9,988 research outputs found
On the Azimuthal Stability of Shock Waves around Black Holes
Analytical studies and numerical simulations of time dependent axially
symmetric flows onto black holes have shown that it is possible to produce
stationary shock waves with a stable position both for ideal inviscid and for
moderately viscous accretion disks.
We perform several two dimensional numerical simulations of accretion flows
in the equatorial plane to study shock stability against non-axisymmetric
azimuthal perturbations. We find a peculiar new result. A very small
perturbation seems to produce an instability as it crosses the shock, but after
some small oscillations, the shock wave suddenly transforms into an asymmetric
closed pattern, and it stabilizes with a finite radial extent, despite the
inflow and outflow boundary conditions are perfectly symmetric. The main
characteristics of the final flow are: 1) The deformed shock rotates steadily
without any damping. It is a permanent feature and the thermal energy content
and the emitted energy vary periodically with time. 2) This behavior is also
stable against further perturbations. 3) The average shock is still very strong
and well defined, and its average radial distance is somewhat larger than that
of the original axially symmetric circular shock. 4) Shocks obtained with
larger angular momentum exhibit more frequencies and beating phenomena. 5) The
oscillations occur in a wide range of parameters, so this new effect may have
relevant observational consequences, like (quasi) periodic oscillations, for
the accretion of matter onto black holes. Typical time scales for the periods
are 0.01 and 1000 seconds for black holes with 10 and 1 million solar mass,
respectively.Comment: 15 pages, 7 figures, accepted by the Astrophysical Journa
Delocalized Entanglement of Atoms in optical Lattices
We show how to detect and quantify entanglement of atoms in optical lattices
in terms of correlations functions of the momentum distribution. These
distributions can be measured directly in the experiments. We introduce two
kinds of entanglement measures related to the position and the spin of the
atoms
Stellar winds, dead zones, and coronal mass ejections
Axisymmetric stellar wind solutions are presented, obtained by numerically
solving the ideal magnetohydrodynamic (MHD) equations. Stationary solutions are
critically analysed using the knowledge of the flux functions. These flux
functions enter in the general variational principle governing all axisymmetric
stationary ideal MHD equilibria. The magnetized wind solutions for
(differentially) rotating stars contain both a `wind' and a `dead' zone. We
illustrate the influence of the magnetic field topology on the wind
acceleration pattern, by varying the coronal field strength and the extent of
the dead zone. This is evident from the resulting variations in the location
and appearance of the critical curves where the wind speed equals the slow,
Alfven, and fast speed. Larger dead zones cause effective, fairly isotropic
acceleration to super-Alfvenic velocities as the polar, open field lines are
forced to fan out rapidly with radial distance. A higher field strength moves
the Alfven transition outwards. In the ecliptic, the wind outflow is clearly
modulated by the extent of the dead zone. The combined effect of a fast stellar
rotation and an equatorial `dead' zone in a bipolar field configuration can
lead to efficient thermo-centrifugal equatorial winds. Such winds show both a
strong poleward collimation and some equatorward streamline bending due to
significant toroidal field pressure at mid-latitudes. We discuss how coronal
mass ejections are then simulated on top of the transonic outflows.Comment: scheduled for Astrophys. J. 530 #2, Febr.20 2000 issue. 9 figures (as
6 jpeg and 8 eps files
The Diverse Properties of GPS Sources
We discuss the morphology and kinematics of five gigahertz-peaked spectrum
(GPS) sources that have been observed with the VLBA. We find a wide range of
observed properties including core-jet structure, superluminal motion,
variability, extended structure, and polarization, all of which appear to
deviate from commonly-accepted GPS paradigms (e.g., O'Dea 1998). We suggest
that the observed low frequency cutoff in GPS sources may be primarily due to
free-free absorption rather than synchrotron self-absorption.Comment: Proceedings of the 6th European VLBI Network Symposium, Ros E.,
Porcas R.W., Lobanov, A.P., & Zensus, J.A. (eds), MPIfR, Bonn, Germany. (4
pages, 5 figures, needs evn2002.cls style file
Life Cycle Assessment, Life cycle thinkingβ Hungarian case study
LCA and life cycle thinking are used in a variety of applications to help decision. Both of them enable identification and quantification of environmental impacts of a product, process or human activity. This study gives a summary about Hungarian LCA practice and shows trends of further development, and some practical applications from environmental impact of the municipal waste treatment to evaluate regional sustainability.ΠΡΠ΅Π½ΠΊΠ° ΠΆΠΈΠ·Π½Π΅Π½Π½ΠΎΠ³ΠΎ ΡΠΈΠΊΠ»Π° ΠΈ ΡΠ°Π·ΠΌΡΡΠ»Π΅Π½ΠΈΠ΅ ΠΎ ΠΆΠΈΠ·Π½Π΅Π½Π½ΠΎΠΌ ΡΠΈΠΊΠ»Π΅ ΠΊΠ°ΠΊ ΡΡΠ΅Π΄ΡΡΠ²Π΅, ΠΊΠΎΡΠΎΡΡΠ΅ ΡΠΏΠΎΡΠΎΠ±ΡΠ²ΡΡΡ ΠΏΡΠΈΠ½ΡΡΠΈΡ ΠΏΡΠ°Π²ΠΈΠ»ΡΠ½ΠΎΠ³ΠΎ ΡΠ΅ΡΠ΅Π½ΠΈΡ. ΠΠ½ΠΈ ΠΎΠ±Π° ΠΏΠΎΠ·Π²ΠΎΠ»ΡΡΡ ΠΈΠ΄Π΅Π½ΡΠΈΡΠΈΠΊΠ°ΡΠΈΡ ΠΈ ΠΎΠΏΡΠ΅Π΄Π΅Π»Π΅Π½ΠΈΠ΅ ΠΊΠΎΠ»ΠΈΡΠ΅ΡΡΠ²Π° Π²ΠΎΠ·Π΄Π΅ΠΉΡΡΠ²ΠΈΠΉ Π½Π° ΠΎΠΊΡΡΠΆΠ°ΡΡΡΡ ΡΡΠ΅Π΄Ρ ΠΏΡΠΎΠ΄ΡΠΊΡΠ°, ΠΏΡΠΎΡΠ΅ΡΡΠ° ΠΈΠ»ΠΈ Π΄Π΅ΡΡΠ΅Π»ΡΠ½ΠΎΡΡΠΈ ΡΠ΅Π»ΠΎΠ²Π΅ΠΊΠ°. Π‘ΡΠ°ΡΡΡ ΠΎΠΏΠΈΡΡΠ²Π°Π΅Ρ ΠΈΡΡΠ»Π΅Π΄ΠΎΠ²Π°Π½ΠΈΠ΅ Π²Π΅Π½Π³Π΅ΡΡΠΊΠΎΠΉ ΠΏΡΠ°ΠΊΡΠΈΠΊΠΈ LCA ΠΈ ΠΏΠΎΠΊΠ°Π·ΡΠ²Π°Π΅Ρ ΡΠ΅Π½Π΄Π΅Π½ΡΠΈΠΈ Π΄Π°Π»ΡΠ½Π΅ΠΉΡΠ΅Π³ΠΎ ΡΠ°Π·Π²ΠΈΡΠΈΡ, ΠΈ ΠΎΠΏΠΈΡΡΠ²Π°Π΅Ρ Π½Π΅ΠΊΠΎΡΠΎΡΡΠ΅ ΠΏΡΠ°ΠΊΡΠΈΡΠ΅ΡΠΊΠΈΠ΅ ΠΏΡΠΈΠΌΠ΅ΡΡ ΠΎΡ ΠΎΡΠ΅Π½ΠΊΠΈ Π²ΠΎΠ·Π΄Π΅ΠΉΡΡΠ²ΠΈΡ ΡΡΠΈΠ»ΠΈΠ·Π°ΡΠΈΠΈ ΠΊΠΎΠΌΠΌΡΠ½Π°Π»ΡΠ½ΡΡ
ΠΎΡΡ
ΠΎΠ΄ΠΎΠ²
The QCD equation of state at finite T/\mu on the lattice
We present N_t=4 lattice results for the equation of state of 2+1 flavour
staggered, dynamical QCD at finite temperature and chemical potential. We use
the overlap improving multi-parameter reweighting technique to extend the
equation of state for non-vanishing chemical potentials. The results are
obtained along the line of constant physics. Our physical parameters extend in
temperature and baryon chemical potential upto \approx 500-600 MeV.Comment: 13 pages 9 figures, talk given at Finite Density QCD at Nara, Nara,
Japan, 10-12 July 200
- β¦