26,376 research outputs found
R-process and alpha-elements in the Galactic disk: Kinematic correlations
Recent studies of elemental abundances in the Galactic halo and in the
Galactic disk have underscored the possibility to kinematically separate
different Galactic subcomponents. Correlations between the galactocentric
rotation velocity and various element ratios were found, providing an important
means to link different tracers of star formation and metal enrichment to the
Galactic components of different origin (collapse vs. accretion). In the
present work we determine stellar kinematics for a sample of 124 disk stars,
which we derive from their orbits based on radial velocities and proper motions
from the the literature. Our stars form a subsample of the Edvardsson et al.
(1993) sample and we concentrate on three main tracers: (i) Europium as an
r-process element is predominantly produced in Supernovae of type II. (ii)
Likewise, alpha-elements, such as Ca, Si, Mg, are synthesised in SNe II,
contrary to iron, which is being produced preferentially in SNe Ia. (iii) The
s-process element Barium is a measure of the relative contribution of AGB stars
to the Galaxy's enrichment history and has been shown to be an indicator for
distinguishing between thin and thick disk stars. All such studies reveal,
basically, that stars with low galactocentric rotational velocity tend to have
high abundances of alpha-elements and Eu, but lower abundances of, e.g., Ba.Comment: 5 pages, 2 figures, Poster contribution to appear in "Planets To
Cosmology: Essential Science In Hubble's Final Years", proceedings of the May
2004 STScI Symposium, M. Livio (ed.), (Cambridge University Press
Theory of the Anderson impurity model: The Schrieffer--Wolff transformation re--examined
We apply the method of infinitesimal unitary transformations recently
introduced by Wegner to the Anderson single impurity model. It is demonstrated
that this method provides a good approximation scheme for all values of the
on-site interaction , it becomes exact for . We are able to treat an
arbitrary density of states, the only restriction being that the hybridization
should not be the largest parameter in the system. Our approach constitutes a
consistent framework to derive various results usually obtained by either
perturbative renormalization in an expansion in the hybridization~,
Anderson's ``poor man's" scaling approach or the Schrieffer--Wolff unitary
transformation. In contrast to the Schrieffer--Wolff result we find the correct
high--energy cutoff and avoid singularities in the induced couplings. An
important characteristic of our method as compared to the ``poor man's" scaling
approach is that we continuously decouple modes from the impurity that have a
large energy difference from the impurity orbital energies. In the usual
scaling approach this criterion is provided by the energy difference from the
Fermi surface.Comment: Uuencoded gzipped postscript, 26 pages, 5 postscript figure
Optimal Strong Rates of Convergence for a Space-Time Discretization of the Stochastic Allen-Cahn Equation with multiplicative noise
The stochastic Allen-Cahn equation with multiplicative noise involves the
nonlinear drift operator . We use the fact that
satisfies a weak monotonicity property to deduce uniform bounds in strong norms
for solutions of the temporal, as well as of the spatio-temporal discretization
of the problem. This weak monotonicity property then allows for the estimate for all
small , where is the strong variational solution of the
stochastic Allen-Cahn equation, while solves a
structure preserving finite element based space-time discretization of the
problem on a temporal mesh of size which
covers
Dynamic features of successive upwelling events in the Baltic Sea - a numerical case study
Coastal upwelling often reveals itself during the thermal stratification season as an abrupt sea surface temperature (SST) drop. Its intensity depends not only on the magnitude of an upwelling-favourable wind impulse but also on the temperature stratification of the water column during the initial stage of the event. When a "chain" of upwelling events is taking place, one event may play a part in forming the initial stratification for the next one; consequently, SST may drop significantly even with a reduced wind impulse.
Two upwelling events were simulated on the Polish coast in August 1996 using a three-dimensional, baroclinic prognostic model. The model results proved to be in good agreement with in situ observations and satellite data. Comparison of the simulated upwelling events show that the first one required a wind impulse of 28000 kg m-1 s-1 to reach its mature, full form, whereas an impulse of only 7500 kg m-1 s-1 was sufficient to bring about a significant drop in SST at the end of the second event. In practical applications like operational modelling, the initial stratification conditions prior to an upwelling event should be described with care in order to be able to simulate the coming event with very good accuracy
Transport across a carbon nanotube quantum dot contacted with ferromagnetic leads: experiment and non-perturbative modeling
We present measurements of tunneling magneto-resistance (TMR) in single-wall
carbon nanotubes attached to ferromagnetic contacts in the Coulomb blockade
regime. Strong variations of the TMR with gate voltage over a range of four
conductance resonances, including a peculiar double-dip signature, are
observed. The data is compared to calculations in the "dressed second order"
(DSO) framework. In this non-perturbative theory, conductance peak positions
and linewidths are affected by charge fluctuations incorporating the properties
of the carbon nanotube quantum dot and the ferromagnetic leads. The theory is
able to qualitatively reproduce the experimental data.Comment: 14 pages, 13 figure
Multiplicative combinatorial properties of return time sets in minimal dynamical systems
We investigate the relationship between the dynamical properties of minimal
topological dynamical systems and the multiplicative combinatorial properties
of return time sets arising from those systems. In particular, we prove that
for a residual sets of points in any minimal system, the set of return times to
any non-empty, open set contains arbitrarily long geometric progressions. Under
the separate assumptions of total minimality and distality, we prove that
return time sets have positive multiplicative upper Banach density along
and along multiplicative subsemigroups of ,
respectively. The primary motivation for this work is the long-standing open
question of whether or not syndetic subsets of the positive integers contain
arbitrarily long geometric progressions; our main result is some evidence for
an affirmative answer to this question.Comment: 32 page
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