5,658 research outputs found
Elemental Abundances of Solar Sibling Candidates
Dynamical information along with survey data on metallicity and in some cases
age have been used recently by some authors to search for candidates of stars
that were born in the cluster where the Sun formed. We have acquired high
resolution, high signal-to-noise ratio spectra for 30 of these objects to
determine, using detailed elemental abundance analysis, if they could be true
solar siblings. Only two of the candidates are found to have solar chemical
composition. Updated modeling of the stars' past orbits in a realistic Galactic
potential reveals that one of them, HD162826, satisfies both chemical and
dynamical conditions for being a sibling of the Sun. Measurements of
rare-element abundances for this star further confirm its solar composition,
with the only possible exception of Sm. Analysis of long-term high-precision
radial velocity data rules out the presence of hot Jupiters and confirms that
this star is not in a binary system. We find that chemical tagging does not
necessarily benefit from studying as many elements as possible, but instead
from identifying and carefully measuring the abundances of those elements which
show large star-to-star scatter at a given metallicity. Future searches
employing data products from ongoing massive astrometric and spectroscopic
surveys can be optimized by acknowledging this fact.Comment: ApJ, in press. Tables 2 and 4 are available in full in the "Other
formats: source" downloa
On the tidal evolution of the orbits of low-mass satellites around black holes
Low-mass satellites, like asteroids and comets, are expected to be present
around the black hole at the Galactic center. We consider small bodies orbiting
a black hole, and we study the evolution of their orbits due to tidal
interaction with the black hole. In this paper we investigate the consequences
of the existence of plunging orbits when a black hole is present. We are
interested in finding the conditions that exist when capture occurs. The main
difference between the Keplerian and black hole cases is in the existence of
plunging orbits. Orbital evolution, leading from bound to plunging orbits, goes
through a final unstable circular orbit. On this orbit, tidal energy is
released on a characteristic black hole timescale. This process may be relevant
for explaining how small, compact clumps of material can be brought onto
plunging orbits, where they may produce individual short duration accretion
events. The available energy and the characteristic timescale are consistent
with energy released and the timescale typical of Galactic flares.Comment: 7 pages, 6 figure
Rigorous approach to the comparison between experiment and theory in Casimir force measurements
In most experiments on the Casimir force the comparison between measurement
data and theory was done using the concept of the root-mean-square deviation, a
procedure that has been criticized in literature. Here we propose a special
statistical analysis which should be performed separately for the experimental
data and for the results of the theoretical computations. In so doing, the
random, systematic, and total experimental errors are found as functions of
separation, taking into account the distribution laws for each error at 95%
confidence. Independently, all theoretical errors are combined to obtain the
total theoretical error at the same confidence. Finally, the confidence
interval for the differences between theoretical and experimental values is
obtained as a function of separation. This rigorous approach is applied to two
recent experiments on the Casimir effect.Comment: 10 pages, iopart.cls is used, to appear in J. Phys. A (special issue:
Proceedings of QFEXT05, Barcelona, Sept. 5-9, 2005
Breast cancer in lesbians and bisexual women: Systematic review of incidence, prevalence and risk studies
This article is made available through the Brunel Open Access Publishing Fund. © 2013 Meads and Moore; licensee BioMed Central Ltd. This is an open access article distributed under the terms of the
Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use,
distribution, and reproduction in any medium, provided the original work is properly cited.Background: The UK Parliamentary Enquiry and USA Institute of Medicine state that lesbians may be at a higher risk of breast cancer but there is insufficient information. Lesbians and bisexual (LB) women have behavioural risk-factors at higher rates compared to heterosexuals such as increased alcohol intake and higher stress levels. Conversely, breast cancer rates are higher in more affluent women yet income levels in LB women are relatively low. This systematic review investigated all evidence on whether there is, or likely to be, higher rates of breast cancer in LB women. Methods: Cochrane library (CDSR, CENTRAL, HTA, DARE, NHSEED), MEDLINE, EMBASE, PsychINFO, CAB abstracts, Web of Science (SCI, SSCI), SIGLE and Social Care Online databases were searched to October 2013. Unpublished research and specific lesbian, gay and bisexual websites were checked, as were citation lists of relevant papers. Included were studies in LB populations reporting breast cancer incidence or prevalence rates, risk model results or risk-factor estimates. Inclusions, data-extraction and quality assessment were by two reviewers with disagreements resolved by discussion. Results: Searches found 198 references. No incidence rates were found. Nine studies gave prevalence estimates - two showed higher, four showed no differences, one showed mixed results depending on definitions, one had no comparison group and one gave no sample size. All studies were small with poor methodological and/or reporting quality. One incidence modelling study suggested a higher rate. Four risk modelling studies were found, one Rosner-Colditz and three Gail models. Three suggested higher and one lower rate in LB compared to heterosexual women. Six risk-factor estimates suggested higher risk and one no difference between LB and heterosexual women. Conclusions: The only realistic way to establish rates in LB women would be to collect sexual orientation within routine statistics, including cancer registry data, or from large cohort studies
First-principles study of the structural energetics of PdTi and PtTi
The structural energetics of PdTi and PtTi have been studied using
first-principles density-functional theory with pseudopotentials and a
plane-wave basis. We predict that in both materials, the experimentally
reported orthorhombic phase will undergo a low-temperature phase
transition to a monoclinic ground state. Within a soft-mode framework,
we relate the structure to the cubic structure, observed at high
temperature, and the structure to via phonon modes strongly
coupled to strain. In contrast to NiTi, the structure is extremely close
to hcp. We draw on the analogy to the bcc-hcp transition to suggest likely
transition mechanisms in the present case.Comment: 8 pages 5 figure
Yukawa potentials in systems with partial periodic boundary conditions I : Ewald sums for quasi-two dimensional systems
Yukawa potentials are often used as effective potentials for systems as
colloids, plasmas, etc. When the Debye screening length is large, the Yukawa
potential tends to the non-screened Coulomb potential ; in this small screening
limit, or Coulomb limit, the potential is long ranged. As it is well known in
computer simulation, a simple truncation of the long ranged potential and the
minimum image convention are insufficient to obtain accurate numerical data on
systems. The Ewald method for bulk systems, i.e. with periodic boundary
conditions in all three directions of the space, has already been derived for
Yukawa potential [cf. Y., Rosenfeld, {\it Mol. Phys.}, \bm{88}, 1357, (1996)
and G., Salin and J.-M., Caillol, {\it J. Chem. Phys.}, \bm{113}, 10459,
(2000)], but for systems with partial periodic boundary conditions, the Ewald
sums have only recently been obtained [M., Mazars, {\it J. Chem. Phys.}, {\bf
126}, 056101 (2007)]. In this paper, we provide a closed derivation of the
Ewald sums for Yukawa potentials in systems with periodic boundary conditions
in only two directions and for any value of the Debye length. A special
attention is paid to the Coulomb limit and its relation with the
electroneutrality of systems.Comment: 40 pages, 5 figures and 4 table
Tidal effects on small bodies by massive black holes
The compact radio source Sagittarius A (Sgr A) at the centre of our Galaxy
harbours a supermassive black hole, whose mass has been measured from stellar
orbital motions. Sgr A is therefore the nearest laboratory where super-massive
black hole astrophysics can be tested, and the environment of black holes can
be investigated. Since it is not an active galactic nucleus, it also offers the
possibility of observing the capture of small objects that may orbit the
central black hole. We study the effects of the strong gravitational field of
the black hole on small objects, such as a comet or an asteroid. We also
explore the idea that the flares detected in Sgr A might be produced by the
final accretion of single, dense objects with mass of the order of 10^20 g, and
that their timing is not a characteristic of the sources, but rather of the
space-time of the central galactic black hole in which they are moving. We find
that tidal effects are strong enough to melt the solid object, and present
calculations of the temporal evolution of the light curve of infalling objects
as a function of various parameters. Our modelling of tidal disruption suggests
that during tidal squeezing, the conditions for synchrotron radiation can be
met. We show that the light curve of a flare can be deduced from dynamical
properties of geodesic orbits around black holes and that it depends only
weakly on the physical properties of the source.Comment: 10 pages, 14 figures, A&A accepte
Elemental abundance differences in the 16 Cygni binary system: a signature of gas giant planet formation?
The atmospheric parameters of the components of the 16Cygni binary system, in
which the secondary has a gas giant planet detected, are measured accurately
using high quality observational data. Abundances relative to solar are
obtained for 25 elements with a mean error of 0.023 dex. The fact that 16CygA
has about four times more lithium than 16CygB is normal considering the
slightly different masses of the stars. The abundance patterns of 16CygA and B,
relative to iron, are typical of that observed in most of the so-called solar
twin stars, with the exception of the heavy elements (Z>30), which can,
however, be explained by Galactic chemical evolution. Differential (A-B)
abundances are measured with even higher precision (0.018 dex, on average). We
find that 16CygA is more metal-rich than 16CygB by 0.041+/-0.007 dex. On an
element-to-element basis, no correlation between the A-B abundance differences
and dust condensation temperature (Tc) is detected. Based on these results, we
conclude that if the process of planet formation around 16CygB is responsible
for the observed abundance pattern, the formation of gas giants produces a
constant downwards shift in the photospheric abundance of metals, without a Tc
correlation. The latter would be produced by the formation of terrestrial
planets instead, as suggested by other recent works on precise elemental
abundances. Nevertheless, a scenario consistent with these observations
requires the convective envelopes of 1 Msun stars to reach their present-day
sizes about three times quicker than predicted by standard stellar evolution
models.Comment: ApJ, in pres
Advances in the Use of LTS and HTS SQUIDS in Electromagnetic NDE
Of the electromagnetic sensors currently under investigation for nondestructive evaluation (NDE), the superconducting quantum interference device (SQUID) arguably has the greatest potential. The characteristics [1] which make it suitable for eddy current NDE are: high sensitivity even in large ambient fields (detection of sub-nT signals); operation from very low frequencies (a few Hz or less) to very high frequencies (potentially MHz) permitting detection of surface and subsurface flaws; and high spatial resolution. Spatial resolution is related to the physical size of the device, which is often less than 1 mm square, even when the need to maintain its other properties is taken into account. This often allows the SQUID to be treated theoretically and practically as an ideal point sensor. However, it must be operated in a cryogenic environment: low temperature superconductor (LTS) SQUIDs need liquid helium and liquid nitrogen (LN2) is needed even for high temperature superconductor (HTS) SQUIDs. This makes it difficult to reduce the specimen-to-sensor stand-off below approximately 1 mm.</p
Microultrasound characterisation of <i>ex vivo</i> porcine tissue for ultrasound capsule endoscopy
Gastrointestinal (GI) disease development and progression is often characterised by cellular and tissue architectural changes within the mucosa and sub-mucosa layers. Current clinical capsule endoscopy and other approaches are heavily reliant on optical techniques which cannot detect disease progression below the surface layer of the tissue. To enhance the ability of clinicians to detect cellular changes earlier and more confidently, both quantitative and qualitative microultrasound (μUS) techniques are investigated in healthy ex vivo porcine GI tissue. This work is based on the use of single-element, focussed μUS transducers made with micromoulded piezocomposite operating at around 48 MHz.
To explore the possibility that μUS can detect Crohn's disease and other inflammatory bowel diseases, ex vivo porcine small bowel tissue samples were cannulised and perfused with phosphate-buffered saline followed by various dilutions of polystyrene microspheres. Comparison with fluorescent imaging showed that the microspheres had infiltrated the microvasculature of the samples and that μUS was able to successfully detect this as a mimic of inflammation. Samples without microspheres were analysed using quantitative ultrasound to assess mechanical properties. Attenuation coefficients of 1.78 ± 0.66 dB/mm and 1.92 ± 0.77 dB/mm were obtained from reference samples which were surgically separated from the muscle layer. Six intact samples were segmented using a software algorithm and the acoustic impedance, Z, for varying tissue thicknesses, and backscattering coefficient, BSC, were calculated using the reference attenuation values and tabulated
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