673 research outputs found
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
Report of IAU Commission 30 on Radial Velocities (2006-2009)
Brief summaries are given on the following subjects: Radial velocities and
exoplanets (Toward Earth-mass planets; Retired A stars and their planets;
Current status and prospects); Toward higher radial velocity precision; Radial
velocities and asteroseismology; Radial velocities in Galactic and
extragalactic clusters; Radial velocities for field giants; Galactic structure
-- Large surveys (The Geneva-Copenhagen Survey; Sloan Digital Sky Survey;
RAVE); Working groups (WG on radial velocity standards; WG on stellar radial
velocity bibliography; WG on the catalogue of orbital elements of spectroscopic
binaries [SB9]).Comment: 11 pages, to appear in the IAU Transactions Vol. XXVIIA, Reports on
Astronomy 2006-2009, ed. Karel van der Hucht. Editor: G. Torre
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
Asymptotically cylindrical 7-manifolds of holonomy G_2 with applications to compact irreducible G_2-manifolds
We construct examples of exponentially asymptotically cylindrical Riemannian
7-manifolds with holonomy group equal to G_2. To our knowledge, these are the
first such examples. We also obtain exponentially asymptotically cylindrical
coassociative calibrated submanifolds. Finally, we apply our results to show
that one of the compact G_2-manifolds constructed by Joyce by desingularisation
of a flat orbifold T^7/\Gamma can be deformed to one of the compact
G_2-manifolds obtainable as a generalized connected sum of two exponentially
asymptotically cylindrical SU(3)-manifolds via the method given by the first
author (math.DG/0012189).Comment: 36 pages; v2: corrected trivial typos; v3: some arguments corrected
and improved; v4: a number of improvements on presentation, paritularly in
sections 4 and 6, including an added picture
Conformally flat spacetimes and Weyl frames
We discuss the concepts of Weyl and Riemann frames in the context of metric
theories of gravity and state the fact that they are completely equivalent as
far as geodesic motion is concerned. We apply this result to conformally flat
spacetimes and show that a new picture arises when a Riemannian spacetime is
taken by means of geometrical gauge transformations into a Minkowskian flat
spacetime. We find out that in the Weyl frame gravity is described by a scalar
field. We give some examples of how conformally flat spacetime configurations
look when viewed from the standpoint of a Weyl frame. We show that in the
non-relativistic and weak field regime the Weyl scalar field may be identified
with the Newtonian gravitational potential. We suggest an equation for the
scalar field by varying the Einstein-Hilbert action restricted to the class of
conformally-flat spacetimes. We revisit Einstein and Fokker's interpretation of
Nordstr\"om scalar gravity theory and draw an analogy between this approach and
the Weyl gauge formalism. We briefly take a look at two-dimensional gravity as
viewed in the Weyl frame and address the question of quantizing a conformally
flat spacetime by going to the Weyl frame.Comment: LATEX - 18 page
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
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
Analogs of the double-Reissner-Nordstrom solution in magnetostatics and dilaton gravity: mathematical description and some physical properties
In this paper we consider a magnetic analog of the double-Reissner-Nordstrom
solution and construct the corresponding magnetic potential A_\varphi in the
explicit form. The behavior of the resulting solution under the Harrison
transformation then naturally singles out the asymmetric black diholes -
configurations composed of two non-extreme black holes possessing unequal
masses, and charges equal in magnitude but opposite in sign - as its most
general subclass for which equilibrium of the black-hole constituents can be
achieved with the aid of the external magnetic (or electric) field. We also
generalize the double-Reissner-Nordstrom solution to the dilaton gravity with
arbitrary dilaton coupling, yielding as the result the 4-dimensional
double-Gibbons-Maeda spacetime. The study of some physical properties of the
solutions obtained leads, in particular, to very simple formulas for the areas
of the horizons and surface gravities.Comment: 18 pages, 1 figure; title changed, typos corrected; a considerably
extended version which includes the discussion of the magnetostatic case and
the explicit formula for the magnetic potentia
First stars XVI. STIS/HST abundances of heavy-elements in the uranium-rich star CS 31082-001
Detailed abundances of the elements produced by r-process nucleosynthesis in
various circumstances are our best observational clues to their origin, since
the site(s) of r-element production is(are) still not known with certainty. A
small fraction of extremely metal-poor (EMP) stars exhibit excesses of heavy
neutron-capture elements produced in the r-process, and CS 31082-001 is among
the 4 well-known r-process-enhanced EMP stars. Observations with HST/STIS
provide abundances for elements observable only from the UV region. Here we aim
to supplement the optical data with abundances from near-UV spectroscopy of the
first and second peak of the r-elements, which are crucial to giving insight
into the nucleosynthesis of the elements beyond iron. The UVES spectrum
provided additional measurements, thereby improving the previous results. The
spectra were analyzed with the OSMARCS LTE model atmosphere and with a
consistent approach based on the spectrum synthesis code Turbospectrum to
derive abundances of heavy elements in CS 31082-001, using updated oscillator
strengths from the recent literature. We computed synthetic spectra for all
lines of the elements of interest, checking for proper intensities and possible
blends. We combined the abundances of heavy elements derived in previous works
with the derivation of abundances from all reliable new list of lines, for the
first and second peaks of r-elements. We were able to derive new abundances for
23 n-elements, 6 of them - Ge, Mo, Lu, Ta, W, and Re - were not available in
previous works, making this star the most complete r-II object studied, with a
total of 37 detections of n-capture elements. We also present the first NLTE+3D
lead abundance in this star. The results provide improved constraints on the
nature of the r-process.Comment: 20 pages, 21 figures, 3 tables. Accepted for publication in Astronomy
and Astrophysic
Lattice dynamics and reduced thermal conductivity of filled skutterudites
The great reduction in thermal conductivity of skutterudites upon filling the
``void'' sites with Rare Earth (RE) ions is key to their favorable
thermoelectric properties but remains to be understood. Using lattice dynamic
models based on first principles calculations, we address the most popular
microscopic mechanism, reduction via rattling ions. The model withstands
inelastic neutron scattering and specific heat measurements, and refutes
hypotheses of an anharmonic RE potential and of two distinct localized RE
vibrations of disparate frequencies. It does indicate a strong hybridization
between bare La vibrations and certain Sb-like phonon branches, suggesting
anharmonic scattering by harmonic RE motions as an important mechanism for
suppression of heat conductivity.Comment: modified version resubmitted to PRB. Results unchanged, text changed
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