27,220 research outputs found
Angular momentum transport and element mixing in the stellar interior I. Application to the rotating Sun
The purpose of this work was to obtain diffusion coefficient for the magnetic
angular momentum transport and material transport in a rotating solar model. We
assumed that the transport of both angular momentum and chemical elements
caused by magnetic fields could be treated as a diffusion process. The
diffusion coefficient depends on the stellar radius, angular velocity, and the
configuration of magnetic fields. By using of this coefficient, it is found
that our model becomes more consistent with the helioseismic results of total
angular momentum, angular momentum density, and the rotation rate in a
radiative region than the one without magnetic fields. Not only can the
magnetic fields redistribute angular momentum efficiently, but they can also
strengthen the coupling between the radiative and convective zones. As a
result, the sharp gradient of the rotation rate is reduced at the bottom of the
convective zone. The thickness of the layer of sharp radial change in the
rotation rate is about 0.036 in our model. Furthermore, the
difference of the sound-speed square between the seismic Sun and the model is
improved by mixing the material that is associated with angular momentum
transport.Comment: 8 pages, 2 figure
Sovereign default and monetary policy tradeoffs
The paper is organized around the following question: when the economy moves from a debt-GDP level where the probability of default is nil to a higher level—the “fiscal limit”—where the default probability is non-negligible, how do the effects of routine monetary operations designed to achieve macroeconomic stabilization change? We find that the specification of the monetary policy rule plays a critical role. Consider a central bank that targets the risky rate. When the economy is near its fiscal limit, a transitory monetary policy contraction leads to a sustained rise in inflation, even though monetary policy actively targets inflation and fiscal policy passively adjusts taxes to stabilize debt. If the central bank targets the riskfree rate, on the other hand, the same transitory monetary contraction keeps inflation under control but leads output to contract for a prolonged period of time. The comparison shows that sovereign default risk puts into sharp relief the tradeoff between inflation and output stabilization
Motility-driven glass and jamming transitions in biological tissues
Cell motion inside dense tissues governs many biological processes, including
embryonic development and cancer metastasis, and recent experiments suggest
that these tissues exhibit collective glassy behavior. To make quantitative
predictions about glass transitions in tissues, we study a self-propelled
Voronoi (SPV) model that simultaneously captures polarized cell motility and
multi-body cell-cell interactions in a confluent tissue, where there are no
gaps between cells. We demonstrate that the model exhibits a jamming transition
from a solid-like state to a fluid-like state that is controlled by three
parameters: the single-cell motile speed, the persistence time of single-cell
tracks, and a target shape index that characterizes the competition between
cell-cell adhesion and cortical tension. In contrast to traditional particulate
glasses, we are able to identify an experimentally accessible structural order
parameter that specifies the entire jamming surface as a function of model
parameters. We demonstrate that a continuum Soft Glassy Rheology model
precisely captures this transition in the limit of small persistence times, and
explain how it fails in the limit of large persistence times. These results
provide a framework for understanding the collective solid-to-liquid
transitions that have been observed in embryonic development and cancer
progression, which may be associated with Epithelial-to-Mesenchymal transition
in these tissues.Comment: accepted for publication in Physical Review X, 201
Solar Models with Revised Abundances and Opacities
Using reconstructed opacities, we construct solar models with low
heavy-element abundance. Rotational mixing and enhanced diffusion of helium and
heavy elements are used to reconcile the recently observed abundances with
helioseismology. The sound speed and density of models where the relative and
absolute diffusion coefficients for helium and heavy elements have been
increased agree with seismically inferred values at better than the 0.005 and
0.02 fractional level respectively. However, the surface helium abundance of
the enhanced diffusion model is too low. The low helium problem in the enhanced
diffusion model can be solved to a great extent by rotational mixing. The
surface helium and the convection zone depth of rotating model M04R3, which has
a surface Z of 0.0154, agree with the seismic results at the levels of 1
and 3 respectively. M04R3 is almost as good as the standard
model M98. Some discrepancies between the models constructed in accord with the
new element abundances and seismic constraints can be solved individually, but
it seems difficult to resolve them as a whole scenario.Comment: 10 pages, 1 figur
Detection and classification of faults in pitch-regulated wind turbine generators using normal behaviour models based on performance curves
Uncertain Fiscal Consolidations
The paper explores the macroeconomic consequences of fiscal consolidations whose timing and composition are uncertain. Drawing on the evidence in Alesina and Ardagna (2010), we emphasize whether or not the fiscal consolidation is driven by tax rises or expenditure cuts. We find that the composition of the fiscal consolidation, its duration, the monetary policy stance, the level of government debt and expectations over the likelihood and composition of fiscal consolidations all matter in determining the extent to which a given consolidation is expansionary and/or successful in stabilizing government debt.government debt, budget reform, monetary-fiscal policy interactions
Ly Leaks in the Absorption Spectra of High Redshift QSOs
Spectra of high redshift QSOs show deep Gunn-Peterson absorptions on the blue
sides of the \Lya emissions lines. They can be decomposed into components
called \Lya leaks, defined to be emissive regions in complementary to otherwise
zero-fluxed absorption gaps. Just like \Lya absorption forests at low
redshifts, \Lya leaks are both easy to find in observations and containing rich
sets of statistical properties that can be used to study the early evolution of
the IGM. Among all properties of a leak profile, we investigate its equivalent
width in this paper, since it is weakly affected by instrumental resolution and
noise. Using 10 Keck QSO spectra at , we have measured the number
density distribution function , defined to be the number of leaks per
equivalent width and per redshift , in the redshift range .
These new observational statistics, in both the differential and cumulative
forms, fit well to hydro numerical simulations of uniform ionizing background
in the CDM cosmology. In this model, Ly leaks are mainly due
to low density voids. It supports the early studies that the IGM at
would still be in a highly ionized state with neutral hydrogen fraction . Measurements of at would be effective to probe the
reionization of the IGM.Comment: 3 figs, accepted by ApJ
Matrix Product Representation of Locality Preserving Unitaries
The matrix product representation provides a useful formalism to study not
only entangled states, but also entangled operators in one dimension. In this
paper, we focus on unitary transformations and show that matrix product
operators that are unitary provides a necessary and sufficient representation
of 1D unitaries that preserve locality. That is, we show that matrix product
operators that are unitary are guaranteed to preserve locality by mapping local
operators to local operators while at the same time all locality preserving
unitaries can be represented in a matrix product way. Moreover, we show that
the matrix product representation gives a straight-forward way to extract the
GNVW index defined in Ref.\cite{Gross2012} for classifying 1D locality
preserving unitaries. The key to our discussion is a set of `fixed point'
conditions which characterize the form of the matrix product unitary operators
after blocking sites. Finally, we show that if the unitary condition is relaxed
and only required for certain system sizes, the matrix product operator
formalism allows more possibilities than locality preserving unitaries. In
particular, we give an example of a simple matrix product operator which is
unitary only for odd system sizes, does not preserve locality and carries a
`fractional' index as compared to their locality preserving counterparts.Comment: 14 page
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