3,146 research outputs found
Population III star formation in a Lambda CDM universe, II: Effects of a photodissociating background
We examine aspects of primordial star formation in the presence of a
molecular hydrogen-dissociating ultraviolet background. We compare a set of AMR
hydrodynamic cosmological simulations using a single cosmological realization
but with a range of ultraviolet background strengths in the Lyman-Werner band.
This allows us to study the effects of Lyman-Werner radiation on suppressing H2
cooling at low densities as well as the high-density evolution of the
collapsing core in a self-consistent cosmological framework. We find that the
addition of a photodissociating background results in a delay of 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. We find that, contrary to previous results, Population III
star formation is not suppressed for J, but occurs even with
backgrounds as high as J. We find that H2 cooling leads to collapse
despite the depressed core molecular hydrogen fractions due to the elevated H2
cooling rates at K. We observe a relationship between the
strength of the photodissociating background and the rate of accretion onto the
evolving protostellar cloud core, with higher LW background fluxes resulting in
higher accretion rates. Finally, we find that the collapsing halo cores in our
simulations do not fragment at densities below cm
regardless of the strength of the LW background, suggesting that Population III
stars forming in halos with T K may still form in isolation.Comment: 46 pages, 14 figures (9 color). Accepted by the Astrophysical
Journal, some minor revision
Bryophytes of Uganda : 4., new and additional records, 2
20 hepatics and 16 mosses are reported new to Uganda, 1 moss being also new to Africa. A further 6 taxa are recorded for the second time from Uganda
British Bryological Society expedition to Mulanje Mountain, Malawi : 13., new and other unpublished records
Further results are provided of the 1991 British Bryological Society Expedition to Mulanje Mt., Malawi including 168 taxa of bryophyte, comprising 72 taxa of liverwort (38 new to Malawi) and 96 taxa of mosses (45 new to Malawi)
Bryophytes of Uganda : 2., new and interesting records
51 hepatics and 46 mosses are reported new to Uganda, including one moss new to Africa, one hepatic and two mosses new to mainland Africa, and 2 hepatics that are otherwise known only from their type collection
Positron detection in silica monoliths for miniaturised quality control of PET radiotracers
We demonstrate the use of the miniaturised Medipix positron sensor for detection of the clinical PET radiotracer, [â¶âžGa]gallium-citrate, on a silica-based monolith, towards microfluidic quality control. The system achieved a far superior signal-to-noise ratio compared to conventional sodium iodide-based radio-HPLC detection and allowed real-time visualisation of positrons in the monolith
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
On the Discovery of Monocular Rivalry by Tscherning in 1898:Translation and Review
Monocular rivalry was named by Breese in 1899. He made prolonged observation of superimposed orthogonal gratings; they fluctuated in clarity with either one or the other grating occasionally being visible alone. A year earlier, Tscherning observed similar fluctuations with a grid of vertical and horizontal lines and with other stimuli; we draw attention to his prior account. Monocular rivalry has since been shown to occur with a wide variety of superimposed patterns with several independent rediscoveries of it. We also argue that Helmholtz described some phenomenon other than monocular rivalry in 1867
Improving the scalability of parallel N-body applications with an event driven constraint based execution model
The scalability and efficiency of graph applications are significantly
constrained by conventional systems and their supporting programming models.
Technology trends like multicore, manycore, and heterogeneous system
architectures are introducing further challenges and possibilities for emerging
application domains such as graph applications. This paper explores the space
of effective parallel execution of ephemeral graphs that are dynamically
generated using the Barnes-Hut algorithm to exemplify dynamic workloads. The
workloads are expressed using the semantics of an Exascale computing execution
model called ParalleX. For comparison, results using conventional execution
model semantics are also presented. We find improved load balancing during
runtime and automatic parallelism discovery improving efficiency using the
advanced semantics for Exascale computing.Comment: 11 figure
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, âsayableâ 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
Bryophytes of Uganda : 6., new and additional records, 3.
12 hepatics and 32 mosses are reported new to Uganda, 1 moss being also new to Africa, and 1 liverwort new to mainland Africa. Ectropothecium plumigerum (Broth.) HedenĂ€s is a new combination (basionym: Isopterygium plumigerum Broth.) with a new synonym Taxicaulis plumirameus MĂŒll.Hal. nom. nud., and Taxiphyllum maniae (Renauld & Paris) M. Fleisch. is a new synonym of Taxiphyllum taxirameum (Mitt.) M.Fleisch. Three mosses are removed from the Uganda list
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