1,597 research outputs found
On Simulating the Proton-Irradiation of O and HO Ices Using Astrochemical-type Models, with Implications for Bulk Reactivity
Many astrochemical models today explicitly consider the species that comprise
the bulk of interstellar dust grain ice-mantles separately from those in the
top few monolayers. Bombardment of these ices by ionizing radiation - whether
in the form of cosmic rays, stellar winds, or radionuclide emission -
represents an astrochemically viable means of driving a rich chemistry even in
the bulk of the ice-mantle, now supported by a large body of work in laboratory
astrophysics. In this study, using an existing rate equation-based
astrochemical code modified to include a method of considering radiation
chemistry recently developed by us, we attempted to simulate two such studies
in which (a) pure O ice at 5 K and, (b) pure HO ice at 16 K and 77 K,
were bombarded by keV H ions.
Our aims are twofold: (1) to test the capability of our newly developed
method to replicate the results of ice-irradiation experiments, and (2) to
determine in such a well-constrained system how bulk chemistry is best handled
using the same gas-grain codes that are used to model the interstellar medium
(ISM). We find that our modified astrochemical model is able to reproduce both
the abundance of O in the 5 K pure O ice, as well as both the abundance
of HO in the 16 K water ice and the previously noted decrease of
hydrogen peroxide at higher temperatures. However, these results require the
assumption that radicals and other reactive species produced via radiolysis
react quickly and non-diffusively with neighbors in the ice.Comment: ApJ, accepted. 30 pages, 5 figure
Tailoring correlations of the local density of states in disordered photonic materials
We present experimental evidence for the different mechanisms driving the
fluctuations of the local density of states (LDOS) in disordered photonic
systems. We establish a clear link between the microscopic structure of the
material and the frequency correlation function of LDOS accessed by a
near-field hyperspectral imaging technique. We show, in particular, that short-
and long-range frequency correlations of LDOS are controlled by different
physical processes (multiple or single scattering processes, respectively) that
can be---to some extent---manipulated independently. We also demonstrate that
the single scattering contribution to LDOS fluctuations is sensitive to
subwavelength features of the material and, in particular, to the correlation
length of its dielectric function. Our work paves a way towards a complete
control of statistical properties of disordered photonic systems, allowing for
designing materials with predefined correlations of LDOS.Comment: 5+9 pages, 5+6 figures. Fixed confusion of references between the
main text and the supplemental material in version
Controlling selectivity in alkene oxidation : anion driven syn-dihydroxylation or epoxidation catalysed by [Iron(III)(Pyridine-Containing Ligand)] complexes
The introduction of a pyridine moiety into the skeleton of a polyazamacrocyclic ligand affects both thermodynamic properties and coordination kinetics of the resulting metal complexes.1 These features have engendered a great interest of the scientific community. Much of the efforts in the use of macrocyclic pyridine containing ligands have been devoted to the study of catalytic oxidation reactions.2 We report here the synthesis and characterisation of [Fe(III)Pc-L\u2019s)] complexes (Pc-L = Pyridine-Containing Ligand) and their catalytic applications in alkene oxidation reactions using H2O2 as the terminal oxidant under mild conditions (Figure). Depending on the anion employed for the synthesis of the iron(III) metal complex, we observed a completely reversed selectivity. When X = OTf, a selective syn-dihydroxylation reaction was observed. On the other hand, employing X = Cl, we obtained the epoxide as the major product. It should be pointed out that under otherwise identical reaction conditions, using FeCl3\ub76H2O as catalyst in the absence of the ligand, no reaction was observed
State update algorithm for associative elastic-plastic pressure-insensitive materials by incremental energy minimization
This work presents a new state update algorithm for small-strain associative elastic-plastic
constitutive models, treating in a unified manner a wide class of deviatoric yield functions with linear or
nonlinear strain-hardening. The algorithm is based on an incremental energy minimization approach, in the
framework of generalized standard materials with convex free energy and dissipation potential. An efficient
method for the computation of the latter, its gradient and its Hessian is provided, using Haigh-Westergaard
stress invariants. Numerical results on a single material point loading history and finite element simulations are
reported to prove the effectiveness and the versatility of the method. Its merit turns out to be complementary
to the classical return map strategy, because no convergence difficulties arise if the stress is close to high
curvature points of the yield surface
Single-cell microfluidic impedance cytometry: From raw signals to cell phenotypes using data analytics
The biophysical analysis of single-cells by microfluidic impedance cytometry is emerging as a label-free and high-throughput means to stratify the heterogeneity of cellular systems based on their electrophysiology. Emerging applications range from fundamental life-science and drug assessment research to point-of-care diagnostics and precision medicine. Recently, novel chip designs and data analytic strategies are laying the foundation for multiparametric cell characterization and subpopulation distinction, which are essential to understand biological function, follow disease progression and monitor cell behaviour in microsystems. In this tutorial review, we present a comparative survey of the approaches to elucidate cellular and subcellular features from impedance cytometry data, covering the related subjects of device design, data analytics (i.e., signal processing, dielectric modelling, population clustering), and phenotyping applications. We give special emphasis to the exciting recent developments of the technique (timeframe 2017-2020) and provide our perspective on future challenges and directions. Its synergistic application with microfluidic separation, sensor science and machine learning can form an essential tool-kit for label-free quantification and isolation of subpopulations to stratify heterogeneous biosystems
The N2D+/N2H+ ratio as an evolutionary tracer of Class 0 protostars
Deuterated ions are abundant in cold (T=10 K), dense (n=10^5 cm^-3) regions,
in which CO is frozen out onto dust grains. In such environments, the deuterium
fractionation of such ions can exceed the elemental abundance ratio of D/H by a
factor of 10^4. In this paper we use the deuterium fractionation to investigate
the evolutionary state of Class 0 protostars. In a sample of 20 protostellar
objects, we found a clear correlation between the N2D+/N2H+ ratio and
evolutionary tracers. As expected, the coolest, i.e. the youngest, objects show
the largest deuterium fractionation. Furthermore, we find that sources with a
high N2D+/N2H+ ratio show clear indication for infall.Comment: 19 pages, 12 figures, accepted by A&
Near-field distribution and propagation of scattering resonances in Vogel spiral arrays of dielectric nanopillars
In this work, we employ scanning near-field optical microscopy, full-vector finite difference time domain numerical simulations and fractional Fourier transformation to investigate the near-field and propagation behavior of the electromagnetic energy scattered at 1.56µm by dielectric arrays of silicon nitride nanopillars with chiral 1-Vogel spiral geometry. In particular, we experimentally study the spatial evolution of scattered radiation and demonstrate near-field coupling between adjacent nanopillars along the parastichies arms. Moreover, by measuring the spatial distribution of the scattered radiation at different heights from the array plane, we demonstrate a characteristic rotation of the scattered field pattern consistent with net transfer of orbital angular momentum in the Fresnel zone, within a few micrometers from the plane of the array. Our experimental results agree with the simulations we performed and may be of interest to nanophotonics applications
Physical and chemical conditions in methanol maser selected hot-cores and UCHII regions
We present the results of a targeted 3-mm spectral line survey towards the
eighty-three 6.67 GHz methanol maser selected star forming clumps observed by
Purcell et al. 2006. In addition to the previously reported measurements of
HCO+ (1 - 0), H13CO+ (1 - 0), and CH3CN (5 - 4) & (6 -5), we used the Mopra
antenna to detect emission lines of N2H+ (1 - 0), HCN (1 - 0) and HNC (1 - 0)
towards 82/83 clumps (99 per cent), and CH3OH (2 - 1) towards 78/83 clumps (94
per cent). The molecular line data have been used to derive virial and LTE
masses, rotational temperatures and chemical abundances in the clumps, and
these properties have been compared between sub-samples associated with
different indicators of evolution. The greatest differences are found between
clumps associated with 8.6 GHz radio emission, indicating the presence of an
Ultra-Compact HII region, and `isolated' masers (without associated radio
emission), and between clumps exhibiting CH3CN emission and those without. In
particular, thermal CH3OH is found to be brighter and more abundant in
Ultra-Compact HII (UCHII) regions and in sources with detected CH3CN, and may
constitute a crude molecular clock in single dish observations. Clumps
associated with 8.6 GHz radio emission tend to be more massive and more
luminous than clumps without radio emission. This is likely because the most
massive clumps evolve so rapidly that a Hyper-Compact HII or UCHII region is
the first visible tracer of star-formation. The gas-mass to sub-mm/IR
luminosity relation for the combined sample was found to be L proportional to
M**0.68, considerably shallower than expected for massive main-sequence stars
Binary Formation in Star-Forming Clouds with Various Metallicities
Cloud evolution for various metallicities is investigated by
three-dimensional nested grid simulations, in which the initial ratio of
rotational to gravitational energy of the host cloud \beta_0 (=10^-1 - 10^-6)
and cloud metallicity Z (=0 - Z_\odot) are parameters. Starting from a central
number density of n = 10^4 cm^-3, cloud evolution for 48 models is calculated
until the protostar is formed (n \simeq 10^23 cm^-3) or fragmentation occurs.
The fragmentation condition depends both on the initial rotational energy and
cloud metallicity. Cloud rotation promotes fragmentation, while fragmentation
tends to be suppressed in clouds with higher metallicity. Fragmentation occurs
when \beta_0 > 10^-3 in clouds with solar metallicity, while fragmentation
occurs when \beta_0 > 10^-5 in the primordial gas cloud. Clouds with lower
metallicity have larger probability of fragmentation, which indicates that the
binary frequency is a decreasing function of cloud metallicity. Thus, the
binary frequency at the early universe (or lower metallicity environment) is
higher than at present day (or higher metallicity environment). In addition,
binary stars born from low-metallicity clouds have shorter orbital periods than
those from high-metallicity clouds. These trends are explained in terms of the
thermal history of the collapsing cloud.Comment: 11 pages, 2 figures, Submitted to ApJL, For high resolution figures
see http://astro3.sci.hokudai.ac.jp/~machida/binary-metal.pd
Exact results for hydrogen recombination on dust grain surfaces
The recombination of hydrogen in the interstellar medium, taking place on
surfaces of microscopic dust grains, is an essential process in the evolution
of chemical complexity in interstellar clouds. The H_2 formation process has
been studied theoretically, and in recent years also by laboratory experiments.
The experimental results were analyzed using a rate equation model. The
parameters of the surface, that are relevant to H_2 formation, were obtained
and used in order to calculate the recombination rate under interstellar
conditions. However, it turned out that due to the microscopic size of the dust
grains and the low density of H atoms, the rate equations may not always apply.
A master equation approach that provides a good description of the H_2
formation process was proposed. It takes into account both the discrete nature
of the H atoms and the fluctuations in the number of atoms on a grain. In this
paper we present a comprehensive analysis of the H_2 formation process, under
steady state conditions, using an exact solution of the master equation. This
solution provides an exact result for the hydrogen recombination rate and its
dependence on the flux, the surface temperature and the grain size. The results
are compared with those obtained from the rate equations. The relevant length
scales in the problem are identified and the parameter space is divided into
two domains. One domain, characterized by first order kinetics, exhibits high
efficiency of H_2 formation. In the other domain, characterized by second order
kinetics, the efficiency of H_2 formation is low. In each of these domains we
identify the range of parameters in which, the rate equations do not account
correctly for the recombination rate. and the master equation is needed.Comment: 23 pages + 8 figure
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