14,310 research outputs found
[C II] emission from galactic nuclei in the presence of X-rays
The luminosity of [C II] is used to probe the star formation rate in
galaxies, but the correlation breaks down in some active galactic nuclei
(AGNs). Models of the [C II] emission from galactic nuclei do not include the
influence of X-rays on the carbon ionization balance, which may be a factor in
reducing the [C II] luminosity. We calculate the [C II] luminosity in galactic
nuclei under the influence of bright sources of X-rays. We solve the balance
equation of the ionization states of carbon as a function of X-ray flux,
electron, atomic hydrogen, and molecular hydrogen density. These are input to
models of [CII] emission from the interstellar medium (ISM) in galactic nuclei.
We also solve the distribution of the ionization states of oxygen and nitrogen
in highly ionized regions. We find that the dense warm ionized medium (WIM) and
dense photon dominated regions (PDRs) dominate the [C II] emission when no
X-rays are present. The X-rays in galactic nuclei can affect strongly the C
abundance in the WIM converting some fraction to C and higher ionization
states and thus reducing its [C II] luminosity. For an X-ray luminosity >
10 erg/s the [C II] luminosity can be suppressed by a factor of a few,
and for very strong sources, >10 erg/s, such as found for many AGNs by
an order of magnitude. Comparison of the model with extragalactic sources shows
that the [C II] to far-infrared ratio declines for an X-ray luminosity
>10 erg/s, in reasonable agreement with our model.Comment: 16 pages and 14 figures, accepted for publication in A&
Dynamics of Large-Scale Plastic Deformation and the Necking Instability in Amorphous Solids
We use the shear transformation zone (STZ) theory of dynamic plasticity to
study the necking instability in a two-dimensional strip of amorphous solid.
Our Eulerian description of large-scale deformation allows us to follow the
instability far into the nonlinear regime. We find a strong rate dependence;
the higher the applied strain rate, the further the strip extends before the
onset of instability. The material hardens outside the necking region, but the
description of plastic flow within the neck is distinctly different from that
of conventional time-independent theories of plasticity.Comment: 4 pages, 3 figures (eps), revtex4, added references, changed and
added content, resubmitted to PR
Unified derivation of phase-field models for alloy solidification from a grand-potential functional
In the literature, two quite different phase-field formulations for the
problem of alloy solidification can be found. In the first, the material in the
diffuse interfaces is assumed to be in an intermediate state between solid and
liquid, with a unique local composition. In the second, the interface is seen
as a mixture of two phases that each retain their macroscopic properties, and a
separate concentration field for each phase is introduced. It is shown here
that both types of models can be obtained by the standard variational procedure
if a grand-potential functional is used as a starting point instead of a
free-energy functional. The dynamical variable is then the chemical potential
instead of the composition. In this framework, a complete analogy with
phase-field models for the solidification of a pure substance can be
established. This analogy is then exploited to formulate quantitative
phase-field models for alloys with arbitrary phase diagrams. The precision of
the method is illustrated by numerical simulations with varying interface
thickness.Comment: 36 pages, 1 figur
Steady-state, effective-temperature dynamics in a glassy material
We present an STZ-based analysis of numerical simulations by Haxton and Liu
(HL). The extensive HL data sharply test the basic assumptions of the STZ
theory, especially the central role played by the effective disorder
temperature as a dynamical state variable. We find that the theory survives
these tests, and that the HL data provide important and interesting constraints
on some of its specific ingredients. Our most surprising conclusion is that,
when driven at various constant shear rates in the low-temperature glassy
state, the HL system exhibits a classic glass transition, including
super-Arrhenius behavior, as a function of the effective temperature.Comment: 9 pages, 6 figure
A microscopic model for solidification
We present a novel picture of a non isothermal solidification process
starting from a molecular level, where the microscopic origin of the basic
mechanisms and of the instabilities characterizing the approach to equilibrium
is rendered more apparent than in existing approaches based on coarse grained
free energy functionals \`a la Landau.
The system is composed by a lattice of Potts spins, which change their state
according to the stochastic dynamics proposed some time ago by Creutz. Such a
method is extended to include the presence of latent heat and thermal
conduction.
Not only the model agrees with previous continuum treatments, but it allows
to introduce in a consistent fashion the microscopic stochastic fluctuations.
These play an important role in nucleating the growing solid phase in the melt.
The approach is also very satisfactory from the quantitative point of view
since the relevant growth regimes are fully characterized in terms of scaling
exponents.Comment: 7 pages Latex +3 figures.p
Group Self‐Interest vs. Equity:Explaining Support for Horizontal Redistribution in (Former) Competitive Clientelist States
Extant literature links intergroup disparities, or horizontal inequalities, in Sub‐Saharan Africa to the unequal representation of ethnic groups in central power, who accumulate wealth at the expense of politically marginalized groups. Over time, these politically‐induced inequalities have trapped some ethnic groups in positions of relative disadvantage. Group‐based, or horizontal, redistribution can help redress these inequalities yet require popular support if they are not to contribute to intergroup tensions. In this article, we examine how people’s experiences of political exclusion, on the one hand, and their attributional beliefs about the causes of political exclusion, on the other, condition support for government policies aimed at eradicating economic inequalities between different ethnic groups. We argue that people are more likely to be supportive of horizontal redistribution either when (H1a) they belong to ethnic groups that have not had access to central power, and/or (H1b) feel that their ethnic group is politically marginalized (and thus stands to gain); or when (H2) they attribute the political exclusion of the politically marginalized group(s) that stand(s) to benefit from these policies to the legacies of colonialism and clientelism (thus seeking to foster equity). To test our hypotheses, we examine these issues in the context of Kenya, a society with politically salient ethnic cleavages and a history of clientelism. Based on a unique online survey involving 2,286 Kenyans, we show that, notwithstanding group self‐interest being at play, there is strong support for horizontal redistribution across groups
Airborne measurements of cloud forming nuclei and aerosol particles at Kennedy Space Center, Florida
Results of airborne measurements of the sizes and concentrations of aerosol particles, ice nuclei, and cloud condensation nuclei that were taken at Kennedy Space Center, Florida, are presented along with a detailed description of the instrumentation and measuring capabilities of the University of Washington airborne measuring facility (Douglas B-23). Airborne measurements made at Ft. Collins, Colorado, and Little Rock, Arkansas, during the ferry of the B-23 are presented. The particle concentrations differed significantly between the clean air over Ft. Collins and the hazy air over Little Rock and Kennedy Space Center. The concentrations of cloud condensation nuclei over Kennedy Space Center were typical of polluted eastern seaboard air. Three different instruments were used to measure ice nuclei: one used filters to collect the particles, and the others used optical and acoustical methods to detect ice crystals grown in portable cloud chambers. A comparison of the ice nucleus counts, which are in good agreement, is presented
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