2,492 research outputs found
On the pollution of white dwarfs by exo-Oort cloud comets
A large fraction of white dwarfs (WDs) have metal-polluted atmospheres, which
are produced by accreting material from remnant planetary systems. The
composition of the accreted debris broadly resembles that of rocky Solar System
objects. Volatile-enriched debris with compositions similar to long-period
comets (LPCs) is rarely observed. We attempt to reconcile this dearth of
volatiles with the premise that exo-Oort clouds (XOCs) occur around a large
fraction of planet-hosting stars. We estimate the comet accretion rate from an
XOC analytically, adapting the 'loss cone' theory of LPC delivery in the Solar
System. We investigate the dynamical evolution of an XOC during late stellar
evolution. Using numerical simulations, we show that 1 to 30 per cent of XOC
objects remain bound after anisotropic stellar mass loss imparting a WD natal
kick of 1 km/s. We also characterize the surviving comets' distribution
function. Surviving planets orbiting a WD can prevent the accretion of XOC
comets by the star. A planet's 'dynamical barrier' is effective at preventing
comet accretion if the energy kick imparted by the planet exceeds the comet's
orbital binding energy. By modifying the loss cone theory, we calculate the
amount by which a planet reduces the WD's accretion rate. We suggest that the
scarcity of volatile-enriched debris in polluted WDs is caused by an unseen
population of 10-100 AU scale giant planets acting as barriers to incoming
LPCs. Finally, we constrain the amount of volatiles delivered to a planet in
the habitable zone of an old, cool WD.Comment: 18 pages, 12 figures; submitted to MNRAS. Comments welcome
A New Method of Measuring 81Kr and 85Kr Abundances in Environmental Samples
We demonstrate a new method for determining the 81Kr/Kr ratio in
environmental samples based upon two measurements: the 85Kr/81Kr ratio measured
by Atom Trap Trace Analysis (ATTA) and the 85Kr/Kr ratio measured by Low-Level
Counting (LLC). This method can be used to determine the mean residence time of
groundwater in the range of 10^5 - 10^6 a. It requires a sample of 100 micro-l
STP of Kr extracted from approximately two tons of water. With modern
atmospheric Kr samples, we demonstrate that the ratios measured by ATTA and LLC
are directly proportional to each other within the measurement error of +/-
10%; we calibrate the 81Kr/Kr ratio of modern air measured using this method;
and we show that the 81Kr/Kr ratios of samples extracted from air before and
after the development of the nuclear industry are identical within the
measurement error
Developing social capital in implementing a complex intervention: a process evaluation of the early implementation of a suicide prevention intervention in four European countries
<p>Background: Variation in the implementation of complex multilevel interventions can impact on their delivery and outcomes. Few suicide prevention interventions, especially multilevel interventions, have included evaluation of both the process of implementation as well as outcomes. Such evaluation is essential for the replication of interventions, for interpreting and understanding outcomes, and for improving implementation science. This paper reports on a process evaluation of the early implementation stage of an optimised suicide prevention programme (OSPI-Europe) implemented in four European countries.</p>
<p>Methods: The process analysis was conducted within the framework of a realist evaluation methodology, and involved case studies of the process of implementation in four European countries. Datasets include: repeated questionnaires to track progress of implementation including delivery of individual activities and their intensity; serial interviews and focus groups with stakeholder groups; and detailed observations at OSPI implementation team meetings.</p>
<p>Results: Analysis of local contexts in each of the four countries revealed that the advisory group was a key mechanism that had a substantial impact on the ease of implementation of OSPI interventions, particularly on their ability to recruit to training interventions. However, simply recruiting representatives of key organisations into an advisory group is not sufficient to achieve impact on the delivery of interventions. In order to maximise the potential of high level âgatekeepersâ, it is necessary to first transform them into OSPI stakeholders. Motivations for OSPI participation as a stakeholder included: personal affinity with the shared goals and target groups within OSPI; the complementary and participatory nature of OSPI that adds value to pre-existing suicide prevention initiatives; and reciprocal reward for participants through access to the extended network capacity that organisations could accrue for themselves and their organisations from participation in OSPI.</p>
<p>Conclusions: Exploring the role of advisory groups and the meaning of participation for these participants revealed some key areas for best practice in implementation: careful planning of the composition of the advisory group to access target groups; the importance of establishing common goals; the importance of acknowledging and complementing existing experience and activity; and facilitating an equivalence of benefit from network participation.</p>
Searches for Stable Strangelets in Ordinary Matter: Overview and a Recent Example
Our knowledge on the possible existence in nature of stable exotic particles
depends solely upon experimental observation. Guided by this general principle
and motivated by theoretical hypotheses on the existence of stable particles of
strange quark matter, a variety of experimental searches have been performed.
We provide an introduction to the theoretical hypotheses, an overview of the
past searches, and a more detailed description of a recent search for
helium-like strangelets in the Earth's atmosphere using a sensitive laser
spectroscopy method
The Information Geometry of the One-Dimensional Potts Model
In various statistical-mechanical models the introduction of a metric onto
the space of parameters (e.g. the temperature variable, , and the
external field variable, , in the case of spin models) gives an alternative
perspective on the phase structure. For the one-dimensional Ising model the
scalar curvature, , of this metric can be calculated explicitly in
the thermodynamic limit and is found to be . This is positive definite and, for
physical fields and temperatures, diverges only at the zero-temperature,
zero-field ``critical point'' of the model.
In this note we calculate for the one-dimensional -state Potts
model, finding an expression of the form , where is the Potts
analogue of . This is no longer positive
definite, but once again it diverges only at the critical point in the space of
real parameters. We remark, however, that a naive analytic continuation to
complex field reveals a further divergence in the Ising and Potts curvatures at
the Lee-Yang edge.Comment: 9 pages + 4 eps figure
Construction and Expected Performance of the Hadron Blind Detector for the PHENIX Experiment at RHIC
A new Hadron Blind Detector (HBD) for electron identification in high density
hadron environment has been installed in the PHENIX detector at RHIC in the
fall of 2006. The HBD will identify low momentum electron-positron pairs to
reduce the combinatorial background in the mass spectrum, mainly
in the low-mass region below 1 GeV/c. The HBD is a windowless
proximity-focusing Cherenkov detector with a radiator length of 50 cm, a CsI
photocathode and three layers of Gas Electron Multipliers (GEM). The HBD uses
pure CF as a radiator and a detector gas. Construction details and the
expected performance of the detector are described.Comment: QM2006 proceedings, 4 pages 3 figure
Coulomb energy contribution to the excitation energy in Th and enhanced effect of variation
We calculated the contribution of Coulomb energy to the spacing between the
ground and first excited state of Th nucleus as a function of the
deformation parameter . We show that despite the fact that the odd
particle is a neutron, the change in Coulomb energy between these two states
can reach several hundreds KeV.This means that the effect of the variation of
the fine structure constant may be enhanced
times in the 7.6 eV "nuclear clock" transition
between the ground and first excited states in the Th nucleus.Comment: 6 pages,2 figure
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