940 research outputs found
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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
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
The role of binaries in the enrichment of the early Galactic halo. II. Carbon-Enhanced Metal-Poor Stars - CEMP-no stars
The detailed composition of most metal-poor halo stars has been found to be
very uniform. However, a fraction of 20-70% (increasing with decreasing
metallicity) exhibit dramatic enhancements in their abundances of carbon - the
so-called carbon-enhanced metal-poor (CEMP) stars. A key question for Galactic
chemical evolution models is whether this non-standard composition reflects
that of the stellar natal clouds, or is due to local, post-birth mass transfer
of chemically processed material from a binary companion; CEMP stars should
then all be members of binary systems. Our aim is to determine the frequency
and orbital parameters of binaries among CEMP stars with and without
over-abundances of neutron-capture elements - CEMP-s and CEMP-no stars,
respectively - as a test of this local mass-transfer scenario. This paper
discusses a sample of 24 CEMP-no stars, while a subsequent paper will consider
a similar sample of CEMP-s stars. Most programme stars exhibit no statistically
significant radial-velocit variation over this period and appear to be single,
while four are found to be binaries with orbital periods of 300-2,000 days and
normal eccentricity; the binary frequency for the sample is 17+-9%. The single
stars mostly belong to the recently-identified ``low-C band'', while the
binaries have higher absolute carbon abundances. We conclude that the
nucleosynthetic process responsible for the strong carbon excess in these
ancient stars is unrelated to their binary status; the carbon was imprinted on
their natal molecular clouds in the early Galactic ISM by an even earlier,
external source, strongly indicating that the CEMP-no stars are likely bona
fide second-generation stars. We discuss potential production sites for carbon
and its transfer across interstellar distances in the early ISM, and
implications for the composition of high-redshift DLA systems. Abridged.Comment: 16 pages, 5 figures, accepted for publication in Astronomy and
Astrophysic
Time in Care for Older People Living in Nursing Homes
In order to measure actual care needs in relation to resources required to fulfill these needs, an instrument (Time in Care) with which to evaluate care needs and determine the time needed for various care activities has been developed with the aim of assessing nursing intensity in municipal care for older people. Interreliability (ICC = 0.854) of time measurements (n = 10'546) of 32 nursing activities in relation to evaluated care levels in two nursing homes (staff n = 81) has been determined. Nursing intensity for both periods at the two nursing homes comprised on average a direct care time of 75 (45%) and 101 (42%) minutes, respectively. Work time was measured according to actual schedule (462 hours per nursing home during two weeks). Given that the need for care was high, one must further investigate if the quality of care the recipients received was sufficiently addressed
A method for atomistic spin dynamics simulations: implementation and examples
We present a method for performing atomistic spin dynamic simulations. A
comprehensive summary of all pertinent details for performing the simulations
such as equations of motions, models for including temperature, methods of
extracting data and numerical schemes for performing the simulations is given.
The method can be applied in a first principles mode, where all interatomic
exchange is calculated self-consistently, or it can be applied with frozen
parameters estimated from experiments or calculated for a fixed
spin-configuration. Areas of potential applications to different magnetic
questions are also discussed. The method is finally applied to one situation
where the macrospin model breaks down; magnetic switching in ultra strong
fields.Comment: 14 pages, 19 figure
Hamilton-Jacobi formalism for Linearized Gravity
In this work we study the theory of linearized gravity via the
Hamilton-Jacobi formalism. We make a brief review of this theory and its
Lagrangian description, as well as a review of the Hamilton-Jacobi approach for
singular systems. Then we apply this formalism to analyze the constraint
structure of the linearized gravity in instant and front-form dynamics.Comment: To be published in Classical and Quantum Gravit
The Geneva-Copenhagen Survey of the Solar Neighbourhood
We report on a new survey of metallicities, ages, and Galactic orbits for a complete, magnitude-limited, and kinematically unbiased all-sky sample of 16 682 nearby F- and G-dwarfs. Our ∼ 63 000 new, accurate radial velocities for nearly 13 500 of the stars, combined with Hipparcos parallaxes and Tycho-2 proper motions, complete the kinematic data for 14 139 stars and allow us to identify most of the binary stars in the sample. Isochrone ages have been determined whenever reliable results are possible, with particular attention to realistic error estimates. Among the basic properties of the Galactic disk that can be reinvestigated from our data are the metallicity distribution of G-dwarfs and the age-metallicity and age-velocity relations of the solar neighbourhood. We confirm the lack of metal-poor G-dwarfs relative to classical model predictions (the 'G-dwarf problem'), the near-constancy of the mean metallicity since the formation of the thin disk, and the appearance of the kinematic signature of the thick disk ∼ 10 Gyr ag
Multipole decomposition of LDA+ energy and its application to actinides compounds
A general reformulation of the exchange energy of -shell is applied in
the analysis of the magnetic structure of various actinides compounds in the
framework of LDA+U method. The calculations are performed in an efficient
scheme with essentially only one free parameter, the screening length. The
results are analysed in terms of different polarisation channels, due to
different multipoles. Generally it is found that the spin-orbital polarisation
is dominating. This can be viewed as a strong enhancement of the spin-orbit
coupling in these systems. This leads to a drastic decrease in spin
polarisation, in accordance with experiments. The calculations are able to
correctly differentiate magnetic and non-magnetic Pu system. Finally, in all
magnetic systems a new multipolar order is observed, whose polarisation energy
is often larger in magnitude than that of spin polarisation.Comment: Fixed some references and picture
Kaluza-Klein towers for real vector fields in flat space
We consider a free real vector field propagating in a five dimensional flat
space with its fifth dimension compactified either on a strip or on a circle
and perform a Kalaza Klein reduction which breaks SO(4,1) invariance while
reserving SO(3,1) invariance. Taking into account the Lorenz gauge condition,
we obtain from the most general hermiticity conditions for the relevant
operators all the allowed boundary conditions which have to be imposed on the
fields in the extra-dimension. The physical Kaluza-Klein mass towers, which
result in a four-dimensional brane, are determined in the different distinct
allowed cases. They depend on the bulk mass, on the parameters of the boundary
conditions and on the extra parameter present in the Lagrangian. In general,
they involve vector states together with accompanying scalar states.Comment: 28 pages, 4 independent table
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