16,786 research outputs found
Efficiency of Nonlinear Particle Acceleration at Cosmic Structure Shocks
We have calculated the evolution of cosmic ray (CR) modified astrophysical
shocks for a wide range of shock Mach numbers and shock speeds through
numerical simulations of diffusive shock acceleration (DSA) in 1D quasi-
parallel plane shocks. The simulations include thermal leakage injection of
seed CRs, as well as pre-existing, upstream CR populations. Bohm-like diffusion
is assumed. We model shocks similar to those expected around cosmic structure
pancakes as well as other accretion shocks driven by flows with upstream gas
temperatures in the range K and shock Mach numbers spanning
. We show that CR modified shocks evolve to time-asymptotic states
by the time injected particles are accelerated to moderately relativistic
energies (p/mc \gsim 1), and that two shocks with the same Mach number, but
with different shock speeds, evolve qualitatively similarly when the results
are presented in terms of a characteristic diffusion length and diffusion time.
For these models the time asymptotic value for the CR acceleration efficiency
is controlled mainly by shock Mach number. The modeled high Mach number shocks
all evolve towards efficiencies %, regardless of the upstream CR
pressure. On the other hand, the upstream CR pressure increases the overall CR
energy in moderate strength shocks (). (abridged)Comment: 23 pages, 12 ps figures, accepted for Astrophysical Journal (Feb. 10,
2005
Effect of sintering temperature under high pressure in the uperconductivity for MgB2
We report the effect of the sintering temperature on the superconductivity of
MgB2 pellets prepared under a high pressure of 3 GPa. The superconducting
properties of the non-heated MgB2 in this high pressure were poor. However, as
the sintering temperature increased, the superconducting properties were vastly
enhanced, which was shown by the narrow transition width for the resistivity
and the low-field magnetizations. This shows that heat treatment under high
pressure is essential to improve superconducting properties. These changes were
found to be closely related to changes in the surface morphology observed using
scanning electron microscopy.Comment: 3 Pages including 3 figure
Cosmological Adaptive Mesh Refinement
We describe a grid-based numerical method for 3D hydrodynamic cosmological
simulations which is adaptive in space and time and combines the best features
of higher order--accurate Godunov schemes for Eulerian hydrodynamics with
adaptive particle--mesh methods for collisionless particles. The basis for our
method is the structured adaptive mesh refinement (AMR) algorithm of Berger &
Collela (1989), which we have extended to cosmological hydro + N-body
simulations. The resulting multiscale hybrid method is a powerful alternative
to particle-based methods in current use. The choices we have made in
constructing this algorithm are discussed, and its performance on the Zeldovich
pancake test problem is given. We present a sample application of our method to
the problem of first structure formation. We have achieved a spatial dynamic
range in a 3D multispecies gas + dark matter
calculation, which is sufficient to resolve the formation of primordial
protostellar cloud cores starting from linear matter fluctuations in an
expanding FRW universe.Comment: 14 pages, 3 figures (incl. one large color PS) to appear in
"Numerical Astrophysics 1998", eds. S. Miyama & K. Tomisaka, Tokyo, March
10-13, 199
Astrophysical Fluids of Novae: High Resolution Pre-decay X-ray spectrum of V4743 Sagittarii
Eight X-ray observations of V4743 Sgr (2002), observed with Chandra and
XMM-Newton are presented. The nova turned off some time between days 301.9 and
371, and the X-ray flux subsequently decreased from day 301.9 to 526 following
an exponential decline time scale of days. We use the absorption
lines present in the SSS spectrum for diagnostic purposes, and characterize the
physics and the dynamics of the expanding atmosphere during the explosion of
the nova. The information extracted from this first stage is then used as input
for computing full photoionization models of the ejecta in V4743 Sgr. The SSS
spectrum is modeled with a simple black-body and multiplicative Gaussian lines,
which provides us of a general kinematical picture of the system, before it
decays to its faint phase (Ness et al. 2003). In the grating spectra taken
between days 180.4 and 370, we can resolve the line profiles of absorption
lines arising from H-like and He-like C, N, and O, including transitions
involving higher principal quantum numbers. Except for a few interstellar
lines, all lines are significantly blue-shifted, yielding velocities between
1000 and 6000 km/s which implies an ongoing mass loss. It is shown that
significant expansion and mass loss occur during this phase of the explosion,
at a rate . Our measurements show that the efficiency of the amount of
energy used for the motion of the ejecta, defined as the ratio between the
kinetic luminosity and the radiated luminosity , is
of the order of one.Comment: 25 pages, 9 figures. Accepted in book: Recent Advances in Fluid
Dynamics with Environmental Applications, pp.365-39
Microstructure and pinning properties of hexagonal-disc shaped single crystalline MgB2
We synthesized hexagonal-disc-shaped MgB2 single crystals under high-pressure
conditions and analyzed the microstructure and pinning properties. The lattice
constants and the Laue pattern of the crystals from X-ray micro-diffraction
showed the crystal symmetry of MgB2. A thorough crystallographic mapping within
a single crystal showed that the edge and c-axis of hexagonal-disc shape
exactly matched the (10-10) and the (0001) directions of the MgB2 phase. Thus,
these well-shaped single crystals may be the best candidates for studying the
direction dependences of the physical properties. The magnetization curve and
the magnetic hysteresis for these single crystals showed the existence of a
wide reversible region and weak pinning properties, which supported our single
crystals being very clean.Comment: 5 pages, 3 figures. submitted to Phys. Rev.
Strongly Correlated Fractional Quantum Hall Line Junctions
We have studied a clean finite-length line junction between interacting
counterpropagating single-branch fractional-quantum-Hall edge channels. Exact
solutions for low-lying excitations and transport properties are obtained when
the two edges belong to quantum Hall systems with different filling factors and
interact via the long-range Coulomb interaction. Charging effects due to the
coupling to external edge-channel leads are fully taken into account.
Conductances and power laws in the current-voltage characteristics of tunneling
are strongly affected by inter-edge correlations.Comment: 4 pages, 1 figure, RevTex4, typos corrected + references added, to
appear in Phys. Rev. Let
Persistent currents in mesoscopic rings with a quantum dot
Using the Anderson model in the Kondo regime, we calculate the persistent
current j in a ring with an embedded quantum dot (QD) as a function of the
Aharonov-Bohm flux Phi for different ring length L, temperature T and
broadening of the conduction states delta . For T=delta =0 and L >> xi, where
xi is the Kondo screening length, Lj tends to the value for a non interacting
ideal ring, while it is suppressed for a side coupled QD. For any L/xi, Lj is
also suppressed when either T or delta increase above a fraction of the level
spacing which depends on Phi.Comment: 5 pages, 6 figures, submitted to Phys. Rev. B, (Refs. added
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