647 research outputs found
Studien zur Geschichte der oberpfälzischen Münzen des Hauses Wittelsbach. 1329-1794 (Schluss.)
Enhanced inverse bremsstrahlung heating rates in a strong laser field
Test particle studies of electron scattering on ions, in an oscillatory
electromagnetic field have shown that standard theoretical assumptions of small
angle collisions and phase independent orbits are incorrect for electron
trajectories with drift velocities smaller than quiver velocity amplitude. This
leads to significant enhancement of the electron energy gain and the inverse
bremsstrahlung heating rate in strong laser fields. Nonlinear processes such as
Coulomb focusing and correlated collisions of electrons being brought back to
the same ion by the oscillatory field are responsible for large angle, head-on
scattering processes. The statistical importance of these trajectories has been
examined for mono-energetic beam-like, Maxwellian and highly anisotropic
electron distribution functions. A new scaling of the inverse bremsstrahlung
heating rate with drift velocity and laser intensity is discussed.Comment: 12 pages, 12 figure
Non-stationary Rayleigh-Taylor instability in supernovae ejecta
The Rayleigh-Taylor instability plays an important role in the dynamics of
several astronomical objects, in particular, in supernovae (SN) evolution. In
this paper we develop an analytical approach to study the stability analysis of
spherical expansion of the SN ejecta by using a special transformation in the
co-moving coordinate frame. We first study a non-stationary spherical expansion
of a gas shell under the pressure of a central source. Then we analyze its
stability with respect to a no radial, non spherically symmetric perturbation
of the of the shell. We consider the case where the polytropic constant of the
SN shell is and we examine the evolution of a arbitrary shell
perturbation. The dispersion relation is derived. The growth rate of the
perturbation is found and its temporal and spatial evolution is discussed. The
stability domain depends on the ejecta shell thickness, its acceleration, and
the perturbation wavelength.Comment: 16 page
Laser-Cluster-Interaction in a Nanoplasma-Model with Inclusion of Lowered Ionization Energies
The interaction of intense laser fields with silver and argon clusters is
investigated theoretically using a modified nanoplasma model. Single pulse and
double pulse excitations are considered. The influence of the dense cluster
environment on the inner ionization processes is studied including the lowering
of the ionization energies. There are considerable changes in the dynamics of
the laser-cluster interaction. Especially, for silver clusters, the lowering of
the ionization energies leads to increased yields of highly charged ions.Comment: 10 pages, 11 figure
Optical and X-ray clusters as tracers of the supercluster-void network. I Superclusters of Abell and X-ray clusters
We study the distribution of X-ray selected clusters of galaxies with respect
to superclusters determined by Abell clusters of galaxies and show that the
distribution of X-ray clusters follows the supercluster-void network determined
by Abell clusters. We find that in this network X-ray clusters are more
strongly clustered than other clusters. Poor, non-Abell X-ray clusters follow
the supercluster-void network as well: these clusters are embedded in
superclusters determined by rich clusters and populate filaments between them.
We present a new catalog of superclusters of Abell clusters out to a redshift
of z_{lim}=0.13, a catalog of X-ray clusters located in superclusters
determined by Abell clusters, and a list of additional superclusters of X-ray
clusters.Comment: LaTex (sty files added), 16 pages, 3 ps figures, submitted to
Astronomical Journal. Animations of the 3D distribution of superclusters of
Abell and X-ray clusters at http://www.aai.ee/~maret/SCLVnet.ht
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Assessing regional-scale variability in deforestation and forest degradation rates in a tropical biodiversity hotspot
Deforestation and forest degradation are major drivers of global environmental change and tropical forests are subjected to unprecedented pressures from both.For most tropical zones, deforestation rates are averaged across entire countries,often without highlighting regional differentiation. There are also very few estimates of forest degradation, either averaged or localized for the tropics. We quantified regional and country-wide changes in deforestation and forest degradation rates for Madagascar from Landsat temporal data (in two intervals,1994–2002 and 2002–2014). To our knowledge, this is the first country-wide estimate of forest degradation for Madagascar. We also performed an intensity analysis to categorize the magnitude and speed of transitions between forest,vegetation matrix, cultivated land and exposed surface. We found significant regional heterogeneity in deforestation and forest degradation. Deforestation rates decreased annually in lowland evergreen moist forest by -0.24% and in all other vegetation zones. Forest degradation rates had annual increases in the same period in lowland evergreen moist forest (0.09%), littoral forest (0.06%)but decreased in medium altitude moist evergreen forest (-0.25%), dry deciduous forest (-0.23%) and scelrophyllous woodland (-0.61%) in the same period. Despite these regional differences, higher rates of deforestation and forest degradation were consistently driven by rapid and large-sized conversions of largely intact forest to cultivated lands and exposed surfaces, most of which occurred between 1994 and 2002. These results suggest that while targeted conservation projects may have reduced forest degradation rates in some areas (e.g.medium altitude moist evergreen forest), the drivers of land cover change remain intense in relatively neglected regions. We advocate a more balanced approach to future conservation initiatives, one recognizing that deforestation and forest degradation, particularly in tropical Africa, are often driven by region-specific conditions and therefore require conservation policies tailored for local conditions
Muscle Activity and User-Perceived Exertion During Immersive Virtual Reality Exergaming Incorporating an Adaptive Cable Resistance System
International Journal of Exercise Science 15(7): 261-275, 2022. The purpose of this exploratory study was to characterize muscle activation via surface electromyography (sEMG), user-perceived exertion, and enjoyment during a 30-minute session of immersive virtual reality (IVR) cable resistance exergaming. Ten healthy, college-aged males completed a signature 30-minute exergaming session using an IVR adaptive cable resistance system that incorporated six traditional compound exercises. Muscle activation (sEMG) was captured during the session with a wearable sEMG system. Rated of Perceived Exertion (RPE) and Physical Activity Enjoyment Scale (PACES) were recorded following the session. Pectoralis major showed the highest activation during chest press, deltoids showed the highest activation on overhead press, latissimus dorsi showed the highest activation during lat pulldown and row exercises, hamstrings were the most activated muscles during Romanian deadlift, and glutes showed the highest activity during squats. RPE and PACES mean scores were 14 (1) and 4.27 (0.38), respectively. IVR exergaming with resistance cable training provides an enjoyable experience and distracts practitioners from exertion while exercising at a high intensity. Results from this study suggest similar muscle activation responses compared to traditional resistance exercises as demonstrated with prior evidence. This novel form of exercise might have important repercussions for improving health outcomes among those who find it challenging to adhere to and enjoy exercise routines, as well as with little knowledge on how to progress in their resistance training. Further investigations are needed to explore long-term adaptations and to assess if IVR exergaming has additional benefits compared to traditional resistance training
Phase transitions in self-gravitating systems. Self-gravitating fermions and hard spheres models
We discuss the nature of phase transitions in self-gravitating systems both
in the microcanonical and in the canonical ensemble. We avoid the divergence of
the gravitational potential at short distances by considering the case of
self-gravitating fermions and hard spheres models. Three kinds of phase
transitions (of zeroth, first and second order) are evidenced. They separate a
``gaseous'' phase with a smoothly varying distribution of matter from a
``condensed'' phase with a core-halo structure. We propose a simple analytical
model to describe these phase transitions. We determine the value of energy (in
the microcanonical ensemble) and temperature (in the canonical ensemble) at the
transition point and we study their dependance with the degeneracy parameter
(for fermions) or with the size of the particles (for a hard spheres gas).
Scaling laws are obtained analytically in the asymptotic limit of a small short
distance cut-off. Our analytical model captures the essential physics of the
problem and compares remarkably well with the full numerical solutions.Comment: Submitted to Phys. Rev. E. New material adde
Density profiles of dark matter haloes: diversity and dependence on environment
(Abridged) We study the outer density profiles of dark matter haloes
predicted by a generalized secondary infall model and observed in a N-body
cosmological simulation of a \Lambda CDM model. We find substantial systematic
variations in shapes and concentrations of the halo profiles as well as a
strong correlation of the profiles with the environment. In the N-body
simulation, the average outer slope of the density profiles, \beta (\rho\propto
r^{-\beta}), of isolated haloes is \approx 2.9; 68% of these haloes have values
of \beta between 2.5 and 3.8. Haloes in dense environments of clusters are more
concentrated and exhibit a broad distribution of \beta with values larger than
for isolated haloes . Contrary to what one may expect, the haloes contained
within groups and galaxy systems are less concentrated and have flatter outer
density profiles than the isolated haloes. The concentration decreases with
M_h, but its scatter for a given mass is substantial. The mass and circular
velocity of the haloes are strongly correlated: M_h \propto V_m^{\alpha} with
\alpha ~ 3.3 (isolated) and ~3.5 (haloes in clusters). For M_h=10^12M_sun the
rms deviations from these relations are \Delta logM_h=0.12 and 0.18,
respectively. Approximately 30% of the haloes are contained within larger
haloes or have massive companions (larger than ~0.3 the mass of the current
halo) within 3 virial radii. The remaining 70% of the haloes are isolated
objects. The distribution of \beta as well as the concentration-mass and
M_h-V_m relations for the isolated haloes agree very well with the predictions
of our seminumerical approach which is based on a generalization of the
secondary infall model and on the extended Press-Schechter formalism.Comment: 14 pages, 11 figures included, uses mn.sty, accepted by MNRAS. Minor
modifications, new and updated reference
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