Search results for "ORBIT"
showing 10 items of 1104 documents
KIC 8462852: Will the Trojans return in 2021?
2017
KIC 8462852 stood out among more than 100,000 stars in the Kepler catalogue because of the strange features of its light curve: a wide, asymmetric dimming taking up to 15 per cent of the light at D793 and a period of multiple, narrow dimmings happening approximately 700 days later. Several models have been proposed to account for this abnormal behaviour, most of which require either unlikely causes or a finely-tuned timing. We aim at offering a relatively natural solution, invoking only phenomena that have been previously observed, although perhaps in larger or more massive versions. We model the system using a large, ringed body whose transit produces the first dimming and a swarm of Troja…
Evaluation of the LSA-SAF gross primary production product derived from SEVIRI/MSG data (MGPP)
2020
The objective of this study is to describe a completely new 10-day gross primary production (GPP) product (MGPP LSA-411) based on data from the geostationary SEVIRI/MSG satellite within the LSA SAF (Land Surface Analysis SAF) as part of the SAF (Satellite Application Facility) network of EUMETSAT. The methodology relies on the Monteith approach. It considers that GPP is proportional to the absorbed photosynthetically active radiation APAR and the proportionality factor is known as the light use efficiency ε. A parameterization of this factor is proposed as the product of a εmax, corresponding to the canopy functioning under optimal conditions, and a coefficient quantifying the reduction of …
Reflections on the Hohmann Transfer
2004
Walter Hohmann was a civil engineer who studied orbital maneuvers in his spare time. In 1925, he published an important book (Ref. 1) containing his main result, namely, that the most economical transfer from a circular orbit to another circular orbit is achieved via an elliptical trajectory bitangent to the terminal orbits. With the advent of the space program some three decades later, the Hohmann transfer maneuver became the most fundamental maneuver in space. In this work, we present a complete study of the Hohmann transfer maneuver. After revisiting its known properties, we present a number of supplementary properties which are essential to the qualitative understanding of the maneuver.…
Development and optimization of confirmatory liquid chromatography—Orbitrap mass spectrometry method for the determination of 17 anticoccidials in po…
2018
A new sensitive and selective multi-residue method based on liquid chromatography - Orbitrap high resolution mass spectrometry (LC-Orbitrap-HRMS) was developed and validated for the determination of 17 anticoccidials in poultry and eggs. Instrumental parameters were optimized by the means of statistical experimental designs to improve the sensitivity, precision, and repeatability of the method. Further optimization of auto-tuned MS parameters led to an increase of signal intensity by 10% to 99% for 16 out of 17 analytes. The sample preparation procedure included extraction from muscle tissue and egg samples with acetonitrile, followed by preconcentration, reconstitution, and filtration. Val…
Comparative density-functional LCAO and plane-wave calculations ofLaMnO3surfaces
2005
We compare two approaches to the atomic, electronic, and magnetic structures of LaMnO3 bulk and the (001), (110) surfaces—hybrid B3PW with optimized LCAO basis set (CRYSTAL-2003 code) and GGA-PW91 with plane-wave basis set (VASP 4.6 code). Combining our calculations with those available in the literature, we demonstrate that combination of nonlocal exchange and correlation used in hybrid functionals allows to reproduce the experimental magnetic coupling constants Jab and Jc as well as the optical gap. Surface calculations performed by both methods using slab models show that the antiferromagnetic (AF) and ferromagnetic (FM) (001) surfaces have lower surface energies than the FM (110) surfac…
Energetics of hydride and electron pair attachment to EX30/+ (E=B, C, Al, Si and X=F, Cl, Br, I) and the study of bonding trends among EX30/+, EX32−/…
2004
Abstract A theoretical gas-phase “ligand-free” or “electron pair affinity” (EPA) approach, based on CCSD(T)/(SDB-)cc-pVTZ//MP2/(SDB-)cc-pVTZ electronic structure calculations, is introduced as a possible means for determining Lewis acidity trends among planar EX30/+ (E = B, C, Al, Si; X = F, Cl, Br, I) species. In this treatment, the free electron pair is considered to be an extreme Lewis base. The calculated EPA values are compared with experimental Lewis acidities, previously calculated fluoride ion affinity (FIA) and hydride ion affinity (HA) trends, and are found to exhibit reasonable correlations in all cases. The bonding in the planar and trigonal pyramidal conformations of EX30/+ and…
Quantum Chemical Calculations Show that the Uranium Molecule U2 Has a Quintuple Bond.
2005
Covalent bonding is commonly described by Lewis's theory1, with an electron pair shared between two atoms constituting one full bond. Beginning with the valence bond description2 for the hydrogen molecule, quantum chemists have further explored the fundamental nature of the chemical bond for atoms throughout the periodic table, confirming that most molecules are indeed held together by one electron pair for each bond. But more complex binding may occur when large numbers of atomic orbitals can participate in bond formation. Such behaviour is common with transition metals. When involving heavy actinide elements, metal–metal bonds might prove particularly complicated. To date, evidence for ac…
Two- and three-state conical intersections in the electron capture dissociation of disulfides: The importance of multireference calculations
2011
The SS bond cleavage produced upon electron attachment to disulfides was generally assumed to be an adiabatic process because the added electron occupies the σ*(SS) antibonding orbital. This is clearly the case in the parent HSSH compound, but not necessarily in XSSX′ derivatives, where the substituents X and X′ are different. Through the use of MS-CASPT2 calculations, we have shown that the dissociation of the SS two-center-three-electron bond in these asymmetric XSSX′ compounds requires the interaction of at least two states, in order to localize the extra electron in one of the fragments upon dissociation. This is actually the case for the CH3SSNH2 derivative, where the most favorable di…
Stability of Borane−Adduct Complexes: A G-2 Molecular Orbital Study
1997
Complexation energies of H3BXHn and [H3BXHn-1]- complexes (X = N, O, F, P, S, and Cl) (n = 3, 2, 1) have been computed at the G-2 level of theory. The formation of H3BXH3 (X = N, P) is found to be more favored than the formations of H3BXH2 (X = O, S) and H3BXH (X = F, Cl). The qualitative features of the molecular orbital interaction (the correlation diagrams) of H3BNH3 (C3v symmetry group), H3BOH2 (Cs symmetry group), and H3BFH (Cs symmetry group) complexes are presented. These diagrams show that the σ character of the B−X bond decreases and the π character increases when the electronegativity of X increases and indicate that the B−X bond cannot be treated only in terms of the simplest mod…
Nuclear magnetic shielding constants in the CC2 model
1997
Abstract Test calculations of nuclear magnetic shielding constants in the CC2 model are performed using the gauge-including atomic orbital approach. Absolute shielding constants are reported for reprsentative first-row hydrides, a few multiply bonded molecules and some challenging cases. The performance of CC2 is analyzed by comparison with experimental data and results from calculations employing more sophisticated treatments of electron correlation. In most cases, CC2 shieldings and chemical shifts are close to those obtained at second-order perturbation theory, despite the fact that the CC2 model includes an approximate treatment of orbital relaxation effects at the correlated level.