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A time-dependent coordinate transformation of a constant coeffcient hyperbolic equation which results in a variable coeffcient problem is considered. By using the energy method, we derive well-posed boundary conditions for the continuous problem. It is shown that the number of boundary conditions depend on the coordinate transformation. By using Summation-by-Parts (SBP) operators for the space discretization and weak boundary conditions, an energy stable finite dieffrence scheme is obtained. We also show how to construct a time-dependent penalty formulation that automatically imposes the right number of boundary conditions. Numerical calculations corroborate the stability and accuracy of the approximations
Stellar substructures in the solar neighbourhood. III. Kinematic group 2 in the Geneva-Copenhagen survey
From correlations between orbital parameters, several new coherent groups of
stars were recently identified in the Galactic disc and suggested to correspond
to remnants of disrupted satellites. To reconstruct their origin at least three
main observational parameters - kinematics, chemical composition and age - must
be known. We determine detailed elemental abundances in stars belonging to the
so-called Group 2 of the Geneva-Copenhagen Survey and compare the chemical
composition with Galactic thin- and thick-disc stars, as well as with the
Arcturus and AF06 streams. The aim is to search for chemical signatures that
might give information about the formation history of this kinematic group of
stars. High-resolution spectra were obtained with the FIES spectrograph at the
Nordic Optical Telescope, La Palma, and were analysed with a differential model
atmosphere method. Comparison stars were observed and analysed with the same
method. The average value of [Fe/H] for the 32 stars of Group 2 is -0.42 +-
0.10 dex. The investigated group consists mainly of two 8- and 12-Gyr-old
stellar populations. Abundances of oxygen, alpha-elements, and
r-process-dominated elements are higher than in Galactic thin-disc dwarfs. This
elemental abundance pattern has similar characteristics as that of the Galactic
thick-disc. The similarity in chemical composition of stars in Group 2 with
that in stars of the thick-disc might suggest that their formation histories
are linked. The chemical composition together with the kinematic properties and
ages of stars in the investigated stars provides evidence of their common
origin and possible relation to an ancient merging event. A gas-rich satellite
merger scenario is proposed as the most likely origin. Groups 2 and 3 of the
Geneva-Copenhagen Survey might have originated in the same merging event.Comment: 17 pages, 13 figures, accepted for publication in Astronomy &
Astrophysics, 201
The double-Reissner-Nordstrom solution and the interaction force between two spherically symmetric charged particles
The physical representation of the general double-Reissner-Nordstrom solution
is obtained by rewriting the N=2 Breton-Manko-Aguilar electrostatic solution in
the Varzugin-Chistyakov parametrization (M_i, Q_i, R). A concise analytical
formula is derived for the interaction force between two arbitrary
Reissner-Nordstrom constituents, and an example of the equilibrium
configuration involving two oppositely charged particles which confirms earlier
Bonnor's prediction of the existence of such configurations is given.Comment: 14 pages, 1 figure; submitted to Physical Review
Stellar substructures in the solar neighbourhood IV. Kinematic Group 1 in the Geneva-Copenhagen survey
We determine detailed elemental abundances in stars belonging to the
so-called Group 1 of the Geneva-Copenhagen survey (GCS) and compare the
chemical composition with the Galactic thin- and thick-disc stars, with the GCS
Group 2 and Group 3 stars, as well as with several kinematic streams of similar
metallicities. The aim is to search for chemical signatures that might give
information about the formation history of this kinematic group of stars.
High-resolution spectra were obtained with the Fibre-fed Echelle Spectrograph
(FIES) spectrograph at the Nordic Optical Telescope, La Palma, and were
analysed with a differential model atmosphere method. Comparison stars were
observed and analysed with the same method. The average value of [Fe/H] for the
37 stars of Group 1 is -0.20 +- 0.14 dex. Investigated Group 1 stars can be
separated into three age subgroups. Along with the main 8- and 12-Gyr-old
populations, a subgroup of stars younger than 5 Gyr can be separated as well.
Abundances of oxygen, alpha-elements, and r-process dominated elements are
higher than in Galactic thin-disc dwarfs. This elemental abundance pattern has
similar characteristics to that of the Galactic thick disc and differs slightly
from those in Hercules, Arcturus, and AF06 stellar streams. The similar
chemical composition of stars in Group 1, as well as in Group 2 and 3, with
that in stars of the thick disc might suggest that their formation histories
are linked. The chemical composition pattern together with the kinematic
properties and ages of stars in the investigated GCS groups provide evidence of
their common origin and possible relation to an ancient merging event. A
gas-rich satellite merger scenario is proposed as the most likely origin.Comment: 17 pages, 13 figures, accepted for publication in Astronomy &
Astrophysics, 201
Two kinds of procedural semantics for privative modification
In this paper we present two kinds of procedural semantics for privative modification. We do this for three reasons. The first reason is to launch a tough test case to gauge the degree of substantial agreement between a constructivist and a realist interpretation of procedural semantics; the second is to extend Martin-L ̈f’s Constructive Type Theory to privative modification, which is characteristic of natural language; the third reason is to sketch a positive characterization of privation
Atomistic spin dynamics of the CuMn spin glass alloy
We demonstrate the use of Langevin spin dynamics for studying dynamical
properties of an archetypical spin glass system. Simulations are performed on
CuMn (20% Mn) where we study the relaxation that follows a sudden quench of the
system to the low temperature phase. The system is modeled by a Heisenberg
Hamiltonian where the Heisenberg interaction parameters are calculated by means
of first-principles density functional theory. Simulations are performed by
numerically solving the Langevin equations of motion for the atomic spins. It
is shown that dynamics is governed, to a large degree, by the damping parameter
in the equations of motion and the system size. For large damping and large
system sizes we observe the typical aging regime.Comment: 18 pages, 9 figure
Lead abundance in the uranium star CS 31082-001
In a previous paper we were able to measure the abundance of uranium and
thorium in the very-metal poor halo giant BPS CS 31082-001, but only obtained
an upper limit for the abundance of lead (Pb). We have got from ESO 17 hours of
additional exposure on this star in order to secure a detection of the minimum
amount of lead expected to be present in CS 31082-001, the amount arising from
the decay of the original content of Th and U in the star. We report here this
successful detection. We find an LTE abundance log(Pb/H)+12=-0.55 \pm 0.15 dex,
one dex below the upper limits given by other authors for the similar stars CS
22892-052 and BD +17d3248, also enhanced in r-process elements. From the
observed present abundances of Th and U in the star, the expected amount of Pb
produced by the decay of 232Th, and 238U alone, over 12-15 Gyr is -0.73\pm 0.17
dex. The decay of 235U is more difficult to estimate, but is probably slightly
below the contribution of 238U, making the contribution of the 3 actinides only
slightly below, or even equal to, the measured abundance. The contribution from
the decay of 234U has was not included, for lack of published data. In this
sense our determination is a lower limit to the contribution of actinides to
lead production. We comment this result, and we note that if a NLTE analysis,
not yet possible, doubles our observed abundance, the decay of the 3 actinides
will still represent 50 per cent of the total lead, a proportion higher than
the values considered so far in the literature.Comment: 4 pages, LateX, A&A Letters Accepte
Dynamics of diluted magnetic semiconductors from atomistic spin dynamics simulations: Mn doped GaAs as a case study
The dynamical behavior of the magnetism of diluted magnetic semiconductors
(DMS) has been investigated by means of atomistic spin dynamics simulations.
The conclusions drawn from the study are argued to be general for DMS systems
in the low concentration limit, although all simulations are done for 5%
Mn-doped GaAs with various concentrations of As antisite defects. The
magnetization curve, , and the Curie temperature have been
calculated, and are found to be in good correspondence to results from Monte
Carlo simulations and experiments. Furthermore, equilibrium and non-equilibrium
behavior of the magnetic pair correlation function have been extracted. The
dynamics of DMS systems reveals a substantial short ranged magnetic order even
at temperatures at or above the ordering temperature, with a non-vanishing pair
correlation function extending up to several atomic shells. For the high As
antisite concentrations the simulations show a short ranged anti-ferromagnetic
coupling, and a weakened long ranged ferromagnetic coupling. For sufficiently
large concentrations we do not observe any long ranged ferromagnetic
correlation. A typical dynamical response shows that starting from a random
orientation of moments, the spin-correlation develops very fast ( 1ps)
extending up to 15 atomic shells. Above 10 ps in the simulations, the
pair correlation is observed to extend over some 40 atomic shells. The
autocorrelation function has been calculated and compared with ferromagnets
like bcc Fe and spin-glass materials. We find no evidence in our simulations
for a spin-glass behaviour, for any concentration of As antisites. Instead the
magnetic response is better described as slow dynamics, at least when compared
to that of a regular ferromagnet like bcc Fe.Comment: 24 pages, 15 figure
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