25,213 research outputs found
Microwave emission from spinning dust in circumstellar disks
In the high density environments of circumstellar disks dust grains are
expected to grow to large sizes by coagulation. Somewhat unexpectedly, recent
near-IR observations of PAH features from disks around Herbig Ae/Be stars
demonstrate that substantial amount of dust mass in these disks (up to several
tens of per cent of the total carbon content) can be locked up in particles
with sizes ranging from several to tens of nanometers. We investigate the
possibility of detecting the electric dipole emission produced by these
nanoparticles as they spin at thermal rates (tens of GHz) in cold gas. We show
that such emission peaks in the microwave range and dominates over the thermal
disk emission at \nu 5 % of the
total carbon abundance is locked up in nanoparticles. We test the sensitivity
of this prediction to various stellar and disk parameters and show that if the
potential contamination of the spinning dust component by the free-free and/or
synchrotron emission can be removed, then the best chances of detecting this
emission would be in disks with small opacity, having SEDs with steep sub-mm
slopes (which minimizes thermal disk emission at GHz frequencies). Detection of
the spinning dust emission would provide important evidence for the existence,
properties, and origin of the population of small dust particles in
protoplanetary disks, with possible ramifications for planet formation.Comment: 9 pages, 3 figures, submitted to Ap
A Stochastic Immersed Boundary Method for Fluid-Structure Dynamics at Microscopic Length Scales
In this work it is shown how the immersed boundary method of (Peskin2002) for
modeling flexible structures immersed in a fluid can be extended to include
thermal fluctuations. A stochastic numerical method is proposed which deals
with stiffness in the system of equations by handling systematically the
statistical contributions of the fastest dynamics of the fluid and immersed
structures over long time steps. An important feature of the numerical method
is that time steps can be taken in which the degrees of freedom of the fluid
are completely underresolved, partially resolved, or fully resolved while
retaining a good level of accuracy. Error estimates in each of these regimes
are given for the method. A number of theoretical and numerical checks are
furthermore performed to assess its physical fidelity. For a conservative
force, the method is found to simulate particles with the correct Boltzmann
equilibrium statistics. It is shown in three dimensions that the diffusion of
immersed particles simulated with the method has the correct scaling in the
physical parameters. The method is also shown to reproduce a well-known
hydrodynamic effect of a Brownian particle in which the velocity
autocorrelation function exhibits an algebraic tau^(-3/2) decay for long times.
A few preliminary results are presented for more complex systems which
demonstrate some potential application areas of the method.Comment: 52 pages, 11 figures, published in journal of computational physic
The cognitive demands of second order manual control: Applications of the event related brain potential
Three experiments are described in which tracking difficulty is varied in the presence of a covert tone discrimination task. Event related brain potentials (ERPs) elicited by the tones are employed as an index of the resource demands of tracking. The ERP measure reflected the control order variation, and this variable was thereby assumed to compete for perceptual/central processing resources. A fine-grained analysis of the results suggested that the primary demands of second order tracking involve the central processing operations of maintaining a more complex internal model of the dynamic system, rather than the perceptual demands of higher derivative perception. Experiment 3 varied tracking bandwidth in random input tracking, and the ERP was unaffected. Bandwidth was then inferred to compete for response-related processing resources that are independent of the ERP
NMR evidence for a strong modulation of the Bose-Einstein Condensate in BaCuSiO
We present a Cu and Si NMR study of the quasi-2D coupled
spin 1/2 dimer compound BaCuSiO in the magnetic field range 13-26 T and
at temperatures as low as 50 mK. NMR data in the gapped phase reveal that below
90 K different intra-dimer exchange couplings and different gaps
( = 1.16) exist in every second plane along
the c-axis, in addition to a planar incommensurate (IC) modulation. Si
spectra in the field induced magnetic ordered phase reveal that close to the
quantum critical point at = 23.35 T the average boson density
of the Bose-Einstein condensate is strongly modulated along the
c-axis with a density ratio for every second plane
. An IC modulation of the local
density is also present in each plane. This adds new constraints for the
understanding of the 2D value = 1 of the critical exponent describing
the phase boundary
Extragalactic H3O+: Some Consequences
We discuss some implications of our recent detection of extragalactic H3O+:
the location of the gas in M82, the origin of energetic radiation in M82, and
the possible feedback effects of star formation on the cosmic ray flux in
galaxies.Comment: Five pages, one figure; contribution to proceedings of conference
"Far-infrared observations of the interstellar medium", December 2007, Bad
Honne
Revivals of quantum wave-packets in graphene
We investigate the propagation of wave-packets on graphene in a perpendicular
magnetic field and the appearance of collapses and revivals in the
time-evolution of an initially localised wave-packet. The wave-packet evolution
in graphene differs drastically from the one in an electron gas and shows a
rich revival structure similar to the dynamics of highly excited Rydberg
states.
We present a novel numerical wave-packet propagation scheme in order to solve
the effective single-particle Dirac-Hamiltonian of graphene and show how the
collapse and revival dynamics is affected by the presence of disorder. Our
effective numerical method is of general interest for the solution of the Dirac
equation in the presence of potentials and magnetic fields.Comment: 22 pages, 10 figures, 3 movies, to appear in New Journal of Physic
Optimization of radio astronomical observations using Allan variance measurements
Stability tests based on the Allan variance method have become a standard
procedure for the evaluation of the quality of radio-astronomical
instrumentation. They are very simple and simulate the situation when detecting
weak signals buried in large noise fluctuations. For the special conditions
during observations an outline of the basic properties of the Allan variance is
given, and some guidelines how to interpret the results of the measurements are
presented. Based on a rather simple mathematical treatment clear rules for
observations in ``Position-Switch'', ``Beam-'' or ``Frequency-Switch'',
``On-The-Fly-'' and ``Raster-Mapping'' mode are derived. Also, a simple ``rule
of the thumb'' for an estimate of the optimum timing for the observations is
found. The analysis leads to a conclusive strategy how to plan
radio-astronomical observations. Particularly for air- and space-borne
observatories it is very important to determine, how the extremely precious
observing time can be used with maximum efficiency. The analysis should help to
increase the scientific yield in such cases significantly.Comment: 11 pages, 7 figures, Latex, to be published in Astronomy &
Astrophysic
Electron propagation in crossed magnetic and electric fields
Laser-atom interaction can be an efficient mechanism for the production of
coherent electrons. We analyze the dynamics of monoenergetic electrons in the
presence of uniform, perpendicular magnetic and electric fields. The Green
function technique is used to derive analytic results for the field--induced
quantum mechanical drift motion of i) single electrons and ii) a dilute Fermi
gas of electrons. The method yields the drift current and, at the same time it
allows us to quantitatively establish the broadening of the (magnetic) Landau
levels due to the electric field: Level number k is split into k+1 sublevels
that render the th oscillator eigenstate in energy space. Adjacent Landau
levels will overlap if the electric field exceeds a critical strength. Our
observations are relevant for quantum Hall configurations whenever electric
field effects should be taken into account.Comment: 11 pages, 2 figures, submitte
Arbitrary Dimensional Schwarzschild-FRW Black Holes
The metric of arbitrary dimensional Schwarzschild black hole in the
background of Friedman-Robertson-Walker universe is presented in the cosmic
coordinates system. In particular, the arbitrary dimensional Schwarzschild-de
Sitter metric is rewritten in the Schwarzschild coordinates system and basing
on which the even more generalized higher dimensional Schwarzschild-de Sitter
metric with another extra dimensions is found. The generalized solution shows
that the cosmological constant may roots in the extra dimensions of space.Comment: 10 page
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