672 research outputs found
Frequency and Phase Synchronization in Stochastic Systems
The phenomenon of frequency and phase synchronization in stochastic systems
requires a revision of concepts originally phrased in the context of purely
deterministic systems. Various definitions of an instantaneous phase are
presented and compared with each other with special attention payed to their
robustness with respect to noise. We review the results of an analytic approach
describing noise-induced phase synchronization in a thermal two-state system.
In this context exact expressions for the mean frequency and the phase
diffusivity are obtained that together determine the average length of locking
episodes. A recently proposed method to quantify frequency synchronization in
noisy potential systems is presented and exemplified by applying it to the
periodically driven noisy harmonic oscillator. Since this method is based on a
threshold crossing rate pioneered by S.O. Rice the related phase velocity is
termed Rice frequency. Finally, we discuss the relation between the phenomenon
of stochastic resonance and noise-enhanced phase coherence by applying the
developed concepts to the periodically driven bistable Kramers oscillator.Comment: to appear in the Chaos focus issue on "Control, communication, and
synchronization in chaotic dynamical systems
Experimental and Theoretical Determination of Forces and Moments on a Store and on a Store- Pylon Combination Mounted on a 45 Deg Swept- Wing-Fuselage Configuration at a Mach Number of 1.61
Forces and moments of store-pylon combination mounting on swept wing-fuselage configuration in supersonic pressure tunne
The MUCHFUSS photometric campaign
Hot subdwarfs (sdO/Bs) are the helium-burning cores of red giants, which lost
almost all of their hydrogen envelopes. This mass loss is often triggered by
common envelope interactions with close stellar or even substellar companions.
Cool companions like late-type stars or brown dwarfs are detectable via
characteristic light curve variations like reflection effects and often also
eclipses. To search for such objects we obtained multi-band light curves of 26
close sdO/B binary candidates from the MUCHFUSS project with the BUSCA
instrument. We discovered a new eclipsing reflection effect system
(~d) with a low-mass M dwarf companion ().
Three more reflection effect binaries found in the course of the campaign were
already published, two of them are eclipsing systems, in one system only
showing the reflection effect but no eclipses the sdB primary is found to be
pulsating. Amongst the targets without reflection effect a new long-period sdB
pulsator was discovered and irregular light variations were found in two sdO
stars. The found light variations allowed us to constrain the fraction of
reflection effect binaries and the substellar companion fraction around sdB
stars. The minimum fraction of reflection effect systems amongst the close sdB
binaries might be greater than 15\% and the fraction of close substellar
companions in sdB binaries might be as high as . This would result in a
close substellar companion fraction to sdB stars of about 3\%. This fraction is
much higher than the fraction of brown dwarfs around possible progenitor
systems, which are solar-type stars with substellar companions around 1 AU, as
well as close binary white dwarfs with brown dwarf companions. This might be a
hint that common envelope interactions with substellar objects are
preferentially followed by a hot subdwarf phase.Comment: accepted for A&
Tumore des biliären Trakts: Häufigkeit, Diagnostik und Therapie
Die malignen Tumoren des biliären Trakts, unterteilt in Gallenblasen- und Gallengangskarzinome, sind eher selten. Dennoch wurden in den vergangenen Jahren Fortschritt in Diagnostik und Therapie erzielt. Neben Verbesserung der operativen Techniken als einzige kurative Option konnte auch ein chemotherapeutischer Standard bei fortgeschrittenen Tumoren etabliert werden. Multimodal Konzepte stehen im Fokus des Forschungsinteresses
White dwarf and subdwarf stars in the Sloan Digital Sky Survey Data Release 14
White dwarfs carry information on the structure and evolution of the Galaxy,
especially through their luminosity function and initial-to-final mass
relation. Very cool white dwarfs provide insight into the early ages of each
population. Examining the spectra of all stars with proper motion in
the Sloan Digital Sky Survey Data Release 14, we report the classification for
20 088 spectroscopically confirmed white dwarfs, plus 415 hot subdwarfs, and
311 cataclysmic variables. We obtain Teff, log g and mass for hydrogen
atmosphere white dwarf stars (DAs), warm helium atmosphere white dwarfs (DBs),
hot subdwarfs (sdBs and sdOs), and estimate photometric Teff for white dwarf
stars with continuum spectra (DCs). We find 15793 sdAs and 447 dCs between the
white dwarf cooling sequence and the main sequence, especially below Teff=
10000 K; most are likely low-mass metal-poor main sequence stars, but some
could be the result of interacting binary evolution.Comment: 18 pages, 13 figure
The High A(V) Quasar Survey: Reddened quasi-stellar objects selected from optical/near-infrared photometry - II
Quasi-stellar objects (QSOs) whose spectral energy distributions (SEDs) are
reddened by dust either in their host galaxies or in intervening absorber
galaxies are to a large degree missed by optical color selection criteria like
the one used by the Sloan Digital Sky Survey (SDSS). To overcome this bias
against red QSOs, we employ a combined optical and near-infrared color
selection. In this paper, we present a spectroscopic follow-up campaign of a
sample of red candidate QSOs which were selected from the SDSS and the UKIRT
Infrared Deep Sky Survey (UKIDSS). The spectroscopic data and SDSS/UKIDSS
photometry are supplemented by mid-infrared photometry from the Wide-field
Infrared Survey Explorer. In our sample of 159 candidates, 154 (97%) are
confirmed to be QSOs. We use a statistical algorithm to identify sightlines
with plausible intervening absorption systems and identify nine such cases
assuming dust in the absorber similar to Large Magellanic Cloud sightlines. We
find absorption systems toward 30 QSOs, 2 of which are consistent with the
best-fit absorber redshift from the statistical modeling. Furthermore, we
observe a broad range in SED properties of the QSOs as probed by the rest-frame
2 {\mu}m flux. We find QSOs with a strong excess as well as QSOs with a large
deficit at rest-frame 2 {\mu}m relative to a QSO template. Potential solutions
to these discrepancies are discussed. Overall, our study demonstrates the high
efficiency of the optical/near-infrared selection of red QSOs.Comment: 64 pages, 18 figures, 16 pages of tables. Accepted to ApJ
Horizontal Branch evolution, metallicity and sdB stars
Context. Abundance anomalies have been observed in field sdB stars and in
nearly all Horizontal Branch (HB) stars of globular clusters with Teff > 11
000K whatever be the cluster metallicity. Aims. The aim is to determine the
abundance variations to be expected in sdB stars and in HB stars of
metallicities Z \geq 0.0001 and what observed abundances teach us about
hydrodynamical processes competing with atomic diffusion. Methods. Complete
stellar evolution models, including the effects of atomic diffusion and
radiative acceleration, have been computed from the zero age main-sequence for
metallicities of Z0 = 0.0001, 0.001, 0.004 and 0.02. On the HB the masses were
selected to cover the Teff interval from 7000 to 37000K. Some 60 evolutionary
HB models were calculated. The calculations of surface abundance anomalies
during the horizontal branch depend on one parameter, the surface mixed mass.
Results. For sdB stars with Teff 11 000K
in all observed clusters, independent of metallicity, it was found that most
observed abundance anomalies (even up to ~ x 200) were compatible, within error
bars, with expected abundances. A mixed mass of ~1.E-7 M\odot was determined by
comparison with observations. Conclusions. Observations of globular cluster HB
stars with Teff > 11 000K and of sdB stars with Teff < 37 000K suggest that
most observed abundance anomalies can be explained by element separation driven
by radiative acceleration occuring at a mass fraction of ~1.E-7 M\odot. Mass
loss or turbulence appear to limit the separation between 1.E-7 M\odot and the
surface.Comment: Accepted for publication by A&
First Passage Time Densities in Non-Markovian Models with Subthreshold Oscillations
Motivated by the dynamics of resonant neurons we consider a differentiable,
non-Markovian random process and particularly the time after which it
will reach a certain level . The probability density of this first passage
time is expressed as infinite series of integrals over joint probability
densities of and its velocity . Approximating higher order terms
of this series through the lower order ones leads to closed expressions in the
cases of vanishing and moderate correlations between subsequent crossings of
. For a linear oscillator driven by white or coloured Gaussian noise,
which models a resonant neuron, we show that these approximations reproduce the
complex structures of the first passage time densities characteristic for the
underdamped dynamics, where Markovian approximations (giving monotonous first
passage time distribution) fail
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