14,557 research outputs found
Pseudo-High-Order Symplectic Integrators
Symplectic N-body integrators are widely used to study problems in celestial
mechanics. The most popular algorithms are of 2nd and 4th order, requiring 2
and 6 substeps per timestep, respectively. The number of substeps increases
rapidly with order in timestep, rendering higher-order methods impractical.
However, symplectic integrators are often applied to systems in which
perturbations between bodies are a small factor of the force due to a dominant
central mass. In this case, it is possible to create optimized symplectic
algorithms that require fewer substeps per timestep. This is achieved by only
considering error terms of order epsilon, and neglecting those of order
epsilon^2, epsilon^3 etc. Here we devise symplectic algorithms with 4 and 6
substeps per step which effectively behave as 4th and 6th-order integrators
when epsilon is small. These algorithms are more efficient than the usual 2nd
and 4th-order methods when applied to planetary systems.Comment: 14 pages, 5 figures. Accepted for publication in the Astronomical
Journa
Vibration
Physiological and biomechanical responses of humans to vibrations during manned space flight and threshold data on tolerances to various vibrational modes and condition
Sound and noise
Sound and noise problems in space environment and human tolerance criteria at varying frequencies and intensitie
A decreased probability of habitable planet formation around low-mass stars
Smaller terrestrial planets (< 0.3 Earth masses) are less likely to retain
the substantial atmospheres and ongoing tectonic activity probably required to
support life. A key element in determining if sufficiently massive "sustainably
habitable" planets can form is the availability of solid planet-forming
material. We use dynamical simulations of terrestrial planet formation from
planetary embryos and simple scaling arguments to explore the implications of
correlations between terrestrial planet mass, disk mass, and the mass of the
parent star. We assume that the protoplanetary disk mass scales with stellar
mass as Mdisk ~ f Mstar^h, where f measures the relative disk mass, and 1/2 < h
< 2, so that disk mass decreases with decreasing stellar mass. We consider
systems without Jovian planets, based on current models and observations for M
stars. We assume the mass of a planet formed in some annulus of a disk with
given parameters is proportional to the disk mass in that annulus, and show
with a suite of simulations of late-stage accretion that the adopted
prescription is surprisingly accurate. Our results suggest that the fraction of
systems with sufficient disk mass to form > 0.3 Earth mass habitable planets
decreases for low-mass stars for every realistic combination of parameters.
This "habitable fraction" is small for stellar masses below a mass in the
interval 0.5 to 0.8 Solar masses, depending on disk parameters, an interval
that excludes most M stars. Radial mixing and therefore water delivery are
inefficient in lower-mass disks commonly found around low-mass stars, such that
terrestrial planets in the habitable zones of most low-mass stars are likely to
be small and dry.Comment: Accepted to ApJ. 11 pages, 6 figure
Crystallization of the oligopeptide-binding protein AppA from Bacillus subtilis
AppA is the membrane-anchored extracellular receptor component of an ABC transporter responsible for the uptake of oligopeptides into Bacillus subtilis. AppA has been overexpressed as a cleavable maltose-binding protein fusion in Escherichia coli. Following removal of the fusion portion, AppA has been crystallized from morpholino-ethanesulfonic acid-buffered solutions at pH 6.5 containing polyethylene glycol and zinc acetate. A complete X-ray diffraction data set extending to 2.3 Angstrom spacing has been collected
Predicting the movements of permanently installed electrodes on an active landslide using time-lapse geoelectrical resistivity data only
If electrodes move during geoelectrical resistivity monitoring and their new positions are not incorporated in the inversion, then the resulting tomographic images exhibit artefacts that can obscure genuine time-lapse resistivity changes in the subsurface. The effects of electrode movements on time-lapse resistivity tomography are investigated using a simple analytical model and real data. The correspondence between the model and the data is sufficiently good to be able to predict the effects of electrode movements with reasonable accuracy. For the linear electrode arrays and 2D inversions under consideration, the data are much more sensitive to longitudinal than transverse or vertical movements. Consequently the model can be used to invert the longitudinal offsets of the electrodes from their known baseline positions using only the time-lapse ratios of the apparent resistivity data. The example datasets are taken from a permanently installed electrode array on an active lobe of a landslide. Using two sets with different levels of noise and subsurface resistivity changes, it is found that the electrode positions can be recovered to an accuracy of 4 % of the baseline electrode spacing. This is sufficient to correct the artefacts in the resistivity images, and provides for the possibility of monitoring the movement of the landslide and its internal hydraulic processes simultaneously using electrical resistivity tomography only
Surface effects on nanowire transport: numerical investigation using the Boltzmann equation
A direct numerical solution of the steady-state Boltzmann equation in a
cylindrical geometry is reported. Finite-size effects are investigated in large
semiconducting nanowires using the relaxation-time approximation. A nanowire is
modelled as a combination of an interior with local transport parameters
identical to those in the bulk, and a finite surface region across whose width
the carrier density decays radially to zero. The roughness of the surface is
incorporated by using lower relaxation-times there than in the interior.
An argument supported by our numerical results challenges a commonly used
zero-width parametrization of the surface layer. In the non-degenerate limit,
appropriate for moderately doped semiconductors, a finite surface width model
does produce a positive longitudinal magneto-conductance, in agreement with
existing theory. However, the effect is seen to be quite small (a few per cent)
for realistic values of the wire parameters even at the highest practical
magnetic fields. Physical insights emerging from the results are discussed.Comment: 15 pages, 7 figure
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