1,552 research outputs found
Dynamics and Thermodynamics of a Novel Phase of NaAlH4
We characterize a novel orthorhombic phase (gamma) of NaAlH4, discovered
using first-principles molecular dynamics, and discuss its relevance to the
dehydrogenation mechanism. This phase is close in energy to the known
low-temperature structure and becomes the stabler phase above 320 K, thanks to
a larger vibrational entropy associated with AlH4 rotational modes. The
structural similarity of gamma-NaAlH4 to alpha-Na3AlH6 suggests it acts as a
key intermediate during hydrogen release. Findings are consistent with recent
experiments recording an unknown phase during dehydrogenation.Comment: 10 pages, 4 figures, 1 table + supplementary info; In press (Physical
Review Letters
Planet--planet scattering in circumstellar gas disks
Hydrodynamical simulations of two giant planets embedded in a gaseous disk
have shown that in case of a smooth convergent migration they end up trapped
into a mean motion resonance. These findings have led to the conviction that
the onset of dynamical instability causing close encounters between the planets
can occur only after the dissipation of the gas when the eccentricity damping
is over. We show that a system of three giant planets may undergo planet-planet
scattering when the gaseous disk, with density values comparable to that of the
Minimum Mass Solar Nebula, is still interacting with the planets. The
hydrodynamical code FARGO--2D--1D is used to model the evolution ofthe disk and
planets, modified to properly handle close encounters between the massive
bodies. Our simulations predict a variety of different outcomes of the
scattering phase which includes orbital exchange, planet merging and scattering
of a planet in a hyperbolic orbit. This implies thatthe final fate of a
multiplanet system under the action of the disk torques is not necessarily a
packed resonant configuration.Comment: Astronomy and Astrophysics Letters, in pres
Asteroid detection at millimetric wavelengths with the Planck survey
The Planck mission, originally devised for cosmological studies, offers the
opportunity to observe Solar System objects at millimetric and submillimetric
wavelengths. We concentrate in this paper on the asteroids of the Main Belt. We
intend to estimate the number of asteroids that can can be detected during the
mission and to evaluate the strength of their signal. We have rescaled the
instrument sensitivities, calculated by the LFI and HFI teams for sources fixed
in the sky, introducing some degradation factors to properly account for moving
objects. In this way a detection threshold is derived for asteroidal detection
that is related to the diameter of the asteroid and its geocentric distance. We
have developed a numerical code that models the detection of asteroids in the
LFI and HFI channels during the mission. This code perfoprms a detailed
integration of the orbits of the asteroids in the timespan of the mission and
identifies those bodies that fall in the beams of Planck and their signal
stenght. According to our simulations, a total of 397 objects will be observed
by Planck and an asteroidal body will be detected in some beam in 30% of the
total sky scan--circles. A significant fraction (in the range from ~50 to 100
objects) of the 397 asteroids will be observed with a high S/N ratio. Flux
measurements of a large sample of asteroids in the submillimeter and millimeter
range are relevant since they allow to analyze the thermal emission and its
relation to the surface and regolith properties. Furthermore, it will be
possible to check on a wider base the two standard thermal models, based on a
nonrotating or rapidly rotating sphere. Our method can also be used to separate
Solar System sources from cosmological sources in the survey. This work is
based on Planck LFI activities.Comment: Contact person [email protected]. Accepted for pubblication in
New Astronomy (2002). 1 figure in .eps format. Needs elsart.cls style +
harvard.st
Population and Size Distribution of Small Jovian Trojan Asteroids
We present a study of Jovian Trojan objects detected serendipitously during
the course of a sky survey conducted at the University of Hawaii 2.2-meter
telescope. We used a 8192 x 8192 pixel charge-coupled device (CCD) mosaic to
observe 20 deg^2 at locations spread over the L4 Lagrangian swarm and reached a
limiting magnitude V = 22.5 mag (50% of maximum detection efficiency).
Ninety-three Jovian Trojans were detected with radii 2 - 20 km (assumed albedo
0.04). Their differential magnitude distribution has a slope of 0.40 +/- 0.05
corresponding to a power law size distribution index 3.0 +/- 0.3 (1-sigma). The
total number of L4 Trojans with radii > 1 km is of order 1.6 x 10^5 and their
combined mass (dominated by the largest objects) is ~ 10^{-4} M_{Earth}. The
bias-corrected mean inclination is 13.7 +/- 0.5 deg. We also discuss the size
and spatial distribution of the L4 swarm.Comment: 21 pages, 11 figures. AJ, in pres
On the eccentricity of self-gravitating circumstellar disks in eccentric binary systems
We study the evolution of circumstellar massive disks around the primary star
of a binary system focusing on the computation of disk eccentricity. In
particular, we concentrate on its dependence on the binary eccentricity.
Self-gravity is included in our numerical simulations. Our standard model
assumes a semimajor axis for the binary of 30 AU, the most probable value
according to the present binary statistics.Comment: Accepted for publication on A&
Transition state method and Wannier functions
We propose a computational scheme for materials where standard Local Density
Approximation (LDA) fails to produce a satisfactory description of excitation
energies. The method uses Slater's "transition state" approximation and Wannier
functions basis set. We define a correction to LDA functional in such a way
that its variation produces one-electron energies for Wannier functions equal
to the energies obtained in "transition state" constrained LDA calculations. In
the result eigenvalues of the proposed functional could be interpreted as
excitation energies of the system under consideration. The method was applied
to MgO, Si, NiO and BaBiO and gave an improved agreement with experimental
data of energy gap values comparing with LDA.Comment: 13 pages, 6 figures, 1 tabl
Density functional theory in transition-metal chemistry: a self-consistent Hubbard U approach
Transition-metal centers are the active sites for many biological and
inorganic chemical reactions. Notwithstanding this central importance,
density-functional theory calculations based on generalized-gradient
approximations often fail to describe energetics, multiplet structures,
reaction barriers, and geometries around the active sites. We suggest here an
alternative approach, derived from the Hubbard U correction to solid-state
problems, that provides an excellent agreement with correlated-electron quantum
chemistry calculations in test cases that range from the ground state of Fe
and Fe to the addition-elimination of molecular hydrogen on FeO. The
Hubbard U is determined with a novel self-consistent procedure based on a
linear-response approach.Comment: 5 pages, 3 figures, Phys. Rev. Lett., in pres
Band Structure and Quantum Conductance of Nanostructures from Maximally-Localized Wannier Functions: The Case of Functionalized Carbon Nanotubes
We have combined large-scale, -point electronic-structure
calculations with the maximally-localized Wannier functions approach to
calculate efficiently the band structure and the quantum conductance of complex
systems containing thousands of atoms while maintaining full first-principles
accuracy. We have applied this approach to study covalent functionalizations in
metallic single-walled carbon nanotubes. We find that the band structure around
the Fermi energy is much less dependent on the chemical nature of the ligands
than on the functionalization pattern disrupting the conjugation
network. Common aryl functionalizations are more stable when paired with
saturating hydrogens; even when paired, they still act as strong scattering
centers that degrade the ballistic conductance of the nanotubes already at low
degrees of coverage.Comment: To be published in Phys. Rev. Let
Eccentricity of radiative discs in close binary-star systems
Discs in binaries have a complex behavior because of the perturbations of the
companion star. Planet formation in binary-star systems both depend on the
companion star parameters and on the properties of the circumstellar disc. An
eccentric disc may increase the impact velocity of planetesimals and therefore
jeopardize the accumulation process. We model the evolution of discs in close
binaries including the effects of self-gravity and adopting different
prescriptions to model the disc's radiative properties. We focus on the
dynamical properties and evolutionary tracks of the discs. We use the
hydrodynamical code FARGO and we include in the energy equation heating and
cooling effects. Radiative discs have a lower disc eccentricity compared to
locally isothermal discs with same temperature profile. As a consequence, we do
not observe the formation of an internal elliptical low density region as in
locally isothermal disc models. However, the disc eccentricity depends on the
disc mass through the opacities. Akin to locally isothermal disc models,
self-gravity forces the disc's longitude of pericenter to librate about a fixed
orientation with respect to the binary apsidal line (). The disc's
radiative properties play an important role in the evolution of discs in
binaries. A radiative disc has an overall shape and internal structure that are
significantly different compared to a locally isothermal disc with same
temperature profile. This is an important finding both for describing the
evolutionary track of the disc during its progressive mass loss, and for planet
formation since the internal structure of the disc is relevant for
planetesimals growth in binary systems. The non-symmetrical distribution of
mass in these discs causes large eccentricities for planetesimals that may
affect their growth.Comment: accepted for publication in A&A (abstract truncated to comply with
astro-ph rules
A generalized Poisson and Poisson-Boltzmann solver for electrostatic environments
The computational study of chemical reactions in complex, wet environments is
critical for applications in many fields. It is often essential to study
chemical reactions in the presence of applied electrochemical potentials,
taking into account the non-trivial electrostatic screening coming from the
solvent and the electrolytes. As a consequence the electrostatic potential has
to be found by solving the generalized Poisson and the Poisson-Boltzmann
equation for neutral and ionic solutions, respectively. In the present work
solvers for both problems have been developed. A preconditioned conjugate
gradient method has been implemented to the generalized Poisson equation and
the linear regime of the Poisson-Boltzmann, allowing to solve iteratively the
minimization problem with some ten iterations of a ordinary Poisson equation
solver. In addition, a self-consistent procedure enables us to solve the
non-linear Poisson-Boltzmann problem. Both solvers exhibit very high accuracy
and parallel efficiency, and allow for the treatment of different boundary
conditions, as for example surface systems. The solver has been integrated into
the BigDFT and Quantum-ESPRESSO electronic-structure packages and will be
released as an independent program, suitable for integration in other codes
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