536 research outputs found
Shape models and physical properties of asteroids
Despite the large amount of high quality data generated in recent space
encounters with asteroids, the majority of our knowledge about these objects
comes from ground based observations. Asteroids travelling in orbits that are
potentially hazardous for the Earth form an especially interesting group to be
studied. In order to predict their orbital evolution, it is necessary to
investigate their physical properties. This paper briefly describes the data
requirements and different techniques used to solve the lightcurve inversion
problem. Although photometry is the most abundant type of observational data,
models of asteroids can be obtained using various data types and techniques. We
describe the potential of radar imaging and stellar occultation timings to be
combined with disk-integrated photometry in order to reveal information about
physical properties of asteroids.Comment: From Assessment and Mitigation of Asteroid Impact Hazards boo
The indigenous arbuscular mycorrhizal fungal colonisation potential in potato roots is affected by agricultural treatments
ArticlesThere is an urgent need to develop novel approaches to enhance sustainable agriculture
while not reducing cr
op yields. Arbuscular mycorrhizal (AM) fungi establish symbiotic
associations with most crop plants improving plant performance and soil health. This study
investigated the extent of
colonisation
of potato roots by indigenous AM
fungi
in the arable soil
under
conventional
and organic farming systems. Potato roots had greater AM fungal colonisation
levels under
organic
than
conventional
farming, though in general, root colonisation levels were
extremely low
in both farming
systems
. Pota
to root AM
fungal
colonisation
was
lower
with
higher soil P content
and
higher with higher annual C
input.
Trap plant root AM fungal
colonisation was
considerably higher than in field potato roots and showed that soil mycorrhizal
inoculum potential was hig
her in organic than in conventional farming.
Thus, the positive impact
of manure application in organic fields to the potato AM fungal colonisation can be explained by
previous higher total annual C fresh organic matter input and lower soil P content under
treatment.
Furthermore, the natural AM fungal abundance in the soil was sufficient to colonise trap plant
roots, suggesting a low mycorrhizal dependence of the studied potato cultivar
BBC Television ‘The Experiment’ – report of the independent ethics panel
Report by the six-person ‘Independent Ethics Panel’ as Chaired by MP Lembik Opik on the BBC Prison Experiment, conducted by Professor Alex Haslam (Exeter University) and Professor Steve Reicher (St Andrews University) to oversee the day-to-day running of the experiment and safeguard participant interests and wellbeing. BBC (October 2001 to May 2002)
Simulations of the Population of Centaurs I: The Bulk Statistics
Large-scale simulations of the Centaur population are carried out. The
evolution of 23328 particles based on the orbits of 32 well-known Centaurs is
followed for up to 3 Myr in the forward and backward direction under the
influence of the 4 massive planets. The objects exhibit a rich variety of
dynamical behaviour with half-lives ranging from 540 kyr (1996 AR20) to 32 Myr
(2000 FZ53). The mean half-life of the entire sample of Centaurs is 2.7 Myr.
The data are analyzed using a classification scheme based on the controlling
planets at perihelion and aphelion, previously given in Horner et al (2003).
Transfer probabilities are computed and show the main dynamical pathways of the
Centaur population. The total number of Centaurs with diameters larger than 1
km is estimated as roughly 44300, assuming an inward flux of one new
short-period comet every 200 yrs. The flux into the Centaur region from the
Edgeworth-Kuiper belt is estimated to be 1 new object every 125 yrs. Finally,
the flux from the Centaur region to Earth-crossing orbits is 1 new
Earth-crosser every 880 yrsComment: 15 pages, 2 figures, MNRAS in pres
Hazardous near Earth asteroid mitigation campaign planning based on uncertain information on fundamental asteroid characteristics
Given a limited warning time, an asteroid impact mitigation campaign would hinge on uncertainty-based information consisting of remote observational data of the identified Earth-threatening object, general knowledge of near-Earth asteroids (NEAs), and engineering judgment. Due to these ambiguities, the campaign credibility could be profoundly compromised. It is therefore imperative to comprehensively evaluate the inherent uncertainty in deflection and plan the campaign accordingly to ensure successful mitigation. This research demonstrates dual-deflection mitigation campaigns consisting of primary (instantaneous/quasi-instantaneous) and secondary (slow-push) deflection missions, where both deflection efficiency and campaign credibility are taken into account. The results of the dual-deflection campaign analysis show that there are trade-offs between the competing aspects: the launch cost, mission duration, deflection distance, and the confidence in successful deflection. The design approach is found to be useful for multi-deflection campaign planning, allowing us to select the best possible combination of missions from a catalogue of campaign options, without compromising the campaign credibility
Generalized "Quasi-classical" Ground State for an Interacting Two Level System
We treat a system (a molecule or a solid) in which electrons are coupled
linearly to any number and type of harmonic oscillators and which is further
subject to external forces of arbitrary symmetry. With the treatment restricted
to the lowest pair of electronic states, approximate "vibronic"
(vibration-electronic) ground state wave functions are constructed having the
form of simple, closed expressions. The basis of the method is to regard
electronic density operators as classical variables. It extends an earlier
"guessed solution", devised for the dynamical Jahn-Teller effect in cubic
symmetry, to situations having lower (e.g., dihedral) symmetry or without any
symmetry at all. While the proposed solution is expected to be quite close to
the exact one, its formal simplicity allows straightforward calculations of
several interesting quantities, like energies and vibronic reduction (or Ham)
factors. We calculate for dihedral symmetry two different -factors (""
and "") and a -factor. In simplified situations we obtain . The formalism enables quantitative estimates to be made for the dynamical
narrowing of hyperfine lines in the observed ESR spectrum of the dihedral
cyclobutane radical cation.Comment: 28 pages, 4 figure
Aligning molecular studies of mycorrhizal fungal diversity with ecologically important levels of diversity in ecosystems.
Arbuscular mycorrhizal fungi (AMF) occur in the roots of most plants and are an ecologically important component of the soil microbiome. Richness of AMF taxa is a strong driver of plant diversity and productivity, thus providing a rationale for characterizing AMF diversity in natural ecosystems. Consequently, a large number of molecular studies on AMF community composition are currently underway. Most published studies, at best, only address species or genera-level resolution. However, several experimental studies indicate that variation in plant performance is large among plants colonised by different individuals of one AMF species. Thus, there is a potential disparity between how molecular community ecologists are currently describing AMF diversity and the level of AMF diversity that may actually be ecologically relevant. We propose a strategy to find many polymorphic loci that can define within-species genetic variability within AMF, or at any level of resolution desired within the Glomermycota. We propose that allele diversity at the intraspecific level could then be measured for target AMF groups, or at other levels of resolution, in environmental DNA samples. Combining the use of such markers with experimental studies on AMF diversity would help to elucidate the most important level(s) of AMF diversity in plant communities. Our goal is to encourage ecologists who are trying to explain how mycorrhizal fungal communities are structured to take an approach that could also yield meaningful information that is relevant to the diversity, functioning and productivity of ecosystems
Topological Phases near a Triple Degeneracy
We study the pattern of three state topological phases that appear in systems
with real Hamiltonians and wave functions. We give a simple geometric
construction for representing these phases. We then apply our results to
understand previous work on three state phases. We point out that the ``mirror
symmetry'' of wave functions noticed in microwave experiments can be simply
understood in our framework.Comment: 4 pages, 1 figure, to appear in Phys. Rev. Let
Spin Driven Jahn-Teller Distortion in a Pyrochlore system
The ground-state properties of the spin-1 antiferromagnetic Heisenberg model
on the corner-sharing tetrahedra, pyrochlore lattice, is investigated. By
breaking up each spin into a pair of 1/2-spins, the problem is reduced to the
equivalent one of the spin-1/2 tetrahedral network in analogy with the valence
bond solid state in one dimension. The twofold degeneracy of the spin-singlets
of a tetrahedron is lifted by a Jahn-Teller mechanism, leading to a cubic to
tetragonal structural transition. It is proposed that the present mechanism is
responsible for the phase transition observed in the spin-1 spinel compounds
ZnVO and MgVO.Comment: 4 pages, 3 eps figures, REVTeX, to appear in Phys. Rev. Let
Finding the trigger to Iapetus' odd global albedo pattern: Dynamics of dust from Saturn's irregular satellites
The leading face of Saturn's moon Iapetus, Cassini Regio, has an albedo only
one tenth that on its trailing side. The origin of this enigmatic dichotomy has
been debated for over forty years, but with new data, a clearer picture is
emerging. Motivated by Cassini radar and imaging observations, we investigate
Soter's model of dark exogenous dust striking an originally brighter Iapetus by
modeling the dynamics of the dark dust from the ring of the exterior retrograde
satellite Phoebe under the relevant perturbations. In particular, we study the
particles' probabilities of striking Iapetus, as well as their expected spatial
distribution on the Iapetian surface. We find that, of the long-lived particles
(greater than about 5 microns), most particle sizes (greater than about 10
microns) are virtually certain to strike Iapetus, and their calculated
distribution on the surface matches up well with Cassini Regio's extent in its
longitudinal span. The satellite's polar regions are observed to be bright,
presumably because ice is deposited there. Thus, in the latitudinal direction
we estimate polar dust deposition rates to help constrain models of thermal
migration invoked to explain the bright poles (Spencer & Denk 2010). We also
analyze dust originating from other irregular outer moons, determining that a
significant fraction of that material will eventually coat Iapetus--perhaps
explaining why the spectrum of Iapetus' dark material differs somewhat from
that of Phoebe. Finally we track the dust particles that do not strike Iapetus,
and find that most land on Titan, with a smaller fraction hitting Hyperion. As
has been previously conjectured, such exogenous dust, coupled with Hyperion's
chaotic rotation, could produce Hyperion's roughly isotropic, moderate-albedo
surface.Comment: Accepted for publication in Icaru
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