132,341 research outputs found
Precessing Jets and Molecular Outflows: A 3-D Numerical Study
We present 3-D numerical hydrodynamical simulations of precessing supersonic
heavy jets to explore how well they serve as a model for generating molecular
outflows from Young Stellar Objects. The dynamics are studied with a number of
high resolution simulations on a Cartesian grid (128x128x128 zones) using a
high order finite difference method. A range of cone angles and precession
rates were included in the study. Two higher resolution runs (256x256x256
zones) were made for comparison in order to confirm numerical convergence of
global flow characteristics. Morphological, kinematical and dynamical
characteristics of precessing jets are described and compared to important
properties of straight jets and also to observations of YSOs. In order to
examine the robustness of precessing jets as a mean to produce molecular
outflows around Young Stellar Objects, ``synthetic observations'' of the
momentum distributions of the simulated precessing jets are compared to
observations of molecular outflows. It is found that precessing jets match
better the morphology, highly forward driven momentum and momentum
distributions along the long axis of molecular outflows than do wind-driven or
straight jet-driven flow models.Comment: Accepted by ApJ, 31 pages, using aasms.sty, Also available in
postscript with figures via a gzipped tar file at
ftp://s1.msi.umn.edu/pub/afrank/3DJet/3DJet.tar.gz . For information contact
[email protected]
Inelastic Scattering from Core-electrons: a Multiple Scattering Approach
The real-space multiple-scattering (RSMS) approach is applied to model
non-resonant inelastic scattering from deep core electron levels over a broad
energy spectrum. This approach is applicable to aperiodic or periodic systems
alike and incorporates ab initio, self-consistent electronic structure and
final state effects. The approach generalizes to finite momentum transfer a
method used extensively to model x-ray absorption spectra (XAS), and includes
both near edge spectra and extended fine structure. The calculations can be
used to analyze experimental results of inelastic scattering from
core-electrons using either x-ray photons (NRIXS) or electrons (EELS). In the
low momentum transfer region (the dipole limit), these inelastic loss spectra
are proportional to those from XAS. Thus their analysis can provide similar
information about the electronic and structural properties of a system. Results
for finite momentum transfer yield additional information concerning monopole,
quadrupole, and higher couplings. Our results are compared both with experiment
and with other theoretical calculations.Comment: 11 pages, 8 figures. Submitted to Phys. Rev.
The role of institutional and family embeddedness in the failure of Sub-Saharan African migrant family businesses
There is considerable interest among European politicians and policymakers in how to integrate migrants in the local and national economy. Drawing on in-depth qualitative interviews with 20 owners of Sub-Saharan African migrant family businesses (SSAMBs) in the United Kingdom, this article critically examines why SSAMBs fail or underperform. This investigation draws upon three streams of literature – notably migrant business failure, institutional theory and family embeddedness. The findings highlight the challenges of doing business and the reasons for business failure among this group. These are different from other small businesses and include culture, family interference and ethnicity. The main contribution of the article lies in the development of a conceptual model that highlights the relationships between institutional contexts and migrant family business outcomes. The model proposes that institution and family embeddedness results in the enactment of ethnic behaviours that drive migrant businesses into cultural markets leading to business underperformance or failure
Analytic Spectra of CMB Anisotropies and Polarization Generated by Relic Gravitational Waves with Modification due to Neutrino Free-Streaming
We present an analytical calculation of the spectra of CMB anisotropies and
polarizations generated by relic gravitational waves (RGWs). As a substantial
extension to the previous studies, three new ingredients are included in this
work. Firstly, the analytic and are given; especially the
latter can be useful to extract signal of RGWs from the observed data in the
zero multipole method. Secondly, a fitting formula of the decaying factor on
small scales is given, coming from the visibility function around the photon
decoupling. Thirdly, the impacts by the neutrino free-streaming (NFS) is
examined, a process that occurred in the early universe and leaves observable
imprints on CMB via RGWs.
It is found that the analytic and have profiles
agreeing with the numeric ones, except that in a range
and the trough of around have some deviations.
With the new damping factor, the analytic and match with
the numeric ones with the maximum errors only up to the first three
peaks for , improving the previous studies substantially. The
correspondence of the positions of peaks of and those of RGWs are
also demonstrated explicitly. We also find that NFS reduces the amplitudes of
by for and shifts slightly
their peaks to smaller angles. Detailed analyses show that the zero multipoles
, where crosses 0, are shifted to larger values by NFS. This
shifting effect is as important as those causedby different inflation models
and different baryon fractions.Comment: 17 pages, 7 figures. accepted by PR
Spin Hall Conductance of the Two Dimensional Hole Gas in a Perpendicular Magnetic Field
The charge and spin Hall conductance of the two-dimensional hole gas within
the Luttinger model with and without inversion symmetry breaking terms in a
perpendicular magnetic field are studied, and two key phenomena are predicted.
The sign of the spin Hall conductance is modulated periodically by the external
magnetic field, which means a possible application in the future. Furthermore,
a resonant spin Hall conductance in the two-dimensional hole gas with a certain
hole density at a typical magnetic field is indicated, which implies a likely
way to firmly establish the intrinsic spin Hall effect. The charge Hall
conductance is unaffected by the spin-orbit coupling.Comment: accepted for publication in Phys. Rev. B; 6 pages, 4 figure
- …