic field and also manifest in the associated Faraday Rotation maps which
evidence smaller correlation lengths in the collisionless MHD case. These
points are remarkably noticeable for the case mimicking the conditions
prevailing in ICM. Nevertheless, in this study we have neglected the decrease
of pressure anisotropy due to the feedback of the instabilities that naturally
arise in collisionless plasmas at small scales. This decrease may not affect
the statistical imprint differences described above, but should be examined
elsewhere.Comment: 24 pages, 15 figures, MNRAS accepte
Turbulence and the formation of filaments, loops and shock fronts in NGC 1275 in the Perseus Galaxy Cluster
NGC1275, the central galaxy in the Perseus cluster, is the host of gigantic
hot bipolar bubbles inflated by AGN jets observed in the radio as Perseus A. It
presents a spectacular -emitting nebulosity surrounding NGC1275,
with loops and filaments of gas extending to over 50 kpc. The origin of the
filaments is still unknown, but probably correlates with the mechanism
responsible for the giant buoyant bubbles. We present 2.5 and 3-dimensional MHD
simulations of the central region of the cluster in which turbulent energy,
possibly triggered by star formation and supernovae (SNe) explosions is
introduced. The simulations reveal that the turbulence injected by massive
stars could be responsible for the nearly isotropic distribution of filaments
and loops that drag magnetic fields upward as indicated by recent observations.
Weak shell-like shock fronts propagating into the ICM with velocities of
100-500 km/s are found, also resembling the observations. The isotropic outflow
momentum of the turbulence slows the infall of the intracluster medium, thus
limiting further starburst activity in NGC1275. As the turbulence is subsonic
over most of the simulated volume, the turbulent kinetic energy is not
efficiently converted into heat and additional heating is required to suppress
the cooling flow at the core of the cluster. Simulations combining the MHD
turbulence with the AGN outflow can reproduce the temperature radial profile
observed around NGC1275. While the AGN mechanism is the main heating source,
the supernovae are crucial to isotropize the energy distribution.Comment: accepted by ApJ Letter
Noether's Symmetry Theorem for Variational and Optimal Control Problems with Time Delay
We extend the DuBois-Reymond necessary optimality condition and Noether's
symmetry theorem to the time delay variational setting. Both Lagrangian and
Hamiltonian versions of Noether's theorem are proved, covering problems of the
calculus of variations and optimal control with delays.Comment: This is a preprint of a paper whose final and definite form will
appear in the international journal Numerical Algebra, Control and
Optimization (NACO). Paper accepted for publication 15-March-201
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