532 research outputs found

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

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    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

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    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

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    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

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    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

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    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

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    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

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    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

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    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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