531 research outputs found
Arbejdsprøve med Vitriolsprøjter paa Lykkensgave 1909.
Arbejdsprøve med Vitriolsprøjter paa Lykkensgave 1909
Comparing multisource harmonized forest types mapping: a case study from central Italy
X-Ray flares in Orion Young Stars. II. Flares, Magnetospheres, and Protoplanetary Disks
We study the properties of powerful X-ray flares from 161 pre-main sequence
(PMS) stars observed with the Chandra X-ray Observatory in the Orion Nebula
region. Relationships between flare properties, protoplanetary disks and
accretion are examined in detail to test models of star-disk interactions at
the inner edge of the accretion disks. Previous studies had found no
differences in flaring between diskfree and accreting systems other than a
small overall diminution of X-ray luminosity in accreting systems. The most
important finding is that X-ray coronal extents in fast-rotating diskfree stars
can significantly exceed the Keplerian corotation radius, whereas X-ray loop
sizes in disky and accreting systems do not exceed the corotation radius. This
is consistent with models of star-disk magnetic interaction where the inner
disk truncates and confines the PMS stellar magnetosphere. We also find two
differences between flares in accreting and diskfree PMS stars. First, a
subclass of super-hot flares with peak plasma temperatures exceeding 100 MK are
preferentially present in accreting systems. Second, we tentatively find that
accreting stars produce flares with shorter durations. Both results may be
consequences of the distortion and destabilization of the stellar magnetosphere
by the interacting disk. Finally, we find no evidence that any flare types,
even slow-rise flat-top flares are produced in star-disk magnetic loops. All
are consistent with enhanced solar long-duration events with both footprints
anchored in the stellar surface.Comment: Accepted for publication in ApJ (07/17/08); 46 pages, 14 figures, 2
table
The optical depth of the Universe to ultrahigh energy cosmic ray scattering in the magnetized large scale structure
This paper provides an analytical description of the transport of ultrahigh
energy cosmic rays in an inhomogeneously magnetized intergalactic medium. This
latter is modeled as a collection of magnetized scattering centers such as
radio cocoons, magnetized galactic winds, clusters or magnetized filaments of
large scale structure, with negligible magnetic fields in between. Magnetic
deflection is no longer a continuous process, it is rather dominated by
scattering events. We study the interaction between high energy cosmic rays and
the scattering agents. We then compute the optical depth of the Universe to
cosmic ray scattering and discuss the phenomological consequences for various
source scenarios. For typical parameters of the scattering centers, the optical
depth is greater than unity at 5x10^{19}eV, but the total angular deflection is
smaller than unity. One important consequence of this scenario is the
possibility that the last scattering center encountered by a cosmic ray be
mistaken with the source of this cosmic ray. In particular, we suggest that
part of the correlation recently reported by the Pierre Auger Observatory may
be affected by such delusion: this experiment may be observing in part the last
scattering surface of ultrahigh energy cosmic rays rather than their source
population. Since the optical depth falls rapidly with increasing energy, one
should probe the arrival directions of the highest energy events beyond
10^{20}eV on an event by event basis to circumvent this effect.Comment: version to appear in PRD; substantial improvements: extended
introduction, sections added on angular images and on direction dependent
effects with sky maps of optical depth, enlarged discussion of Auger results
(conclusions unchanged); 27 pages, 9 figure
Shot Noise Enhancement in Resonant Tunneling Structures in a Magnetic Field
We have observed that the shot noise of tunnel current, I, in
GaSb-AlSb-InAs-AlSb-GaSb double-barrier structure under a magnetic field can
exceed 2qI. The measurements were done at T=4K in fields up to 5T parallel to
the current. The noise enhancement occurred at each of the several
negative-differential conductance regions induced by the tunneling of holes
through Landau levels in the InAs quantum well. The amount of the enhancement
increased with the strength of the negative conductance and reached values up
to 8qI. These results are explained qualitatively by fluctuations of the
density of states in the well, but point out the need for a detailed theory of
shot noise enhancement in resonant tunneling devices.Comment: 4 pages, RevTex, 3 figure
The 1/3-shot noise suppression in diffusive nanowires
We report low-temperature shot noise measurements of short diffusive Au wires
attached to electron reservoirs of varying sizes. The measured noise
suppression factor compared to the classical noise value
strongly depends on the electric heat conductance of the reservoirs. For small
reservoirs injection of hot electrons increases the measured noise and hence
the suppression factor. The universal 1/3-suppression factor can only
asymptotically be reached for macroscopically large and thick electron
reservoirs. A heating model based on the Wiedemann-Franz law is used to explain
this effect.Comment: 10 figure
Shot noise of a quantum dot with non-Fermi liquid correlations
The shot noise of a one-dimensional wire interrupted by two barriers shows
interesting features related to the interplay between Coulomb blockade effects,
Luttinger correlations and discrete excitations. At small bias the Fano factor
reaches the lowest attainable value, 1/2, irrespective of the ratio of the two
junction resistances. At larger voltages this asymmetry is power-law
renormalized by the interaction strength. We discuss how the measurement of
current and these features of the noise allow to extract the Luttinger liquid
parameter.Comment: 4 pages, 3 figures,to be published in Phys. Rev. B. For high
resolution image of Fig.1 see http://server1.fisica.unige.it/~braggio/doc.ht
Full Counting Statistics of Charge Transfer in Coulomb Blockade Systems
Full counting statistics (FCS) of charge transfer in mesoscopic systems has
recently become a subject of significant interest, since it proves to reveal an
important information about the system which can be hardly assessed by other
means. While the previous research mostly addressed the FCS of non- interacting
systems, the present paper deals with the FCS in the limit of strong
interaction. In this Coulomb blockade limit the electron dynamics is known to
be governed by a master equation. We develop a general scheme to evaluate the
FCS in such case, this being the main result of the work presented. We
illustrate the scheme, by applying it to concrete systems. For generic case of
a single resonant level we establish the equivalence of scattering and master
equation approach to FCS. Further we study a single Coulomb blockade island
with two and three leads attached and compare the FCS in this case with our
recent results concerning an open dot either with two and three terminals. We
demonstrate that Coulomb interaction suppresses the relative probabilities of
large current fluctuations.Comment: 17 pages, 16 figure
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