61,159 research outputs found
Two-temperature coronal flow above a thin disk
We extended the disk corona model (Meyer & Meyer-Hofmeister 1994; Meyer, Liu,
& Meyer-Hofmeister 2000a) to the inner region of galactic nuclei by including
different temperatures in ions and electrons as well as Compton cooling. We
found that the mass evaporation rate and hence the fraction of accretion energy
released in the corona depend strongly on the rate of incoming mass flow from
outer edge of the disk, a larger rate leading to more Compton cooling, less
efficient evaporation and a weaker corona. We also found a strong dependence on
the viscosity, higher viscosity leading to an enhanced mass flow in the corona
and therefore more evaporation of gas from the disk below. If we take accretion
rates in units of the Eddington rate our results become independent on the mass
of the central black hole. The model predicts weaker contributions to the hard
X-rays for objects with higher accretion rate like narrow-line Seyfert 1
galaxies (NLS1s), in agreement with observations. For luminous active galactic
nuclei (AGN) strong Compton cooling in the innermost corona is so efficient
that a large amount of additional heating is required to maintain the corona
above the thin disk.Comment: 17 pages, 6 figures. ApJ accepte
Properties of Intercalated 2H-NbSe2, 4Hb-TaS2 and 1T-TaS2
The layered compounds 2H-NbSe, 24Hb-TaS, 2and 1T-TaS2 have been intercalated with organic molecules; and the resulting crystal structure, heat capacity, conductivity, and superconductivity have been studied. The coordination in the disulfide layers was found to be unchanged in the product phase. Resistance minima appear and the superconducting transition temperature is reduced in the NbSe2 complex. Conversely, superconductivity is induced in the 4Hb-TaS2 complex. Corresponding evidence of a large change of the density of states, negative for 2H-NbSe2 and positive for 4Hb-TaS2, was also observed upon intercalation. The transport properties of all the intercalation complexes show a pronounced dependence upon the coordination of the transition metal
Evolution of Supermassive Black Hole Binary and Acceleration of Jet Precession in Galactic Nuclei
Supermassive black hole binary (SMBHB) is expected with the hierarchical
galaxy formation model. Currently, physics processes dominating the evolution
of a SMBHB are unclear. An interesting question is whether we could
observationally determine the evolution of SMBHB and give constraints on the
physical processes. Jet precession have been observed in many AGNs and
generally attributed to disk precession. In this paper we calculate the time
variation of jet precession and conclude that jet precession is accelerated in
SMBHB systems but decelerated in others. The acceleration of jet precession
is related to jet precession timescale and
SMBHB evolution timescale , . Our calculations based on the models
for jet precession and SMBHB evolution show that can be as
high as about with a typical value -0.2 and can be easily detected. We
discussed the differential jet precession for NGC1275 observed in the
literature. If the observed rapid acceleration of jet precession is true, the
jet precession is due to the orbital motion of an unbound SMBHB with mass ratio
. When jets precessed from the ancient bubbles to the currently
active jets, the separation of SMBHB decrease from about to
with an averaged decreasing velocity and evolution timescale . However, if we assume a steady jet precession for many cycles,
the observations implies a hard SMBHB with mass ratio and
separation .Comment: 29 pages, no figure, Accepted for publication in Ap
Acceleration of weakly collisional solar-type winds
One of the basic properties of the solar wind, that is the high speed of the
fast wind, is still not satisfactorily explained. This is mainly due to the
theoretical difficulty of treating weakly collisional plasmas. The fluid
approach implies that the medium is collision dominated and that the particle
velocity distributions are close to Maxwellians. However the electron velocity
distributions observed in the solar wind depart significantly from Maxwellians.
Recent kinetic collisionless models (called exospheric) using velocity
distributions with a suprathermal tail have been able to reproduce the high
speeds of the fast solar wind. In this letter we present new developments of
these models by generalizing them over a large range of corona conditions. We
also present new results obtained by numerical simulations that include
collisions. Both approaches calculate the heat flux self-consistently without
any assumption on the energy transport. We show that both approaches - the
exospheric and the collisional one - yield a similar variation of the wind
speed with the basic parameters of the problem; both produce a fast wind speed
if the coronal electron distribution has a suprathermal tail. This suggests
that exospheric models contain the necessary ingredients for the powering of a
transonic stellar wind, including the fast solar one.Comment: Accepted for publication in The Astrophysical Journal Letters
(accepted: 13 May 2005
Heavy Meson Production in NN Collisions with Polarized Beam and Target -- A new facility for COSY
The study of near--threshold meson production in pp and pd collisions
involving polarized beams and polarized targets offers the rare opportunity to
gain insight into short--range features of the nucleon--nucleon interaction.
The Cooler Synchrotron COSY at FZ--J\"ulich is a unique environment to perform
such studies. Measurements of polarization observables require a cylindrically
symmetrical detector, capable to measure the momenta and the directions of
outgoing charged hadrons. The wide energy range of COSY leads to momenta of
outgoing protons to be detected in a single meson production reaction between
300 and 2500 MeV/c. Scattering angles of protons to be covered extend to about
in the laboratory system. An azimuthal angular coverage of the
device around 98% seems technically achievable. The required magnetic
spectrometer could consist of a superconducting toroid, providing fields around
3 T.Comment: 6 pages, 1 figure, submitted to Czechoslovak Journal of Physic
Emergence of foams from the breakdown of the phase field crystal model
The phase field crystal (PFC) model captures the elastic and topological
properties of crystals with a single scalar field at small undercooling. At
large undercooling, new foam-like behavior emerges. We characterize this foam
phase of the PFC equation and propose a modified PFC equation that may be used
for the simulation of foam dynamics. This minimal model reproduces von
Neumann's rule for two-dimensional dry foams, and Lifshitz-Slyozov coarsening
for wet foams. We also measure the coordination number distribution and find
that its second moment is larger than previously-reported experimental and
theoretical studies of soap froths, a finding that we attribute to the wetness
of the foam increasing with time.Comment: 4 pages, 4 figure
On the turbulent -disks and the intermittent activity in AGN
We consider effects of the MHD turbulence on the viscosity during the
evolution of the thermal-viscous ionization instability in the standard
-accretion disks. We consider the possibility that the accretion onto a
supermassive black hole proceeds through an outer standard accretion disk and
inner, radiatively inefficient and advection dominated flow. In this scenario
we follow the time evolution of the accretion disk in which the viscosity
parameter is constant throughout the whole instability cycle, as
implied by the strength of MHD turbulence. We conclude that the hydrogen
ionization instability is a promising mechanism to explain the intermittent
activity in AGN.Comment: 13 pages, 9 figures; ApJ accepte
A Local One-Zone Model of MHD Turbulence in Dwarf Nova Disks
The evolution of the magnetorotational instability (MRI) during the
transition from outburst to quiescence in a dwarf nova disk is investigated
using three-dimensional MHD simulations. The shearing box approximation is
adopted for the analysis, so that the efficiency of angular momentum transport
is studied in a small local patch of the disk: this is usually referred as to a
one-zone model. To take account of the low ionization fraction of the disk, the
induction equation includes both ohmic dissipation and the Hall effect. We
induce a transition from outburst to quiescence by an instantaneous decrease of
the temperature. The evolution of the MRI during the transition is found to be
very sensitive to the temperature of the quiescent disk. As long as the
temperature is higher than a critical value of about 2000 K, MHD turbulence and
angular momentum transport is sustained by the MRI. However, MHD turbulence
dies away within an orbital time if the temperature falls below this critical
value. In this case, the stress drops off by more than 2 orders of magnitude,
and is dominated by the Reynolds stress associated with the remnant motions
from the outburst. The critical temperature depends slightly on the distance
from the central star and the local density of the disk.Comment: 20 pages, 2 tables, 6 figures, accepted for publication in Ap
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