30,085 research outputs found
Stationary Bianchi type II perfect fluid models
Einstein's field equations for stationary Bianchi type II models with a
perfect fluid source are investigated. The field equations are rewritten as a
system of autonomous first order differential equations. Dimensionless
variables are subsequently introduced for which the reduced phase space is
compact. The system is then studied qualitatively using the theory of dynamical
systems. It is shown that the locally rotationally symmetric models are not
asymptotically self-similar for small values of the independent , tovariable. A
new exact solution is also given.Comment: 6 pages, 1 figure LaTeX. To appear in JM
The micrometeoroid experiment on the OGO 2 satellite
Micrometeoroids in earths dust cloud obtained from OGO-B satellit
The interaction between the Moon and the solar wind
We study the interaction between the Moon and the solar wind using a
three-dimensional hybrid plasma solver. The proton fluxes and electromagnetical
fields are presented for typical solar wind conditions with different magnetic
field directions. We find two different wake structures for an interplanetary
magnetic field that is perpendicular to the solar wind flow, and for one that
is parallell to the flow. The wake for intermediate magnetic field directions
will be a mix of these two extreme conditions. Several features are consistent
with a fluid interaction, e.g., the presence of a rarefaction cone, and an
increased magnetic field in the wake. There are however several kinetic
features of the interaction. We find kinks in the magnetic field at the wake
boundary. There are also density and magnetic field variations in the far wake,
maybe from an ion beam instability related to the wake refill. The results are
compared to observations by the WIND spacecraft during a wake crossing. The
model magnetic field and ion velocities are in agreement with the measurements.
The density and the electron temperature in the central wake are not as well
captured by the model, probably from the lack of electron physics in the hybrid
model.Comment: Accepted for publication in Earth, Planets and Spac
Legg-Calvé-Perthes disease and the risk of injuries requiring hospitalization
Background and purpose Previous studies have suggested that Legg-Calvé-Perthes disease (LCPD) is associated with repetitive trauma, coagulation problems and anatomical abnormalities of the blood supply to the femoral head. The hypothesis that repetitive trauma can affect the blood supply of the femoral head, leading to LCPD, is supported by an animal model. For evidence of an increased risk of repetitive trauma, we investigated whether patients with LCPD have a higher risk for severe injuries requiring hospitalization. Patients and methods We identified 2579 patients with LCPD in Sweden during the period 1964-2005. 13,748 individuals without LCPD were randomly selected from the Swedish general population, matched by year of birth, sex and region (control group). Cox proportional hazard regression estimated the risks. Results Compared to the control group, patients with LCPD had a modestly raised hazard ratio (HR) of 1.2 (95% CI 1.1-1.3) for injury requiring hospitalization. The risks were slightly higher for soft tissue injuries (HR = 1.3, 95% CI:1.1-1.4) than for fractures (HR = 1.1, 95% CI: 1.0-1.3) and more pronounced among females. Compared to the control group, the higher risk for injury only applied to the lower extremities (HR = 1.2, 95% CI: 1.0-1.4) in patients with LCPD. Interpretation Patients with LCPD are vulnerable to injuries which could be interpreted as a marker of hyperactive behavior. It could also implicate that anatomical changes in the bone formation or blood supply of the femoral head - increasing its sensibility for trauma - contribute to the etiology of LCPD. © Copyright: ©Nordic Orthopaedic Federation
Angular Momentum Projected Configuration Interaction with Realistic Hamiltonians
The Projected Configuration Interaction (PCI) method starts from a collection
of mean-field wave functions, and builds up correlated wave functions of good
symmetry. It relies on the Generator Coordinator Method (GCM) techniques, but
it improves the past approaches by a very efficient method of selecting the
basis states. We use the same realistic Hamiltonians and model spaces as the
Configuration Interaction (CI) method, and compare the results with the full CI
calculations in the sd and pf shell. Examples of 24Mg, 28Si, 48Cr, 52Fe and
56Ni are discussed.Comment: 10 pages, 10 figures. Revised version. To be published in Physical
Review
Forest Phytomass and Carbon in European Russia.
Regression equations for basic fractions of forest phytomass have been developed for the European-Ural part of Russia based on available experimental data and publications (preliminarily examined 962 sample plots and aggregations). 8 main forest forming species (pine, spruce, oak, birch, beech, aspen, alder, lime) which were involved into analysis cover in European Russia about 95% of total forested areas. The equations allow to evaluate the ratio between phytomass fractions weight and growing stock by species, age and site indexes. The application of the equations to the Forest State Account data (1988) gives the total (living) phytomass in forest ecosystems of European Russia (forested area of 166.0 Mha, growing stock of 20.28 bln m3) of 15.47 Pg drt matter (density 9.32 kg/m2). The total carbon pool was estimated 7.64 Pg C with average density 4.60 kg C/m2 in 1988. Analysis of uncertainties of data and used methods showed that the results which based on FSA data probably underestimate real values for about 5% with standard error +/- 7-8% with confidential (apriori) probability 0.8-0.9. Used this assumption unbiased estimate of total storage of phytomass in forest vegetation were calculated for 1993 of 16.94 Pg (average density 10.36 kg/m2 ) and for C content respectively (8.37 Tg and 5.03 kg/m2 repectively). The changes of forest phytomass in 1966-1993 were esimated of 4.73 Pg (or about 174 Tg of dry matter/year) and for C - 2.34 Pg, i.e. the European Russian forests provided during 1966-1993 net sink of carbon in forest vegetation about 87 Tg C annually.
Uni-directional polymerization leading to homochirality in the RNA world
The differences between uni-directional and bi-directional polymerization are
considered. The uni-directional case is discussed in the framework of the RNA
world. Similar to earlier models of this type, where polymerization was assumed
to proceed in a bi-directional fashion (presumed to be relevant to peptide
nucleic acids), left-handed and right-handed monomers are produced via an
autocatalysis from an achiral substrate. The details of the bifurcation from a
racemic solution to a homochiral state of either handedness is shown to be
remarkably independent of whether the polymerization in uni-directional or
bi-directional. Slightly larger differences are seen when dissociation is
allowed and the dissociation fragments are being recycled into the achiral
substrate.Comment: 9 pages, 4 figures, submitted to Astrobiolog
Rotating binary Bose-Einstein condensates and vortex clusters in quantum droplets
Quantum droplets may form out of a gaseous Bose-Einstein condensate,
stabilized by quantum fluctuations beyond mean field. We show that multiple
singly-quantized vortices may form in these droplets at moderate angular
momenta in two dimensions. Droplets carrying these precursors of an Abrikosov
lattice remain self-bound for certain timescales after switching off an initial
harmonic confinement. Furthermore, we examine how these vortex-carrying
droplets can be formed in a more pertubation-resistant setting, by starting
from a rotating binary Bose-Einstein condensate and inducing a metastable
persistent current via a non-monotonic trapping potential.Comment: 5 page, 4 figure
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