42,669 research outputs found
Smoothed Particle Magnetohydrodynamics (some shocking results...)
There have been some issues in the past in attempts to simulate magnetic
fields using the Smoothed Particle Hydrodynamics (SPH) method. SPH is well
suited to star formation problems because of its Lagrangian nature. We present
new, stable and conservative methods for magnetohydrodynamics (MHD) in SPH and
present numerical tests on both waves and shocks in one dimension to show that
it gives robust and accurate results.Comment: Kluwer latex, 6 pages, 3 figures; Proceedings of the International
Workshop "Magnetic Fields and Star Formation: Theory vs Observations",
Madrid, 21-25 April 2003. Revised version accepted to proceedings (exact
solutions added, other minor changes
Complementary approaches to the ab initio calculation of melting properties
Several research groups have recently reported {\em ab initio} calculations
of the melting properties of metals based on density functional theory, but
there have been unexpectedly large disagreements between results obtained by
different approaches. We analyze the relations between the two main approaches,
based on calculation of the free energies of solid and liquid and on direct
simulation of the two coexisting phases. Although both approaches rely on the
use of classical reference systems consisting of parameterized empirical
interaction models, we point out that in the free energy approach the final
results are independent of the reference system, whereas in the current form of
the coexistence approach they depend on it. We present a scheme for correcting
the predictions of the coexistence approach for differences between the
reference and {\em ab initio} systems. To illustrate the practical operation of
the scheme, we present calculations of the high-pressure melting properties of
iron using the corrected coexistence approach, which agree closely with earlier
results from the free energy approach. A quantitative assessment is also given
of finite-size errors, which we show can be reduced to a negligible size.Comment: 14 pages, two figure
Ab-initio chemical potentials of solid and liquid solutions and the chemistry of the Earth's core
A general set of methods is presented for calculating chemical potentials in
solid and liquid mixtures using {\em ab initio} techniques based on density
functional theory (DFT). The methods are designed to give an {\em ab initio}
approach to treating chemical equilibrium between coexisting solid and liquid
solutions, and particularly the partitioning ratio of solutes between such
solutions. For the liquid phase, the methods are based on the general technique
of thermodynamic integration, applied to calculate the change of free energy
associated with the continuous interconversion of solvent and solute atoms, the
required thermal averages being computed by DFT molecular dynamics simulation.
For the solid phase, free energies and hence chemical potentials are obtained
using DFT calculation of vibrational frequencies of systems containing
substitutional solute atoms, with anharmonic contributions calculated, where
needed, by thermodynamic integration. The practical use of the methods is
illustrated by applying them to study chemical equilibrium between the outer
liquid and inner solid parts of the Earth's core, modelled as solutions of S,
Si and O in Fe. The calculations place strong constraints on the chemical
composition of the core, and allow an estimate of the temperature at the
inner-core/outer-core boundary.Comment: 19 pages, two figure
A preliminary study of composite orbiter/ lander missions to satellites of the outer planets
Objectives and payload requirements for composite orbiter/lander missions to satellites of outer planet
Preliminary feasibility study of soft-lander missions to the Galilean satellites of Jupiter
Feasibility study of soft lander missions to Galilean satellites of Jupite
Sculplexity: Sculptures of Complexity using 3D printing
We show how to convert models of complex systems such as 2D cellular automata
into a 3D printed object. Our method takes into account the limitations
inherent to 3D printing processes and materials. Our approach automates the
greater part of this task, bypassing the use of CAD software and the need for
manual design. As a proof of concept, a physical object representing a modified
forest fire model was successfully printed. Automated conversion methods
similar to the ones developed here can be used to create objects for research,
for demonstration and teaching, for outreach, or simply for aesthetic pleasure.
As our outputs can be touched, they may be particularly useful for those with
visual disabilities.Comment: Free access to article on European Physics Letter
Phase Lags in the Optical-Infrared Light Curves of AGB Stars
To search for phase lags in the optical-infrared light curves of asymptotic
giant branch stars, we have compared infrared data from the COBE DIRBE
satellite with optical light curves from the AAVSO and other sources. We found
17 examples of phase lags in the time of maximum in the infrared vs. that in
the optical, and 4 stars with no observed lags. There is a clear difference
between the Mira variables and the semi-regulars in the sample, with the
maximum in the optical preceding that in the near-infrared in the Miras, while
in most of the semi-regulars no lags are observed. Comparison to published
theoretical models indicates that the phase lags in the Miras are due to strong
titanium oxide absorption in the visual at stellar maximum, and suggests that
Miras pulsate in the fundamental mode, while at least some semi-regulars are
first overtone pulsators. There is a clear optical-near-infrared phase lag in
the carbon-rich Mira V CrB; this is likely due to C2 and CN absorption
variations in the optical.Comment: AJ, in pres
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