376 research outputs found
Gold catalyzed multicomponent reactions beyond A3 coupling
The preparation of complex architectures has inspired the search for new methods and new processes in organic synthesis. Multicomponent reactions have become an interesting approach to achieve such molecular diversity and complexity. This review intends to illustrate important gold-catalyzed examples for the past ten years leading to interesting skeletons involved in biologically active compounds
A Global Semi-Analytic Model of the First Stars and Galaxies Including Dark Matter Halo Merger Histories
We present a new self-consistent semi-analytic model of the first stars and
galaxies to explore the high-redshift () Population III (PopIII) and
metal-enriched star formation histories. Our model includes the detailed merger
history of dark matter halos generated with Monte Carlo merger trees. We
calibrate the minimum halo mass for PopIII star formation from recent
hydrodynamical cosmological simulations that simultaneously include the
baryon-dark matter streaming velocity, Lyman-Werner (LW) feedback, and
molecular hydrogen self-shielding. We find that the resulting star formation
rate density (SFRD) is dramatically increased compared to calibrations based on
previous simulations (e.g., the PopIII SFRD is over two orders of magnitude
higher at ). We evaluate the effect of the halo-to-halo scatter
in this critical mass and find that it increases the PopIII stellar mass
density by a factor of at . Additionally, we assess the
impact of various semi-analytic/analytic prescriptions for halo assembly and
star formation previously adopted in the literature. For example, we find that
models assuming smooth halo growth computed via abundance matching predict
SFRDs similar to the merger tree model for our fiducial model parameters, but
that they may underestimate the PopIII SFRD in cases of strong LW feedback.
Finally, we simulate sub-volumes of the Universe with our model both to
quantify the reduction in total star formation in numerical simulations due to
a lack of density fluctuations on spatial scales larger than the simulation
box, and to determine spatial fluctuations in SFRD due to the diversity in halo
abundances and merger histories.Comment: Submitted to ApJ -- 21 Pages, 9 Figure
ON SPURIOUS BEHAVIOR OF CFD SIMULATIONS
Spurious behavior in underresolved grids and:or semi-implicit temporal discretizations for four computational fluid dynamics (CFD) simulations are studied. The numerical simulations consist of (a) a 1-D chemically relaxed non-equilibrium flow model, (b) the direct numerical simulation (DNS) of 2D incompressible flow over a backward facing step, (c) a loosely coupled approach for a 2D fluid–structure interaction, and (d) a 3D unsteady compressible flow simulation of vortex breakdown on delta wings. These examples were chosen based on their non-apparent spurious behaviors that were difficult to detect without extensive grid and:or temporal refinement studies and without some knowledge from dynamical systems theory. Studies revealed the various possible dangers of misinterpreting numerical simulation of realistic complex flows that are constrained by available computing power. In large scale computations, underresolved grids, semi-implicit procedures, loosely coupled implicit procedures, and insufficiently long-time integration in DNS are most often unavoidable. Consequently, care must be taken in both computation and in interpretation of the numerical data. The results presented confirm the important role that dynamical systems theory can play in the understanding of the non-linear behavior of numerical algorithms and in aiding the identification of the sources of numerical uncertainties in CFD
Volumetric measurements and simulations of the vortex structures generated by low aspect ratio plunging wings
Volumetric three-component velocimetry measurements have been performed on low aspect ratio wings undergoing a small amplitude pure plunging motion. This study focuses on the vortex flows generated by rectangular and elliptical wings set to a fixed geometric angle of attack of α = 20°. An investigation into the effect of Strouhal number illustrates the highly three-dimensional nature of the leading edge vortex as well as its inherent ability to improve lift performance. Computational simulations show good agreement with experimental results, both demonstrating the complex interaction between leading, trailing, and tip vortices generated in each cycle. The leading edge vortex, in particular, may deform significantly throughout the cycle, in some cases developing strong spanwise undulations. These are at least both Strouhal number and planform dependent. One or two arch-type vortical structures may develop, depending on the aspect ratio and Strouhal number. At sufficiently high Strouhal numbers, a tip vortex ring may also develop, propelling itself away from the wing in the spanwise direction due to self-induced velocity
Phase diagrams of period-4 spin chains consisting of three kinds of spins
We study a period-4 antiferromagnetic mixed quantum spin chain consisting of
three kinds of spins. When the ground state is singlet, the spin magnitudes in
a unit cell are arrayed as (s-t, s, s+t, s) with integer or half-odd integer s
and t (0 <= t < s). The spin Hamiltonian is mapped onto a nonlinear sigma model
(NLSM) in a previously developed method. The resultant NLSM includes only two
independent parameters originating from four exchange constants for fixed s and
t. The topological angle in the NLSM determines the gapless phase boundaries
between disordered phases in the parameter space. The phase diagrams for
various s and t shows rich structures. We systematically explain the phases in
the singlet-cluster-solid picture.Comment: 8 pages (16 figures included
Highly photoluminescent copper carbene complexes based on prompt rather than delayed fluorescence
Linear two-coordinate copper complexes of cyclic (alkyl)(amino)-carbenes (CAAC)CuX (X = halide) show photoluminescence with solid-state quantum yields of up to 96%; in contrast to previously reported Cu photoemitters the emission is independent of temperature over the range T = 4 – 300 K and occurs very efficiently by prompt rather than delayed fluorescence, with lifetimes in the sub-nanosecond range
21-cm cosmology
Imaging the Universe during the first hundreds of millions of years remains
one of the exciting challenges facing modern cosmology. Observations of the
redshifted 21 cm line of atomic hydrogen offer the potential of opening a new
window into this epoch. This would transform our understanding of the formation
of the first stars and galaxies and of the thermal history of the Universe. A
new generation of radio telescopes is being constructed for this purpose with
the first results starting to trickle in. In this review, we detail the physics
that governs the 21 cm signal and describe what might be learnt from upcoming
observations. We also generalize our discussion to intensity mapping of other
atomic and molecular lines.Comment: 64 pages, 20 figures, submitted to Reports on Progress in Physics,
comments welcom
The signature of the first stars in atomic hydrogen at redshift 20
Dark and baryonic matter moved at different velocities in the early Universe,
which strongly suppressed star formation in some regions. This was estimated to
imprint a large-scale fluctuation signal of about 2 mK in the 21-cm spectral
line of atomic hydrogen associated with stars at a redshift of 20, although
this estimate ignored the critical contribution of gas heating due to X-rays
and major enhancements of the suppression. A large velocity difference reduces
the abundance of halos and requires the first stars to form in halos of about a
million solar masses, substantially greater than previously expected. Here we
report a simulation of the distribution of the first stars at z=20 (cosmic age
of ~180 Myr), incorporating all these ingredients within a 400 Mpc box. We find
that the 21-cm signature of these stars is an enhanced (10 mK) fluctuation
signal on the 100-Mpc scale, characterized by a flat power spectrum with
prominent baryon acoustic oscillations. The required sensitivity to see this
signal is achievable with an integration time of a thousand hours with an
instrument like the Murchison Wide-field Array or the Low Frequency Array but
designed to operate in the range of 50-100 MHz.Comment: 27 pages, 5 figures, close (but not exact) match to accepted version.
Basic results unchanged from first submitted version, but justification
strengthened, title and abstract modified, and substantial Supplementary
Material added. Originally first submitted for publication on Oct. 12, 201
- …