1,915 research outputs found
The existence of a 2Po excited state for the e+Ca system
The Configuration Interaction method is used to demonstrate that there is an
electronically stable state of positronic calcium with an orbital angular
momentum of L=1. This prediction relies on the use of an asymptotic series to
estimate the variational limit of the energy. The best estimate of the binding
energy is 37 meV. A discussion of the structure of the system is also
presented.Comment: 4 pages, 2 figures, in press PR
Large dimension Configuration Interaction calculations of positron binding to the group II atoms
The Configuration Interaction (CI) method is applied to the calculation of
the structures of a number of positron binding systems, including e+Be, e+Mg,
e+Ca and e+Sr. These calculations were carried out in orbital spaces containing
about 200 electron and 200 positron orbitals up to l = 12. Despite the very
large dimensions, the binding energy and annihilation rate converge slowly with
l, and the final values do contain an appreciable correction obtained by
extrapolating the calculation to the l to infinity limit. The binding energies
were 0.00317 hartree for e+Be, 0.0170 hartree for e+Mg, 0.0189 hartree for
e+Ca, and 0.0131 hartree for e+Sr.Comment: 13 pages, no figs, revtex format, Submitted to PhysRev
Kinematic Effects of Tidal Interaction on Galaxy Rotation Curves
We use self-consistent N-body models, in conjunction with models of test
particles moving in galaxy potentials, to explore the initial effects of
interactions on the rotation curves of spiral galaxies. Using nearly
self-consistent disk/bulge/halo galaxy models (Kuijken & Dubinski 1995), we
simulate the first pass of galaxies on nearly parabolic orbits; we vary orbit
inclinations, galaxy halo masses and impact parameters. For each simulation, we
mimic observed rotation curves of the model galaxies. Transient
interaction-induced features of the curves include distinctly rising or falling
profiles at large radii and pronounced bumps in the central regions. Remarkably
similar features occur in our statistical sample of optical emission-line
rotation curves of spiral galaxies in tight pairs and n-tuples.Comment: 9 pages, 2 figures, accepted for publication in ApJ Letter
Properties of the triplet metastable states of the alkaline-earth-metal atoms
A configuration interaction approach with a semiempirical model potential for the core was used to study the static and dynamic properties of the alkaline-earth-metal atoms. The alkaline-earth-metal atoms properties determined were the scalar and tensor polarizabilities, the quadrupole moment, some of the oscillator strengths and the dispersion coefficents of the van der Waals intearaction. It was observed that the use of a polarization potential to the experimental binding energy consider the influence of relativistic effects upon the core electron distribution. The resulting polarizabilities, and dispersion parameters for homonuclear pairs of atoms were within 0.1% and within 1-2 % for heavier atoms
Developing evidence-based practice: the role of case-based research
How can practitioners engage in evidence-based practice when the evidence for effectiveness of psychological treatments comes from randomized controlled trials using patient populations different from those encountered in everyday settings and treatment manuals that seem oversimplified and inflexible? The authors argue that important evidence about best practice comes from case-based research, which builds knowledge in a clinically useful manner and complements what is achieved by multivariate research methods. A multidimensional model of the research process is provided that includes clinical practice and case-based research as significant contributors. The authors summarize the principles of case-based research and provide examples of recent technical advances. Finally, the authors suggest ways in which practitioners can apply the case-based approach in researching and publishing their own cases, perhaps in collaboration with university-based researchers
Models of helically symmetric binary systems
Results from helically symmetric scalar field models and first results from a
convergent helically symmetric binary neutron star code are reported here;
these are models stationary in the rotating frame of a source with constant
angular velocity omega. In the scalar field models and the neutron star code,
helical symmetry leads to a system of mixed elliptic-hyperbolic character. The
scalar field models involve nonlinear terms that mimic nonlinear terms of the
Einstein equation. Convergence is strikingly different for different signs of
each nonlinear term; it is typically insensitive to the iterative method used;
and it improves with an outer boundary in the near zone. In the neutron star
code, one has no control on the sign of the source, and convergence has been
achieved only for an outer boundary less than approximately 1 wavelength from
the source or for a code that imposes helical symmetry only inside a near zone
of that size. The inaccuracy of helically symmetric solutions with appropriate
boundary conditions should be comparable to the inaccuracy of a waveless
formalism that neglects gravitational waves; and the (near zone) solutions we
obtain for waveless and helically symmetric BNS codes with the same boundary
conditions nearly coincide.Comment: 19 pages, 7 figures. Expanded version of article to be published in
Class. Quantum Grav. special issue on Numerical Relativit
Gravitational Stirring in Planetary Debris Disks
We describe gravitational stirring models of planetary debris disks using a
new multi-annulus planetesimal evolution code. The current code includes
gravitational stirring and dynamical friction; future studies will include
coagulation, fragmentation, Poynting-Robertson drag, and other physical
processes. We use the results of our calculations to investigate the physical
conditions required for small bodies in a planetesimal disk to reach the
shattering velocity and begin a collisional cascade. Our results demonstrate
that disks composed primarily of bodies with a single size will not undergo a
collisional cascade which produces small dust grains at 30-150 AU on timescales
of 1 Gyr or smaller. Disks with a size distribution of bodies reach conditions
necessary for a collisional cascade in 10 Myr to 1 Gyr if the disk is at least
as massive as a minimum mass solar nebula and if the disk contains objects with
radii of 500 km or larger. The estimated 500 Myr survival time for these disks
is close to the median age of roughly 400 Myr derived for nearby stars with
dusty disks.Comment: 23 pages of text + 16 Figures; to appear in the Astronomical Journal,
January 200
A Lagrangian Integrator for Planetary Accretion and Dynamics (LIPAD)
We presented the first particle based, Lagrangian code that can follow the
collisional/accretional/dynamical evolution of a large number of km-sized
planetesimals through the entire growth process to become planets. We refer to
it as the 'Lagrangian Integrator for Planetary Accretion and Dynamics' or
LIPAD. LIPAD is built on top of SyMBA, which is a symplectic -body
integrator. In order to handle the very large number of planetesimals required
by planet formation simulations, we introduce the concept of a `tracer'
particle. Each tracer is intended to represent a large number of disk particles
on roughly the same orbit and size as one another, and is characterized by
three numbers: the physical radius, the bulk density, and the total mass of the
disk particles represented by the tracer. We developed statistical algorithms
that follow the dynamical and collisional evolution of the tracers due to the
presence of one another. The tracers mainly dynamically interact with the
larger objects (`planetary embryos') in the normal N-body way. LIPAD's greatest
strength is that it can accurately model the wholesale redistribution of
planetesimals due to gravitational interaction with the embryos, which has
recently been shown to significantly affect the growth rate of planetary
embryos . We verify the code via a comprehensive set of tests which compare our
results with those of Eulerian and/or direct N-body codes.Comment: Accepted to the Astronomical Journal. See
http://www.boulder.swri.edu/~hal/LIPAD.html for more detail including
animation
Precautionary advice about mobile phones: Public understandings and intended responses
The official published version can be obtained from the link below - Copyright @ Taylor & FrancisThere is a widespread academic and policy debate about public responses to precaution in public health campaigns. This paper explores these issues in relation to the precautionary stance adopted in the UK around the regulation of mobile telecommunications. The aim of the paper is to examine the nature of attitudes to precaution, and the way in which these, along with other relevant variables, relate to the intention to adopt relevant behaviours. The results from an experimental study (n = 173) indicate that people distinguish between two dimensions of precaution: firstly in relation to its value or necessity per se and secondly as anchored to notions of governance. The two variables differentially relate to other variables including trust and uncertainty, and are predictive of intended behaviour change indirectly, through worry about mobile phone risks. Precautionary advice was generally interpreted as causing concern rather than providing reassurance. The results suggest that precaution may be considered a valuable stance but this does not mean that it is seen as good governance or that it will reduce concern. Whilst the discourse of precaution is aimed at reducing concern, it appears that the uptake of relevant behaviours is largely triggered by worry
Excited states of positronic atoms
The existence and structure of positronic atoms with a total angular momentum of L=1 is studied with the configuration interaction method. Evidence is presented that there is a P-2(o) state of e(+)Ca and P-2,4(o) states of e(+)Be(P-3(o)) that are electronically stable with binding energies of 45 meV and 2.6 meV, respectively. These predictions rely on the use of an asymptotic series analysis to estimate the angular L ->infinity limit of the energy. Incorporating corrections that compensate for the finite range of the radial basis increased the binding energies of e(+)Ca and e(+)Be to 71 meV and 42 meV, respectively
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