462 research outputs found
The Effect of Expansion on Mass Entrainment and Stability of Super-Alfv\'enic Jets
We extend investigations of mass entrainment by jets, which previously have
focused on cylindrical supermagnetosonic jets and expanding trans-Alfv\'enic
jets, to a set of expanding supermagnetosonic jets. We precess these jets at
the origin to excite the helical mode of the Kelvin-Helmholtz (or KH)
instability, in order to compare the results with predictions from linear
stability analysis. We analyze this simulation set for the spatial development
of magnetized mass, which we interpret as jet plus entrained, initially
unmagnetized external mass. As with the previous simulation sets, we find that
the growth of magnetized mass is associated with the growth of the KH
instability through linear, nonlinear, and saturated stages and with the
expansion of magnetized material in simulated observations of the jet. From
comparison of measured wavelengths and wave speeds with the predictions from
linear stability analysis, we see evidence that the KH instability is the
primary cause for mass entrainment in these simulations, and that the expansion
reduces the rate of mass entrainment. This reduced rate can be observed as a
somewhat greater distance between the two transition points separating the
three stages of expansion.Comment: 18 pages, 6 figures, AASTeX, to appear in Nov 1 issue of ApJ (vol
543), postscript versions of Figures 3 and 5 are available at
http://crux.astr.ua.edu/~rosen/supcon/rh.htm
Recollimation Shocks in Magnetized Relativistic Jets
We have performed two-dimensional special-relativistic magnetohydrodynamic
simulations of non-equilibrium over-pressured relativistic jets in cylindrical
geometry. Multiple stationary recollimation shock and rarefaction structures
are produced along the jet by the nonlinear interaction of shocks and
rarefaction waves excited at the interface between the jet and the surrounding
ambient medium. Although initially the jet is kinematically dominated, we have
considered axial, toroidal and helical magnetic fields to investigate the
effects of different magnetic-field topologies and strengths on the
recollimation structures. We find that an axial field introduces a larger
effective gas-pressure and leads to stronger recollimation shocks and
rarefactions, resulting in larger flow variations. The jet boost grows
quadratically with the initial magnetic field. On the other hand, a toroidal
field leads to weaker recollimation shocks and rarefactions, modifying
significantly the jet structure after the first recollimation rarefaction and
shock. The jet boost decreases systematically. For a helical field, instead,
the behaviour depends on the magnetic pitch, with a phenomenology that ranges
between the one seen for axial and toroidal magnetic fields, respectively. In
general, however, a helical magnetic field yields a more complex shock and
rarefaction substructure close to the inlet that significantly modifies the jet
structure. The differences in shock structure resulting from different field
configurations and strengths may have observable consequences for disturbances
propagating through a stationary recollimation shock.Comment: 14 pages, 15 figures and 1 table, accepted for publication in Ap
Erratic Jet Wobbling in the BL Lacertae Object OJ287 Revealed by Sixteen Years of 7mm VLBA Observations
We present the results from an ultra-high-resolution 7mm Very Long Baseline
Array (VLBA) study of the relativistic jet in the BL Lacertae object OJ287 from
1995 to 2011 containing 136 total intensity images. Analysis of the image
sequence reveals a sharp jet-position-angle swing by >100 deg. during
[2004,2006], as viewed in the plane of the sky, that we interpret as the
crossing of the jet from one side of the line of sight to the other during a
softer and longer term swing of the inner jet. Modulating such long term swing,
our images also show for the first time a prominent erratic wobbling behavior
of the innermost ~0.4mas of the jet with fluctuations in position angle of up
to ~40 deg. over time scales ~2yr. This is accompanied by highly superluminal
motions along non-radial trajectories, which reflect the remarkable
non-ballistic nature of the jet plasma on these scales. The erratic nature and
short time scales of the observed behavior rules out scenarios such as binary
black hole systems, accretion disk precession, and interaction with the ambient
medium as possible origins of the phenomenon on the scales probed by our
observations, although such processes may cause longer-term modulation of the
jet direction. We propose that variable asymmetric injection of the jet flow;
perhaps related to turbulence in the accretion disk; coupled with hydrodynamic
instabilities, leads to the non-ballistic dynamics that cause the observed
non-periodic changes in the direction of the inner jet.Comment: Accepted for Publication in The Astrophysical Journal. 11 pages, 6
figures, 4 tables. High resolution images on figure 1 and complete tables 1
and 2 may be provided on reques
A Comparison of the Morphology and Stability of Relativistic and Nonrelativistic Jets
We compare results from a relativistic and a nonrelativistic set of 2D
axisymmetric jet simulations. For a set of five relativistic simulations that
either increase the Lorentz factor or decrease the adiabatic index we compute
nonrelativistic simulations with equal useful power or thrust. We examine these
simulations for morphological and dynamical differences, focusing on the
velocity field, the width of the cocoon, the age of the jets, and the internal
structure of the jet itself. The primary result of these comparisons is that
the velocity field of nonrelativistic jet simulations cannot be scaled up to
give the spatial distribution of Lorentz factors seen in relativistic
simulations. Since the local Lorentz factor plays a major role in determining
the total intensity for parsec scale extragalactic jets, this suggests that a
nonrelativistic simulation cannot yield the proper intensity distribution for a
relativistic jet. Another general result is that each relativistic jet and its
nonrelativistic equivalents have similar ages (in dynamical time units, =
R/a_a, where R is the initial radius of a cylindrical jet and a_a is the sound
speed in the ambient medium). In addition to these comparisons, we have
completed four new relativistic simulations to investigate the effect of
varying thermal pressure on relativistic jets. The simulations generally
confirm that faster (larger Lorentz factor) and colder jets are more stable,
with smaller amplitude and longer wavelength internal variations. The apparent
stability of these jets does not follow from linear normal mode analysis, which
suggests that there are available growing Kelvin-Helmholtz modes. (Abridged.)Comment: 32 pages, AASTEX, to appear in May 10, 1999 issue of ApJ, better
versions of Figures 1 and 6 are available at
http://crux.astr.ua.edu/~rosen/rel/rhdh.htm
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Ewing sarcoma in a child with neurofibromatosis type 1.
We report here on a case of Ewing sarcoma (ES) occurring in a child with neurofibromatosis type 1. The sarcoma had an EWSR1-ERG translocation as well as loss of the remaining wild-type allele of NF1. Loss of the NF1 wild-type allele in the tumor suggests that activation of the Ras pathway contributed to its evolution. Review of available public data suggests that secondary mutations in the Ras pathway are found in ∼3% of ESs. This case suggests that Ras pathway activation may play a role in tumor progression in a subset of ESs
Semiconductor Electrodes: V. The Application of Chemically Vapor Deposited Iron Oxide Films to Photosensitized Electrolysis
ABSTRACT The preparation of polycrystalline n-type Fe203 electrodes by the chemical vapor deposition (CVD) of iron oxide onto Ti and Pt substrates is reported. The behavior of these electrodes in aqueous solutions of different pH in the absence and presence of illumination is shown. Photoassisted electrolysis of water occurs at wavelengths longer than 400 nm and the current vs. wavelength curve for this process is compared to that of a CVD TiO2 electrode. Recently there has been much interest in the utilization of semiconductor electrodes for solar energy conversion by the photoassisted electrolysis of water (1-4). By illuminating an n-type electrode with light energy greater than the bandgap energy, holes are produced in the valence band which, under anodic polarization, migrate to the surface where they can react with a species in solution. Thus with a suitable semiconductor electrode it is possible to oxidize water to oxygen utilizing solar energy. However, only TiO2 (1, 2, 4), SnO2 (5, 6), and SrTiO3 (7) have been shown to be capable of oxygen generation without decomposition of the electrode material. All of these materials have a large bandgap (greater than 3 eV) and thus require u.v. illumination for the reaction. Other semiconducting materials with smaller bandgaps (e.g., GaAs, CdS, Si) decompose under anodic polarization and/or illumination. A nonreactive electrode which shows appreciable photocurrents at wavelengths longer than 415 nm (3.0 eV) has not been reported. Since the bulk of the solar radiation on the surface of the earth is found at longer wavelengths [only about 3% occurs in the 315-400 nm range (3, 8)], a practically useful material for solar energy conversion must be able to utilize this lower energy radiation. We recently reported the preparation of n-type TiO2 electrodes by chemical vapor deposition (CVD) (9). We report here the preparation of iron oxide (Fe203) electrodes by CVD and their use in the photoassisted oxidation of water at potentials less positive than those at platinum and at wavelengths longer than 400 nm. The unavailability of a single crystal of iron oxide prevented comparison of its characteristics with the CVD material, but based on our findings with CVD vs. single crystal TiO2, we would expect generally similar behavior
Resonant Kelvin-Helmholtz modes in sheared relativistic flows
Qualitatively new aspects of the (linear and non-linear) stability of sheared
relativistic (slab) jets are analyzed. The linear problem has been solved for a
wide range of jet models well inside the ultrarelativistic domain (flow Lorentz
factors up to 20; specific internal energies ). As a distinct
feature of our work, we have combined the analytical linear approach with
high-resolution relativistic hydrodynamical simulations, which has allowed us
i) to identify, in the linear regime, resonant modes specific to the
relativistic shear layer ii) to confirm the result of the linear analysis with
numerical simulations and, iii) more interestingly, to follow the instability
development through the non-linear regime. We find that very high-order
reflection modes with dominant growth rates can modify the global, long-term
stability of the relativistic flow. We discuss the dependence of these resonant
modes on the jet flow Lorentz factor and specific internal energy, and on the
shear layer thickness. The results could have potential applications in the
field of extragalactic relativistic jets.Comment: Accepted for publication in Physical Review E. For better quality
images, please check
http://www.mpifr-bonn.mpg.de/staff/mperucho/Research.htm
Faraday rotation and polarization gradients in the jet of 3C~120: Interaction with the external medium and a helical magnetic field?
We present a sequence of 12 monthly polarimetric 15, 22, and 43 GHz VLBA
observations of the radio galaxy 3C 120 revealing a systematic presence of
gradients in Faraday rotation and degree of polarization across and along the
jet. The degree of polarization increases with distance from the core and
toward the jet edges, and has an asymmetric profile in which the northern side
of the jet is more highly polarized. The Faraday rotation measure is also
stratified across the jet width, with larger values for the southern side. We
find a localized region of high Faraday rotation measure superposed on this
structure between approximately 3 and 4 mas from the core, with a peak of about
6000 rad/m^2. Interaction of the jet with the external medium or a cloud would
explain the confined region of enhanced Faraday rotation, as well as the
stratification in degree of polarization and the flaring of superluminal knots
when crossing this region. The data are also consistent with a helical field in
a two-fluid jet model, consisting of an inner, emitting jet and a sheath
containing nonrelativistic electrons. However, this helical magnetic field
model cannot by itself explain the localized region of enhanced Faraday
rotation. The polarization electric vectors, predominantly perpendicular to the
jet axis once corrected for Faraday rotation, require a dominant component
parallel to the jet axis (in the frame of the emitting plasma) for the magnetic
field in the emitting region.Comment: Accepted for publication in ApJ Letters. 4 pages (including 5
figures
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