597 research outputs found
Stellar wind interaction and pick-up ion escape of the Kepler-11 "super-Earths"
We study the interactions between stellar wind and the extended
hydrogen-dominated upper atmospheres of planets and the resulting escape of
planetary pick-up ions from the 5 "super-Earths" in the compact Kepler-11
system and compare the escape rates with the efficiency of the thermal escape
of neutral hydrogen atoms. Assuming the stellar wind of Kepler-11 is similar to
the solar wind, we use a polytropic 1D hydrodynamic wind model to estimate the
wind properties at the planetary orbits. We apply a Direct Simulation Monte
Carlo Model to model the hydrogen coronae and the stellar wind plasma
interaction around Kepler-11b-f within a realistic expected heating efficiency
range of 15-40%. The same model is used to estimate the ion pick-up escape from
the XUV heated and hydrodynamically extended upper atmospheres of Kepler-11b-f.
From the interaction model we study the influence of possible magnetic moments,
calculate the charge exchange and photoionization production rates of planetary
ions and estimate the loss rates of pick-up H+ ions for all five planets. We
compare the results between the five "super-Earths" and in a more general sense
also with the thermal escape rates of the neutral planetary hydrogen atoms. Our
results show that for all Kepler-11b-f exoplanets, a huge neutral hydrogen
corona is formed around the planet. The non-symmetric form of the corona
changes from planet to planet and is defined mostly by radiation pressure and
gravitational effects. Non-thermal escape rates of pick-up ionized hydrogen
atoms for Kepler-11 "super-Earths" vary between approximately 6.4e30 1/s and
4.1e31 1/s depending on the planet's orbital location and assumed heating
efficiency. These values correspond to non-thermal mass loss rates of
approximately 1.07e7 g/s and 6.8e7 g/s respectively, which is a few percent of
the thermal escape rates.Comment: 8 pages, 3 figures, accepted to A&
An \emph{ab initio} method for locating characteristic potential energy minima of liquids
It is possible in principle to probe the many--atom potential surface using
density functional theory (DFT). This will allow us to apply DFT to the
Hamiltonian formulation of atomic motion in monatomic liquids [\textit{Phys.
Rev. E} {\bf 56}, 4179 (1997)]. For a monatomic system, analysis of the
potential surface is facilitated by the random and symmetric classification of
potential energy valleys. Since the random valleys are numerically dominant and
uniform in their macroscopic potential properties, only a few quenches are
necessary to establish these properties. Here we describe an efficient
technique for doing this. Quenches are done from easily generated "stochastic"
configurations, in which the nuclei are distributed uniformly within a
constraint limiting the closeness of approach. For metallic Na with atomic pair
potential interactions, it is shown that quenches from stochastic
configurations and quenches from equilibrium liquid Molecular Dynamics (MD)
configurations produce statistically identical distributions of the structural
potential energy. Again for metallic Na, it is shown that DFT quenches from
stochastic configurations provide the parameters which calibrate the
Hamiltonian. A statistical mechanical analysis shows how the underlying
potential properties can be extracted from the distributions found in quenches
from stochastic configurations
Transit Ly- signatures of terrestrial planets in the habitable zones of M dwarfs
We modeled the transit signatures in the Lya line of a putative Earth-sized
planet orbiting in the HZ of the M dwarf GJ436. We estimated the transit depth
in the Lya line for an exo-Earth with three types of atmospheres: a
hydrogen-dominated atmosphere, a nitrogen-dominated atmosphere, and a
nitrogen-dominated atmosphere with an amount of hydrogen equal to that of the
Earth. We calculated the in-transit absorption they would produce in the Lya
line. We applied it to the out-of-transit Lya observations of GJ 436 obtained
by the HST and compared the calculated in-transit absorption with observational
uncertainties to determine if it would be detectable. To validate the model, we
also used our method to simulate the deep absorption signature observed during
the transit of GJ 436b and showed that our model is capable of reproducing the
observations. We used a DSMC code to model the planetary exospheres. The code
includes several species and traces neutral particles and ions. At the lower
boundary of the DSMC model we assumed an atmosphere density, temperature, and
velocity obtained with a hydrodynamic model for the lower atmosphere. We showed
that for a small rocky Earth-like planet orbiting in the HZ of GJ436 only the
hydrogen-dominated atmosphere is marginally detectable with the STIS/HST.
Neither a pure nitrogen atmosphere nor a nitrogen-dominated atmosphere with an
Earth-like hydrogen concentration in the upper atmosphere are detectable. We
also showed that the Lya observations of GJ436b can be reproduced reasonably
well assuming a hydrogen-dominated atmosphere, both in the blue and red wings
of the Lya line, which indicates that warm Neptune-like planets are a suitable
target for Lya observations. Terrestrial planets can be observed in the Lya
line if they orbit very nearby stars, or if several observational visits are
available.Comment: 17 pages, 12 figures, accepted for publication in Astronomy &
Astrophysic
Liquid state properties from first principles DFT calculations: Static properties
In order to test the Vibration-Transit (V-T) theory of liquid dynamics, ab
initio density functional theory (DFT) calculations of thermodynamic properties
of Na and Cu are performed and compared with experimental data. The
calculations are done for the crystal at T = 0 and T_m, and for the liquid at
T_m. The key theoretical quantities for crystal and liquid are the structural
potential and the dynamical matrix, both as function of volume. The theoretical
equations are presented, as well as details of the DFT computations. The
properties compared with experiment are the equilibrium volume, the isothermal
bulk modulus, the internal energy and the entropy. The agreement of theory with
experiment is uniformly good. Our primary conclusion is that the application of
DFT to V-T theory is feasible, and the resulting liquid calculations achieve
the same level of accuracy as does ab initio lattice dynamics for crystals.
Moreover, given the well established reliability of DFT, the present results
provide a significant confirmation of V-T theory itself.Comment: 9 pages, 3 figures, 5 tables, edited to more closely match published
versio
An economic insight into additive manufacturing system implementation
With an ever growing diffusion of Additive Manufacturing (AM) system in industrial and commercial level, as well as the direct and indirect dynamics which are being introduced resulting from its inclusion as a possible production technology on companies’ portfolio, the need to reconfigure production system and adapt the production strategy becomes even more relevant than before. There are several studies which have emphasized on the importance of a paradigm shift in order to exploit advantages of AM, not only considering changes within design and functionality of the product, but also concerning AM’s impact on the entire value chain (re)configuration. Thus, it is of crucial importance to take into consideration that for this shift to be feasible and manageable, there is a need to include both technical and managerial aspects of manufacturing. This work proposes an economic insight in order to provide a guideline for the proper evaluation of AM system implementation. © IFIP International Federation for Information Processing 2015
Giant magnetic enhancement in Fe/Pd films and its influence on the magnetic interlayer coupling
The magnetic properties of thin Pd fcc(001) films with embedded monolayers of
Fe are investigated by means of first principles density functional theory. The
induced spin polarization in Pd is calculated and analyzed in terms of quantum
interference within the Fe/Pd/Fe bilayer system. An investigation of the
magnetic enhancement effects on the spin polarization is carried out and its
consequences for the magnetic interlayer coupling are discussed. In contrast to
{\it e.g.} the Co/Cu fcc(001) system we find a large effect on the magnetic
interlayer coupling due to magnetic enhancement in the spacer material. In the
case of a single embedded Fe monolayer we find aninduced Pd magnetization
decaying with distance from the magnetic layer as ~ with
. For the bilayer system we find a giant magnetic
enhancement (GME) that oscillates strongly due to interference effects. This
results in a strongly modified magnetic interlayer coupling, both in phase and
magnitude, which may not be described in the pure
Ruderman-Kittel-Kasuya-Yoshida (RKKY) picture. No anti-ferromagnetic coupling
was found and by comparison with magnetically constrained calculations we show
that the overall ferromagnetic coupling can be understood from the strong
polarization of the Pd spacer
Proof of concept of ovine artificial insemination by vaginal deposition of frozen-thawed semen under UK sheep-farming conditions
- …