7,731 research outputs found
The performance of forecast-based monetary policy rules under model uncertainty
We investigate the performance of forecast-based monetary policy rules using five macroeconomic models that reflect a wide range of views on aggregate dynamics. We identify the key characteristics of rules that are robust to model uncertainty: such rules respond to the one-year-ahead inflation forecast and to the current output gap and incorporate a substantial degree of policy inertia. In contrast, rules with longer forecast horizons are less robust and are prone to generating indeterminacy. Finally, we identify a robust benchmark rule that performs very well in all five models over a wide range of policy preferences
Simplified approach to double jumps for fluorescing dipole-dipole interacting atoms
A simplified scheme for the investigation of cooperative effects in the
quantum jump statistics of small numbers of fluorescing atoms and ions in a
trap is presented. It allows the analytic treatment of three dipole-dipole
interacting four-level systems which model the relevant level scheme of Ba+
ions. For the latter, a huge rate of double and triple jumps was reported in a
former experiment and the huge rate was attributed to the dipole-dipole
interaction. Our theoretical results show that the effect of the dipole-dipole
interaction on these rates is at most 5% and that for the parameter values of
the experiment there is practically no effect. Consequently it seems that the
dipole-dipole interaction can be ruled out as a possible explanation for the
huge rates reported in the experiment.Comment: 7 pages, 6 figures, typos corrected, to appear in EPJ D (as highlight
paper
Effect of ion hydration on the first-order transition in the sequential wetting of hexane on brine
In recent experiments, a sequence of changes in the wetting state (`wetting
transitions') has been observed upon increasing the temperature in systems
consisting of pentane on pure water and of hexane on brine. This sequence of
two transitions is brought about by an interplay of short-range and long-range
interactions between substrate and adsorbate. In this work, we argue that the
short-range interaction (contact energy) between hexane and pure water remains
unchanged due to the formation of a depletion layer (a thin `layer' of pure
water which is completely devoid of ions) at the surface of the electrolyte and
that the presence of the salt manifests itself only in a modification of the
long-range interaction between substrate and adsorbate. In a five-layer
calculation considering brine, water, the first layer of adsorbed hexane
molecules, liquid hexane, and vapor, we determine the new long-range
interaction of brine with the adsorbate {\em across} the water `layer'.
According to the recent theory of the excess surface tension of an electrolyte
by Levin and Flores-Mena, this water `layer' is of constant, i.e.\
salt-concentration independent, thickness , with being the
hydrodynamic radius of the ions in water. Our results are in good agreement
with the experimental ones.Comment: 27 pages, 2 tables, 4 figure
Spatially regularized estimation for the analysis of DCE-MRI data
Competing compartment models of different complexities have been used for the quantitative analysis of Dynamic Contrast-Enhanced Magnetic Resonance Imaging data.
We present a spatial Elastic Net approach that allows to estimate the number of compartments for each voxel such that the model complexity is not fixed a priori.
A multi-compartment approach is considered, which is translated into a
restricted least square model selection problem. This is done by
using a set of basis functions for a given set of candidate rate
constants. The form of the basis functions is derived from a kinetic
model and thus describes the contribution of a specific compartment.
Using a spatial Elastic Net estimator, we chose a sparse set of basis functions per voxel, and hence, rate constants of compartments.
The spatial penalty takes into account the voxel structure of an image and performs better than a penalty treating voxels independently.
The proposed estimation method is evaluated for simulated images and applied to an in-vivo data set
Analysis of DCE-MRI Data using a Nonnegative Elastic Net
We present a nonnegative Elastic Net approach for the analysis of Dynamic Contrast-Enhanced Magnetic Resonance Imaging data. A multi-compartment approach is considered, which is translated into a (restricted) least square model
selection problem. This is done by using a set of basis functions for a given set of candidate rate constants. The form of the basis functions is derived from a kinetic model and thus describes the contribution of some compartment. Using the Elastic Net estimator, we chose clusters of basis functions, and hence, rate constants of compartments. As further challenge, the estimator has to be restricted to positive regression parameters, which correspond to transfer rates of the compartments. The proposed estimation method is applied to an in-vivo data set
Gravitational fragmentation in turbulent primordial gas and the initial mass function of Population III stars
We report results from numerical simulations of star formation in the early
universe that focus on the dynamical behavior of metal-free gas under different
initial and environmental conditions. In particular we investigate the role of
turbulence, which is thought to ubiquitously accompany the collapse of
high-redshift halos. We distinguish between two main cases: the birth of
Population III.1 stars - those which form in the pristine halos unaffected by
prior star formation - and the formation of Population III.2 stars - those
forming in halos where the gas is still metal free but has an increased
ionization fraction. This latter case can arise either from exposure to the
intense UV radiation of stellar sources in neighboring halos, or from the high
virial temperatures associated with the formation of massive halos, that is,
those with masses greater than 1e8 solar masses. We find that turbulent
primordial gas is highly susceptible to fragmentation in both cases, even for
turbulence in the subsonic regime, i.e. for rms velocity dispersions as low as
20 % of the sound speed. Contrary to our original expectations, fragmentation
is more vigorous and more widespread in pristine halos compared to pre-ionized
ones. We therefore predict Pop III.1 stars to be on average of somewhat lower
mass, and form in larger groups, than Pop III.2 stars. We find that fragment
masses cover over two orders of magnitude, indicating that the resulting
Population III initial mass function was significantly extended in mass as
well. This prompts the need for a large, high-resolution study of the formation
of dark matter minihalos that is capable of resolving the turbulent flows in
the gas at the moment when the baryons become self-gravitating. This would help
determine which, if any, of the initial conditions presented in our study are
realized in nature.Comment: Accepted for publication in Ap
Formation and evolution of primordial protostellar systems
We investigate the formation of the first stars at the end of the cosmic dark
ages with a suite of three-dimensional, moving mesh simulations that directly
resolve the collapse of the gas beyond the formation of the first protostar at
the centre of a dark matter minihalo. The simulations cover more than 25 orders
of magnitude in density and have a maximum spatial resolution of 0.05 R_sun,
which extends well below the radius of individual protostars and captures their
interaction with the surrounding gas. In analogy to previous studies that
employed sink particles, we find that the Keplerian disc around the primary
protostar fragments into a number of secondary protostars, which is facilitated
by H2 collisional dissociation cooling and collision-induced emission. The
further evolution of the protostellar system is characterized by strong
gravitational torques that transfer angular momentum between the secondary
protostars formed in the disc and the surrounding gas. This leads to the
migration of about half of the secondary protostars to the centre of the cloud
in a free-fall time, where they merge with the primary protostar and enhance
its growth to about five times the mass of the second most massive protostar.
By the same token, a fraction of the protostars obtain angular momentum from
other protostars via N-body interactions and migrate to higher orbits. On
average, only every third protostar survives until the end of the simulation.
However, the number of protostars present at any given time increases
monotonically, suggesting that the system will continue to grow beyond the
limited period of time simulated here.Comment: 19 pages, 13 figures, accepted for publication in MNRAS, movies of
the simulations may be downloaded at http://www.mpa-garching.mpg.de/~tgrei
- …