2,838 research outputs found
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
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
Extensible Component Based Architecture for FLASH, A Massively Parallel, Multiphysics Simulation Code
FLASH is a publicly available high performance application code which has
evolved into a modular, extensible software system from a collection of
unconnected legacy codes. FLASH has been successful because its capabilities
have been driven by the needs of scientific applications, without compromising
maintainability, performance, and usability. In its newest incarnation, FLASH3
consists of inter-operable modules that can be combined to generate different
applications. The FLASH architecture allows arbitrarily many alternative
implementations of its components to co-exist and interchange with each other,
resulting in greater flexibility. Further, a simple and elegant mechanism
exists for customization of code functionality without the need to modify the
core implementation of the source. A built-in unit test framework providing
verifiability, combined with a rigorous software maintenance process, allow the
code to operate simultaneously in the dual mode of production and development.
In this paper we describe the FLASH3 architecture, with emphasis on solutions
to the more challenging conflicts arising from solver complexity, portable
performance requirements, and legacy codes. We also include results from user
surveys conducted in 2005 and 2007, which highlight the success of the code.Comment: 33 pages, 7 figures; revised paper submitted to Parallel Computin
Observation and Modeling of the Solar Transition Region: II. Solutions of the Quasi-Static Loop Model
In the present work we undertake a study of the quasi-static loop model and
the observational consequences of the various solutions found. We obtain the
most general solutions consistent with certain initial conditions. Great care
is exercised in choosing these conditions to be physically plausible (motivated
by observations). We show that the assumptions of previous quasi-static loop
models, such as the models of Rosner, Tucker and Vaiana (1978) and Veseckey,
Antiochos and Underwood (1979), are not necessarily valid for small loops at
transition region temperatures. We find three general classes of solutions for
the quasi-static loop model, which we denote, radiation dominated loops,
conduction dominated loops and classical loops. These solutions are then
compared with observations. Departures from the classical scaling law of RTV
are found for the solutions obtained. It is shown that loops of the type that
we model here can make a significant contribution to lower transition region
emission via thermal conduction from the upper transition region.Comment: 30 pages, 3 figures, Submitted to ApJ, Microsoft Word File 6.0/9
Educational Attainment Moderates the Association Between Hippocampal Volumes and Memory Performances in Healthy Older Adults
Objective: To examine whether educational attainment, as a proxy of cognitive reserve, moderated the association between hippocampal volumes and episodic verbal memory performances in healthy older adults.Methods: Data from 76 community dwelling older adults were included in the present study. Measures of hippocampal volumes (total, left, and right) were obtained using FreeSurfer software. Immediate and delayed verbal recall scores were derived from performances on the California Verbal Learning Test-Second Edition and the Wechsler Memory Scale- Third Edition. Educational attainment was defined by years of education. Linear regression analyses were performed using immediate and delayed recall as dependent variables and hippocampal volumes, years of education, and their interaction terms as independent variables. All analyses were controlled for age, sex, depression, and health status.Results: Total and left Hippocampal volumes had a positive main effect on delayed recall only. Additionally, the interaction between total, left, and right hippocampal volumes and education was a significant predictor for delayed recall performance but not for immediate recall performance. The positive association between hippocampal volumes and delayed recall was greatest in those with more years of education.Conclusion: Larger hippocampal volumes were associated with better delayed verbal recall and the effect on delayed recall was greatest in those with more years of education. Having higher levels of education, or cognitive reserve, may enable individuals to capitalize on greater structural integrity in the hippocampus to support delayed recall in old age. However, longitudinal research is needed to investigate the directionality of these associations
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Application of a Catalytic Asymmetric Povarov Reaction using Chiral Ureas to the Synthesis of a Tetrahydroquinoline Library
A 2328-membered library of 2,3,4-trisubstituted tetrahydroquinolines was produced using a combination of solution- and solid-phase synthesis techniques. A tetrahydroquinoline (THQ) scaffold was prepared via an asymmetric Povarov reaction using cooperative catalysis to generate three contiguous stereogenic centers. A matrix of 4 stereoisomers of the THQ scaffold was prepared to enable the development of stereo/structure-activity relationships (SSAR) upon biological testing. A sparse matrix design strategy was employed to select library members to be synthesized with the goal of generating a diverse collection of tetrahydroquinolines with physicochemical properties suitable for downstream discovery.Chemistry and Chemical Biolog
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