910 research outputs found
The Fermi Surface Effect on Magnetic Interlayer Coupling
The oscillating magnetic interlayer coupling of Fe over spacer layers
consisting of CuPd alloys is investigated by first principles
density functional theory. The amplitude, period and phase of the coupling, as
well as the disorder-induced decay, are analyzed in detail and the consistency
to the Ruderman-Kittel-Kasuya-Yoshida (RKKY) theory is discussed. For the first
time an effect of the Fermi surface nesting strength on the amplitude is
established from first principles calculations. An unexpected variation of the
phase and disorder-induced decay is obtained and the results are discussed in
terms of asymptotics
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&
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
Flow of forces and couples to the cylindrical solids through system asymmetry thin-walled elements of varying stiffness
Задача про передачу силового і моментного навантаження до криволінійного отвору
нескінченної ізотропної пластинки через систему несиметричних відносно серединної площини
тонкостінних елементів змінної жорсткості зведена до системи чотирьох сингулярних інтегрально–
диференціальних рівнянь з ядрами Гільберта. Методом колокації досліджується вплив на напружений
стан пластинки несиметричності підсилень, способу їх розміщення на контурі отвору.The problem of flow of forces and couples to the curvilinear hole in a infinite isotropic plate through
system of asymmetrical by a middle plane the thin-walled elements of varying stiffness reduce to the system of
four singular integral-differential equations with cores of Hilbert. The effect of asymmetry strengthens and
location mode by method of collocation does investigate
Protons in the near-lunar wake observed by the Sub-keV Atom Reflection Analyzer on board Chandrayaan-1
Significant proton fluxes were detected in the near wake region of the Moon
by an ion mass spectrometer on board Chandrayaan-1. The energy of these
nightside protons is slightly higher than the energy of the solar wind protons.
The protons are detected close to the lunar equatorial plane at a
solar zenith angle, i.e., ~50 behind the terminator at a height of
100 km. The protons come from just above the local horizon, and move along the
magnetic field in the solar wind reference frame. We compared the observed
proton flux with the predictions from analytical models of an electrostatic
plasma expansion into a vacuum. The observed velocity was higher than the
velocity predicted by analytical models by a factor of 2 to 3. The simple
analytical models cannot explain the observed ion dynamics along the magnetic
field in the vicinity of the Moon.Comment: 28 pages, 7 figure
- …