248 research outputs found
Distribution Patterns of Grasshoppers and Their Kin in the Boreal Zone
The distribution patterns of Orthoptera are described for the boreal zone. The boreal fauna of Eurasia includes more than 81 species. Many of them are widely distributed. The monotypic genus Paracyphoderris Storozhenko and at least 13 species are endemics or subendemics. About 50 species are known from boreal North America. Four endemic species are distributed very locally. Relationships between the faunas of the Eurasian and North American parts of the boreal zone are relatively weak. The boreal assemblages are usually characterized by the low levels of species diversity and abundance. Grasshoppers and their relatives occupy almost exclusively open habitats, such as different types of meadows, mountain steppes and tundras, clearings, openings, bogs, and stony flood plains. The local endemics and subendemics are found only in some habitats of the eastern part of Eurasia and the north-western part of North America. Retrospective and prospective of the boreal fauna of Orthoptera are also discussed
Locusts and grasshoppers: behavior, ecology, and biogeography
Presentación del número especial de la revista sobre langostas y saltamontes.Facultad de Ciencias Naturales y Muse
Ultra-Sensitive Hot-Electron Nanobolometers for Terahertz Astrophysics
The background-limited spectral imaging of the early Universe requires
spaceborne terahertz (THz) detectors with the sensitivity 2-3 orders of
magnitude better than that of the state-of-the-art bolometers. To realize this
sensitivity without sacrificing operating speed, novel detector designs should
combine an ultrasmall heat capacity of a sensor with its unique thermal
isolation. Quantum effects in thermal transport at nanoscale put strong
limitations on the further improvement of traditional membrane-supported
bolometers. Here we demonstrate an innovative approach by developing
superconducting hot-electron nanobolometers in which the electrons are cooled
only due to a weak electron-phonon interaction. At T<0.1K, the electron-phonon
thermal conductance in these nanodevices becomes less than one percent of the
quantum of thermal conductance. The hot-electron nanobolometers, sufficiently
sensitive for registering single THz photons, are very promising for
submillimeter astronomy and other applications based on quantum calorimetry and
photon counting.Comment: 19 pages, 3 color figure
Sigma Model as a Conformal Field Theory
We discuss the sigma model on the supergroup manifold. We
demonstrate that this theory is exactly conformal. The chiral algebra of this
model is given by some extension of the Virasoro algebra, similar to the
algebra of Zamolodchikov. We also show that all group invariant correlation
functions are coupling constant independent and can be computed in the free
theory. The non invariant correlation functions are highly nontrivial and
coupling dependent. At the end we compare two and three-point correlation
functions of the sigma model with the correlation functions in the
boundary theory of and find a qualitative agreement.Comment: 34 pages, 5 figure
On sets of eigenvalues of matrices with prescribed row sums and prescribed graph
Motivated by a work of Boros, Brualdi, Crama and Hoffman, we consider the
sets of (i) possible Perron roots of nonnegative matrices with prescribed row
sums and associated graph, and (ii) possible eigenvalues of complex matrices
with prescribed associated graph and row sums of the moduli of their entries.
To characterize the set of Perron roots or possible eigenvalues of matrices in
these classes we introduce, following an idea of Al'pin, Elsner and van den
Driessche, the concept of row uniform matrix, which is a nonnegative matrix
where all nonzero entries in every row are equal. Furthermore, we completely
characterize the sets of possible Perron roots of the class of nonnegative
matrices and the set of possible eigenvalues of the class of complex matrices
under study. Extending known results to the reducible case, we derive new sharp
bounds on the set of eigenvalues or Perron roots of matrices when the only
information available is the graph of the matrix and the row sums of the moduli
of its entries. In the last section of the paper a new constructive proof of
the Camion-Hoffman theorem is given.Comment: 22 page
Long-Term Profile Variability in Active Galactic Nuclei with Double-Peaked Balmer Emission Lines
An increasing number of Active Galactic Nuclei (AGNs) exhibit broad,
double-peaked Balmer emission lines,which represent some of the best evidence
for the existence of relatively large-scale accretion disks in AGNs. A set of
20 double-peaked emitters have been monitored for nearly a decade in order to
observe long-term variations in the profiles of the double-peaked Balmer lines.
Variations generally occur on timescales of years, and are attributed to
physical changes in the accretion disk. Here we characterize the variability of
a subset of seven double-peaked emitters in a model independent way. We find
that variability is caused primarily by the presence of one or more discrete
"lumps" of excess emission; over a timescale of a year (and sometimes less)
these lumps change in amplitude and shape, but the projected velocity of these
lumps changes over much longer timescales (several years). We also find that
all of the objects exhibit red peaks that are stronger than the blue peak at
some epochs and/or blueshifts in the overall profile, contrary to the
expectations for a simple, circular accretion disk model, thus emphasizing the
need for asymmetries in the accretion disk. Comparisons with two simple models,
an elliptical accretion disk and a circular disk with a spiral arm, are unable
to reproduce all aspects of the observed variability, although both account for
some of the observed behaviors. Three of the seven objects have robust
estimates of the black hole masses. For these objects the observed variability
timescale is consistent with the expected precession timescale for a spiral
arm, but incompatible with that of an elliptical accretion disk. We suggest
that with the simple modification of allowing the spiral arm to be fragmented,
many of the observed variability patterns could be reproduced.Comment: 74 pages, 4 tables, 35 figure
Weak localization effect on thermomagnetic phenomena
The quantum transport equation (QTE) is extended to study weak localization
(WL) effects on galvanomagnetic and thermomagnetic phenomena. QTE has many
advantages over the linear response method (LRM): (i) particle-hole asymmetry
which is necessary for the Hall effect is taken into account by the
nonequilibrium distribution function, while LRM requires expansion near the
Fermi surface, (ii) when calculating response to the temperature gradient, the
problem of WL correction to the heat current operator is avoided, (iii)
magnetic field is directly introduced to QTE, while the LRM deals with the
vector potential and and special attention should be paid to maintain gauge
invariance, e.g. when calculating the Nernst effect the heat current operator
should be modified to include the external magnetic field. We reproduce in a
very compact form known results for the conductivity, the Hall and the
thermoelectric effects and then we study our main problem, WL correction to the
Nernst coefficient (transverse thermopower).Comment: 20 pages 2 figure
Locusts and grasshoppers: behavior, ecology, and biogeography
Presentación del número especial de la revista sobre langostas y saltamontes.Facultad de Ciencias Naturales y Muse
- …