2,586 research outputs found
3-D Simulations of Protostellar Jets in Stratified Ambient Media
We present fully three-dimensional hydrodynamical simulations of radiative
cooling jets propagating into stratified isothermal ambient media with
power-law density and pressure distributions. The parameters used are mainly
suitable for protostellar jets but results applicable to extragalactic jets are
also presented. Comparisons are made with previous simulations of jets through
homogeneous media. We find that for radiative cooling jets propagating into
regions where the ambient medium has an increasing density (and pressure)
gradient, the ambient gas tends to compress the cold, low-pressure cocoon of
shocked material that surrounds the beam and destroy the bow shock-like
structure at the head. The compressing medium collimates the jet and promotes
the development of Kelvin-Helmholtz instabilities which cause beam focusing,
wiggling and the formation of internal traveling shocks,
, via pinching along the beam. This remarkably resembles the structure of
some observed systems (e.g. Haro 6-5B northern and HH 24G jets). These effects
are larger for jets with smaller density ratio between jet and environment
(tested for =1, 3, and 10) and larger Mach number
(tested for 12 and 24, where is the jet velocity and the
ambient sound speed). In an ambient medium of decreasing density (and
pressure), the beam is poorly collimated and relaxes, becoming faint. This
could explain ''invisible'' jet sections, like the gap between the parent
source and collimated beam (e.g., in HH30 jet). Although, on average, jets
propagating into an increasing (decreasing) density environment are decelerated
(accelerated) by the increasing (decreasing) ram pressure of the ambient
medium, we find that their propagation velocities have an oscillating pattern.Comment: 33 pp, LaTeX file, 13 figures upon request. To appear in the
Astrophys. J., vol 471, nov. 10t
Evidence for a Very Large-Scale Fractal Structure in the Universe from Cobe Measurements
In this work, we analyse the temperature fluctuations of the cosmic microwave
background radiation observed by COBE and show that the distribution can be
fitted by a fractal distribution with a fractal dimension .
This value is in close agreement with the fractal dimension obtained by Coleman
and Pietronero (1992) and Luo and Schramm (1992) from galaxy-galaxy and
cluster-cluster correlations up to . The fact that the
observed temperature fluctuations correspond to scales much larger than and are signatures of the primordial density fluctuations at the
recombination layer suggests that the structure of the matter at the early
universe was already fractal and thus non-homogeneous on those scales. This
result may have important consequences for the theoretical framework that
describes the universe.Comment: 11 pages, postscript file, 2 figures available upon request. To
appear in ApJ Letter
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
Modeling the spectrum of gravitational waves in the primordial Universe
Recent observations from type Ia Supernovae and from cosmic microwave
background (CMB) anisotropies have revealed that most of the matter of the
Universe interacts in a repulsive manner, composing the so-called dark energy
constituent of the Universe. The analysis of cosmic gravitational waves (GW)
represents, besides the CMB temperature and polarization anisotropies, an
additional approach in the determination of parameters that may constrain the
dark energy models and their consistence. In recent work, a generalized
Chaplygin gas model was considered in a flat universe and the corresponding
spectrum of gravitational waves was obtained. The present work adds a massless
gas component to that model and the new spectrum is compared to the previous
one. The Chaplygin gas is also used to simulate a -CDM model by means
of a particular combination of parameters so that the Chaplygin gas and the
-CDM models can be easily distinguished in the theoretical scenarios
here established. The lack of direct observational data is partialy solved when
the signature of the GW on the CMB spectra is determined.Comment: Proc. of the Conference on Magnetic Fields in the Universe: from
laboratories and stars to primordial structures, AIP(NY), eds. E. M. de
Gouveia Dal Pino, G. Lugones & A. Lazarian (2005), in press. (8 pages, 11
figures
Rickettsial Infection Caused by Accidental Conjunctival Inoculation
The most common transmission route of tick-borne Rickettsia is through tick bite; nevertheless, other transmission routes should also be considered. We report a case of rickettsial infection in a 15-year-old boy caused by accidental contamination of the conjunctiva through the infected fluid of a crushed engorged tick removed from a dog. Right eye pain, conjunctival hyperaemia with mucopurulent exudate, chemosis and eyelid oedema were the first signs and symptoms. Two days later, the boy developed fever, myalgia, headache, abdominal pain and was vomiting; physical examination showed multiple cervical adenopathies but no rash. He was treated with doxycycline (200 mg/day) for 7 days with progressive resolution of clinical signs. Rickettsial infection was confirmed by immunofluorescence assay with serological seroconversion in two consecutive samples. Rickettsia conorii or Rickettsia massiliae were the possible causal agents since they are the Rickettsia spp found in the Rhipicephalus sanguineus dog tick in Portugal
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
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