3,230 research outputs found
Population III Star Formation in a Lambda WDM Universe
In this paper we examine aspects of primordial star formation in a gravitino
warm dark matter universe with a cosmological constant. We compare a set of
simulations using a single cosmological realization but with a wide range of
warm dark matter particle masses which have not yet been conclusively ruled out
by observations. The addition of a warm dark matter component to the initial
power spectrum results in a delay in the collapse of high density gas at the
center of the most massive halo in the simulation and, as a result, an increase
in the virial mass of this halo at the onset of baryon collapse. Both of these
effects become more pronounced as the warm dark matter particle mass becomes
smaller. A cosmology using a gravitino warm dark matter power spectrum assuming
a particle mass of m_{WDM} ~ 40keV is effectively indistinguishable from the
cold dark matter case, whereas the m_{WDM} ~ 15 keV case delays star formation
by approx. 10^8 years. There is remarkably little scatter between simulations
in the final properties of the primordial protostar which forms at the center
of the halo, possibly due to the overall low rate of halo mergers which is a
result of the WDM power spectrum. The detailed evolution of the collapsing halo
core in two representative WDM cosmologies is described. At low densities
(n_{b} <= 10^5 cm^{-3}), the evolution of the two calculations is qualitatively
similar, but occurs on significantly different timescales, with the halo in the
lower particle mass calculation taking much longer to evolve over the same
density range and reach runaway collapse. Once the gas in the center of the
halo reaches relatively high densities (n_{b} >= 10^5 cm^{-3}) the overall
evolution is essentially identical in the two calculations.Comment: 36 pages, 12 figures (3 color). Astrophysical Journal, accepte
The prediction and management of aquatic nitrogen pollution across Europe: an introduction to the Integrated Nitrogen in European Catchments project (INCA)
Excess nitrogen in soils, fresh water, estuarine and marine systems contributes to nutrient enrichment in key ecosystems throughout Europe, often leading to detrimental environmental impacts, such as soil acidification or the eutrophication of water bodies. The Integrated Nitrogenmodel for European Catchments (INCA) project aims to develop a generic version of the Integrated Nitrogen in Catchments (INCA) model to simulate the retention and transport of nitrogen within river systems, thereby providing a tool to aid the understanding of nitrogen dynamics and for river-basin management/policy-making. To facilitate the development of the model, 10 partners have tested the INCA model with data collected in study sites located in eight European countries as part of the INCA project. This paper summarises the key nitrogen issues within Europe, describes the main aims and methodology of the INCA project, and sets the project in the context of the current major research initiatives at a European level.</p> <p style='line-height: 20px;'><b>Keywords: </b>Europe, European Union, nitrogen, nitrate, ammonium, river basin management, modelling, water chemistry, acidification, eutrophication, Water Framework Directive, INCA
Gravitational Wave Propagation in Isotropic Cosmologies
We study the propagation of gravitational waves carrying arbitrary
information through isotropic cosmologies. The waves are modelled as small
perturbations of the background Robertson-Walker geometry. The perfect fluid
matter distribution of the isotropic background is, in general, modified by
small anisotropic stresses. For pure gravity waves, in which the perturbed Weyl
tensor is radiative (i.e. type N in the Petrov classification), we construct
explicit examples for which the presence of the anisotropic stress is shown to
be essential and the histories of the wave-fronts in the background
Robertson-Walker geometry are shear-free null hypersurfaces. The examples
derived in this case are analogous to the Bateman waves of electromagnetic
theory.Comment: 27 pages, accepted for publication in Phys.Rev.
The Santa Fe Light Cone Simulation Project: I. Confusion and the WHIM in Upcoming Sunyaev-Zel'dovich Effect Surveys
We present the first results from a new generation of simulated large sky
coverage (~100 square degrees) Sunyaev-Zeldovich effect (SZE) cluster surveys
using the cosmological adaptive mesh refinement N-body/hydro code Enzo. We have
simulated a very large (512^3h^{-3}Mpc^3) volume with unprecedented dynamic
range. We have generated simulated light cones to match the resolution and
sensitivity of current and future SZE instruments. Unlike many previous studies
of this type, our simulation includes unbound gas, where an appreciable
fraction of the baryons in the universe reside.
We have found that cluster line-of-sight overlap may be a significant issue
in upcoming single-dish SZE surveys. Smaller beam surveys (~1 arcmin) have more
than one massive cluster within a beam diameter 5-10% of the time, and a larger
beam experiment like Planck has multiple clusters per beam 60% of the time. We
explore the contribution of unresolved halos and unbound gas to the SZE
signature at the maximum decrement. We find that there is a contribution from
gas outside clusters of ~16% per object on average for upcoming surveys. This
adds both bias and scatter to the deduced value of the integrated SZE, adding
difficulty in accurately calibrating a cluster Y-M relationship.
Finally, we find that in images where objects with M > 5x10^{13} M_{\odot}
have had their SZE signatures removed, roughly a third of the total SZE flux
still remains. This gas exists at least partially in the Warm Hot Intergalactic
Medium (WHIM), and will possibly be detectable with the upcoming generation of
SZE surveys.Comment: 14 pages, 13 figures, version accepted to ApJ. Major revisions mad
Popular critiques of consultancy and a politics of management learning?
In this short article, I argue that popular business discourse on the role of management consultancy in the promotion and translation of management ideas is often critical, informed by more or less implicit ethical and political concerns with employee security, equity, openness and the transparency and legitimacy of responsibility. These concerns are, in part, ‘sayable’ because their object is seen as a scapegoat for management. Nevertheless, combined with the popular form of their expression, they can support and legitimize critical studies of management learning, a discipline which otherwise has become overly concerned with processual and situational phenomena at the expense of broader political dynamics and of the content and consequences of management and management knowledg
Case report of a phantom pheochromocytoma
Plasma free metanephrines or urinary fractionated metanephrines are the biochemical tests of choice for the diagnosis of pheochromocytoma as
they have greater sensitivity and specificity than catecholamines for pheochromocytoma detection. This case highlights the preanalytical factors
which can influence metanephrine measurement and cause a false positive result. It describes a patient with a high pre-test probability of pheochromocytoma
due to hypertension and a past medical history of adrenalectomy for a purported pheochromocytoma in her home country. When
biochemical screening revealed grossly elevated urine normetanephrine in the presence of a previously identified right adrenal lesion, there was
high clinical suspicion of a pheochromocytoma. However, functional imaging did not support this view which prompted additional testing with
plasma metanephrines. Results for plasma and urine metanephrines were discordant and preanalytical drug interference was suspected. Patient
medications were reviewed and sulfasalazine, an anti-inflammatory drug was identified as the most likely analytical interferent. Urinary fractionated
metanephrines were re-analysed using liquid chromatography tandem mass spectrometry (LC-MS/MS) and all metanephrines were within their
reference intervals. This case illustrates how method-specific analytical drug interference prompted unnecessary expensive imaging, heightened
patient anxiety and resulted in lengthy investigations for what turned out to be a phantom pheochromocytoma
Direct measurement of molecular stiffness and damping in confined water layers
We present {\em direct} and {\em linear} measurements of the normal stiffness
and damping of a confined, few molecule thick water layer. The measurements
were obtained by use of a small amplitude (0.36 ), off-resonance
Atomic Force Microscopy (AFM) technique. We measured stiffness and damping
oscillations revealing up to 7 layers separated by 2.56 0.20
. Relaxation times could also be calculated and were found to
indicate a significant slow-down of the dynamics of the system as the confining
separation was reduced. We found that the dynamics of the system is determined
not only by the interfacial pressure, but more significantly by solvation
effects which depend on the exact separation of tip and surface. Thus `
solidification\rq seems to not be merely a result of pressure and confinement,
but depends strongly on how commensurate the confining cavity is with the
molecule size. We were able to model the results by starting from the simple
assumption that the relaxation time depends linearly on the film stiffness.Comment: 7 pages, 6 figures, will be submitted to PR
Population III star formation in a Lambda CDM universe, I: The effect of formation redshift and environment on protostellar accretion rate
(abridged) We perform 12 extremely high resolution adaptive mesh refinement
cosmological hydrodynamic simulations of Population III star formation in a
Lambda CDM universe, varying the box size and large-scale structure, to
understand systematic effects in the formation of primordial protostellar
cores. We find results that are qualitatively similar to those observed
previously. We observe that the threshold halo mass for formation of a
Population III protostar does not evolve significantly with time in the
redshift range studied (33 > z > 19) but exhibits substantial scatter due to
different halo assembly histories: Halos which assembled more slowly develop
cooling cores at lower mass than those that assemble more rapidly, in agreement
with Yoshida et al. (2003). We do, however, observe significant evolution in
the accretion rates of Population III protostars with redshift, with objects
that form later having higher maximum accretion rates, with a variation of two
orders of magnitude (10^-4 - 10^-2 Msolar/year). This can be explained by
considering the evolving virial properties of the halos with redshift and the
physics of molecular hydrogen formation at low densities. Our result implies
that the mass distribution of Population III stars inferred from their
accretion rates may be broader than previously thought, and may evolve with
redshift. Finally, we observe that our collapsing protostellar cloud cores do
not fragment, consistent with previous results, which suggests that Population
III stars which form in halos of mass 10^5 - 10^6 Msun always form in
isolation.Comment: Accepted by The Astrophysical Journal. Some minor changes. 65 pages,
3 tables, 21 figures (3 color). To appear in January 1, 2007 issu
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