14,543 research outputs found
Equicontinuous Families of Markov Operators in View of Asymptotic Stability
Relation between equicontinuity, the so called e property and stability of
Markov operators is studied. In particular, it is shown that any asymptotically
stable
Markov operator with an invariant measure such that the interior of its
support is nonempty satisfies the e property
Formation of wind-captured discs in Supergiant X-ray binaries : consequences for Vela X-1 and Cygnus X-1
In Supergiant X-ray binaries (SgXB), a compact object captures a fraction of
the wind of an O/B supergiant on a close orbit. Proxies exist to evaluate the
efficiency of mass and angular momentum accretion but they depend so
dramatically on the wind speed that given the current uncertainties, they only
set loose constrains. Furthermore, they often bypass the impact of orbital and
shock effects on the flow structure. We study the wind dynamics and the angular
momentum gained as the flow is accreted. We identify the conditions for the
formation of a disc-like structure around the accretor and the observational
consequences for SgXB. We use recent results on the wind launching mechanism to
compute 3D streamlines, accounting for the gravitational and X-ray ionizing
influence of the compact companion on the wind. Once the flow enters the Roche
lobe of the accretor, we solve the hydrodynamics equations with cooling. A
shocked region forms around the accretor as the flow is beamed. For wind speeds
of the order of the orbital speed, the shock is highly asymmetric compared to
the axisymmetric bow shock obtained for a purely planar homogeneous flow. With
net radiative cooling, the flow always circularizes for wind speeds low enough.
Although the donor star does not fill its Roche lobe, the wind can be
significantly beamed and bent by the orbital effects. The net angular momentum
of the accreted flow is then sufficient to form a persistent disc-like
structure. This mechanism could explain the proposed limited outer extension of
the accretion disc in Cygnus X-1 and suggests the presence of a disc at the
outer rim of the neutron star magnetosphere in Vela X-1, with dramatic
consequences on the spinning up of the accretor
The Driving of Hot Star Winds
In the regime of hot stars, winds were not seen as a common thing until the
era of UV astronomy. Since we have access to the UV wavelength range, it has
become clear that winds are not an exotic phenomenon limited to some special
objects, but actually ubiquitous among hot and massive stars. The opacities due
to spectral lines are the decisive ingredient that allows hot, massive stars to
launch powerful winds. While the fundamental principles of these so-called
line-driven winds have been realized decades ago, their proper quantitative
prediction is still a major challenge today. Established theoretical and
empirical descriptions have allowed us to make major progress on all
astrophysical scales. However, we are now reaching their limitations as we
still lack various fundamental insights on the nature of hot star winds,
thereby hampering us from drawing deeper conclusions, not least when dealing
with stellar or sub-stellar companions. This has spawned a new generation of
researchers searching for answers with a yet unprecedented level of detail in
observational and new theoretical approaches.
In these proceedings, the fundamental principles of driving hot star winds
will be briefly reviewed. Starting from the classical CAK theory and its
extensions, over Monte Carlo and recent comoving-frame-based simulations, the
different methods to describe and model the acceleration of hot star winds will
be introduced. The review continues with briefly discussing instabilities as
well as qualitative and quantitative insights for OB- and Wolf-Rayet-star
winds. Moreover, the challenges of companions and their impact on
radiation-driven winds are outlined.Comment: 14 pages, 4 figures, to be published in the Proceedings of the
International Astronomical Union for the IAU Symposium 370 "Winds of Stars
and Exoplanets" (eds. A.A. Vidotto, L. Fossati, J.S. Vink
Surface stress of Ni adlayers on W(110): the critical role of the surface atomic structure
Puzzling trends in surface stress were reported experimentally for Ni/W(110)
as a function of Ni coverage. In order to explain this behavior, we have
performed a density-functional-theory study of the surface stress and atomic
structure of the pseudomorphic and of several different possible 1x7
configurations for this system. For the 1x7 phase, we predict a different, more
regular atomic structure than previously proposed based on surface x-ray
diffraction. At the same time, we reproduce the unexpected experimental change
of surface stress between the pseudomorphic and 1x7 configuration along the
crystallographic surface direction which does not undergo density changes. We
show that the observed behavior in the surface stress is dominated by the
effect of a change in Ni adsorption/coordination sites on the W(110) surface.Comment: 14 pages, 3 figures Published in J. Phys.: Condens. Matter 24 (2012)
13500
A gobal fit to the anomalous magnetic moment, Higgs limit and b->s gamma in the constrained MSSM
New data on the anomalous magnetic moment of the muon together with the b->s
gamma decay rate and Higgs limits are considered within the supergravity
inspired constrained minimal supersymmetric model. We perform a global
statistical chi2 analysis of these data and show that the allowed region of
parameter space is bounded from below by the Higgs limit, which depends on the
trilinear coupling and from above by the anomalous magnetic moment.Comment: 3 pages, To appear in Proc. of SUSY01, Dubna (Russia
Population genetic structure in natural and reintroduced beaver (Castor fiber) populations in Central Europe
Castor fiber Linnaeus, 1758 is the only indigenous species of the genus Castor in Europe and Asia. Due to extensive hunting until the beginning of the 20th century, the distribution of the formerly widespread Eurasian beaver was dramatically reduced. Only a few populations remained and these were in isolated locations, such as the region of the German Elbe River. The loss of genetic diversity in small or captive populations throughgenetic drift and inbreeding is a severe conservation problem. However, the reintroduction of beaver populations from several regions in Europe has shown high viability and populations today are growing fast. In the present study we analysed the population genetic structure of a natural and two reintroduced beaver populations in Germany and Austria. Furthermore, we studied the genetic differentiation between two beaver species, C. fiber and the American beaver (C. canadensis), using RAPD (Random Amplified Polymorphic DNA) as a genetic marker. The reintroduced beaver populations of different origins and the autochthonous population of the Elbe River showed a similar low genetic heterogeneity. There was an overall high genetic similarity in the species C. fiber, and no evidence was found for a clear subspecific structure in the populations studied
Self-similar decay of high Reynolds number Taylor-Couette turbulence
We study the decay of high-Reynolds number Taylor-Couette turbulence, i.e.
the turbulent flow between two coaxial rotating cylinders. To do so, the
rotation of the inner cylinder (Re, the outer cylinder is at
rest) is stopped within 12 s, thus fully removing the energy input to the
system. Using a combination of laser Doppler anemometry and particle image
velocimetry measurements, six decay decades of the kinetic energy could be
captured. First, in the absence of cylinder rotation, the flow-velocity during
the decay does not develop any height dependence in contrast to the well-known
Taylor vortex state. Second, the radial profile of the azimuthal velocity is
found to be self-similar. Nonetheless, the decay of this wall-bounded
inhomogeneous turbulent flow does not follow a strict power law as for decaying
turbulent homogeneous isotropic flows, but it is faster, due to the strong
viscous drag applied by the bounding walls. We theoretically describe the decay
in a quantitative way by taking the effects of additional friction at the walls
into account.Comment: 7 pages, 6 figure
Near-Infrared Kinetic Spectroscopy of the HO_2 and C_2H_5O_2 Self-Reactions and Cross Reactions
The self-reactions and cross reactions of the peroxy radicals HO_2 and C_2H_5O_2 and HO_2 were monitored using simultaneous independent spectroscopic probes to observe each radical species. Wavelength modulation (WM) near-infrared (NIR) spectroscopy was used to detect HO_2, and UV absorption monitored HO_2 and C_2H_5O_2. The temperature dependences of these reactions were investigated over a range of interest to tropospheric chemistry, 221−296 K. The Arrhenius expression determined for the cross reaction, k_2(T) = (6.01^(+1.95)_(−1.47)) × 10^(−13) exp((638 ± 73)/T) cm^3 molecules^(−1) s^(−1) is in agreement with other work from the literature. The measurements of the HO_2 self-reaction agreed with previous work from this lab and were not further refined.(1) The C_2H_5O_2 self-reaction is complicated by secondary production of HO_2. This experiment performed the first direct measurement of the self-reaction rate constant, as well as the branching fraction to the radical channel, in part by measurement of the secondary HO_2. The Arrhenius expression for the self-reaction rate constant is k_3(T) = (1.29^(+0.34)_(−0.27)) × 10^(−13)exp((−23 ± 61)/T) cm^3 molecules^(−1) s^(−1), and the branching fraction value is α = 0.28 ± 0.06, independent of temperature. These values are in disagreement with previous measurements based on end product studies of the branching fraction. The results suggest that better characterization of the products from RO_2 self-reactions are required
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