532 research outputs found

### Compositional Model Repositories via Dynamic Constraint Satisfaction with Order-of-Magnitude Preferences

The predominant knowledge-based approach to automated model construction,
compositional modelling, employs a set of models of particular functional
components. Its inference mechanism takes a scenario describing the constituent
interacting components of a system and translates it into a useful mathematical
model. This paper presents a novel compositional modelling approach aimed at
building model repositories. It furthers the field in two respects. Firstly, it
expands the application domain of compositional modelling to systems that can
not be easily described in terms of interacting functional components, such as
ecological systems. Secondly, it enables the incorporation of user preferences
into the model selection process. These features are achieved by casting the
compositional modelling problem as an activity-based dynamic preference
constraint satisfaction problem, where the dynamic constraints describe the
restrictions imposed over the composition of partial models and the preferences
correspond to those of the user of the automated modeller. In addition, the
preference levels are represented through the use of symbolic values that
differ in orders of magnitude

### 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

### Numerical simulations of stellar winds: polytropic models

We discuss steady-state transonic outflows obtained by direct numerical
solution of the hydrodynamic and magnetohydrodynamic equations. We make use of
the Versatile Advection Code, a software package for solving systems of
(hyperbolic) partial differential equations. We proceed stepwise from a
spherically symmetric, isothermal, unmagnetized, non-rotating Parker wind to
arrive at axisymmetric, polytropic, magnetized, rotating models. These
represent 2D generalisations of the analytical 1D Weber-Davis wind solution,
which we obtain in the process. Axisymmetric wind solutions containing both a
`wind' and a `dead' zone are presented.
Since we are solving for steady-state solutions, we efficiently exploit fully
implicit time stepping. The method allows us to model thermally and/or
magneto-centrifugally driven stellar outflows. We particularly emphasize the
boundary conditions imposed at the stellar surface. For these axisymmetric,
steady-state solutions, we can use the knowledge of the flux functions to
verify the physical correctness of the numerical solutions.Comment: 11 pages, 6 figures, accepted for Astron. Astrophys. 342, to appear
199

### Wind Roche lobe overflow in high mass X-ray binaries : a possible mass transfer mechanism for Ultraluminous X-ray sources

Ultra-luminous X-ray sources (ULX) have so high X-ray luminosities that they
were long thought to be accreting intermediate mass black holes. Yet, some ULX
have been shown to display periodic modulations and coherent pulsations,
suggestive of a neutron star in orbit around a stellar companion and accreting
at super-Eddington rates. In this letter, we propose that the mass transfer in
ULX could be qualitatively the same as in Supergiant X-ray binaries (SgXB),
with a wind from the donor star highly beamed towards the compact object. Since
the star does not fill its Roche lobe, this mass transfer mechanism known as
"wind Roche lobe overflow" can remain stable even for large mass ratios. Based
on realistic acceleration profiles derived from spectral observations and
modeling of the stellar wind, we compute the bulk motion of the wind to
evaluate the fraction of the stellar mass outflow captured by the compact
object. We identify the orbital and stellar conditions for a SgXB to transfer
mass at rates matching the expectations for ULX and show that the transition
from SgXB to ULX luminosity levels is progressive. These results indicate that
a high stellar Roche lobe filling factor is not necessary to funnel large
quantities of material into the Roche lobe of the accretor. Large stellar mass
loss rates such as the ones from the Wolf-Rayet star in M101 ULX-1 or the late
B9 Supergiant in NGC 7793 P13 are enough to lead to a highly beamed wind and a
significantly enhanced mass transfer rate

### Evolution of Magnetic Fields in Supernova Remnants

Supernova remnants (SNR) are now widely believed to be a source of cosmic
rays (CRs) up to an energy of 1 PeV. The magnetic fields required to accelerate
CRs to sufficiently high energies need to be much higher than can result from
compression of the circumstellar medium (CSM) by a factor 4, as is the case in
strong shocks. Non-thermal synchrotron maps of these regions indicate that
indeed the magnetic field is much stronger, and for young SNRs has a dominant
radial component while for old SNRs it is mainly toroidal. How these magnetic
fields get enhanced, or why the field orientation is mainly radial for young
remnants, is not yet fully understood. We use an adaptive mesh refinement MHD
code, AMRVAC, to simulate the evolution of supernova remnants and to see if we
can reproduce a mainly radial magnetic field in early stages of evolution. We
follow the evolution of the SNR with three different configurations of the
initial magnetic field in the CSM: an initially mainly toroidal field, a
turbulent magnetic field, and a field parallel to the symmetry axis. Although
for the latter two topologies a significant radial field component arises at
the contact discontinuity due to the Rayleigh-Taylor instability, no radial
component can be seen out to the forward shock. Ideal MHD appears not
sufficient to explain observations. Possibly a higher compression ratio and
additional turbulence due to dominant presence of CRs can help us to better
reproduce the observations in future studies.Comment: 5 pages, 3 figures. To appear in conference proceedings of "Magnetic
Fields in the Universe II" (2008), RevMexA

### Convective magneto-rotational instabilities in accretion disks

We present a study of instabilities occuring in thick magnetized accretion
disks. We calculate the growth rates of these instabilities and characterise
precisely the contribution of the magneto-rotational and the convective
mechanism. All our calculations are performed in radially stratified disks in
the cylindrical limit. The numerical calculations are performed using the
appropriate local dispersion equation solver discussed in Blokland et al.
(2005). A comparison with recent results by Narayan et al. (2002) shows
excellent agreement with their approximate growth rates only if the disks are
weakly magnetized. However, for disks close to equipartition, the dispersion
equation from Narayan et al. (2002) loses its validity. Our calculations allow
for a quantitative determination of the increase of the growth rate due to the
magneto-rotational mechanism. We find that the increase of the growth rate for
long wavelength convective modes caused by this mechanism is almost neglible.
On the other hand, the growth rate of short wavelength instabilities can be
significantly increased by this mechanism, reaching values up to 60%.Comment: 10 pages, 9 figures, Accepted for publication in Astronomy &
Astrophysic

### 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

### Effect of disorder on the thermal transport and elastic properties in thermoelectric Zn4Sb3

Zn4Sb3 undergoes a phase transition from alpha to beta phase at T1[approximate]250 K. The high temperature beta-Zn4Sb3 phase has been widely investigated as a potential state-of-the-art thermoelectric (TE) material, due to its remarkably low thermal conductivity. We have performed electronic and thermal transport measurements exploring the structural phase transition at 250 K. The alpha to beta phase transition manifests itself by anomalies in the resistivity, thermopower, and specific heat at 250 K as well as by a reduction in the thermal conductivity as Zn4Sb3 changes phase from the ordered alpha to the disordered beta-phase. Moreover, measurements of the elastic constants using resonant ultrasound spectroscopy (RUS) reveal a dramatic softening at the order-disorder transition upon warming. These measurements provide further evidence that the remarkable thermoelectric properties of beta-Zn4Sb3 are tied to the disorder in the crystal structure

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