Philipp Podsiadlowski

Abstract:

We have imaged the X-ray binary SS433 with unprecedented Fourier-plane coverage at 6cm using simultaneously the VLBA, MERLIN, and the VLA, and also at 20cm with the VLBA. At both wavelengths we have securely detected smooth, low-surface brightness emission having the appearance of a `ruff' or collar attached perpendicularly to the well-studied knotty jets in this system, extending over at least a few hundred AU. We interpret this smooth emission as a wind-like outflow from the binary, and discuss its implications for the present evolutionary stage of this system.

Abstract:

We describe the theory and implementation of a three-dimensional fluid dynamics code which we have developed for calculating the surface geometry and circulation currents in the secondaries of interacting binary systems. The main method is based on an Eulerian-Lagrangian scheme to solve the advective and force terms in Euler's equation. Surface normalised spherical polar coordinates are used to allow the accurate modelling of the surface of the star, as is necessary when free surfaces and irradiation effects are to be considered. The potential and its gradient are expressed as sums of Legendre polynomials, which allows a very efficient solution of Poisson's equation. The basic solution scheme, based on operator splitting, is outlined, and standard numerical tests are presented.

Abstract:

We have searched for evidence of emission at 15 microns with ISOCAM and at 90 microns with ISOPHOT from dust orbiting six nearby pulsars, both in binaries and in isolation, located at distances between about 100 to 1000 pc. No emission was detected at any of the pulsar positions, and for the nearest pulsar J0108-1431 the 3 sigma upper limits on the flux density is about 66 mJy at 15 microns and 22.5 mJy at 90 microns. Upper limits on the masses of circumpulsar dust are inferred at a given temperature using a simple modelling of the radiated flux; they are compared to upper limits of orbiting mass obtained with the dust heating model of Foster & Fisher (1996). These results suggest that it is unlikely that any of them have sufficiently massive, circumpulsar discs out of which planets may form in the future.

Abstract:

As has only recently been recognized, X-ray binaries with intermediate-mass secondaries are much more important than previously believed. To assess the relative importance of low- and intermediate-mass X-ray binaries (LMXBs and IMXBs), we have initiated a systematic study of these systems consisting of two parts: an exploration of the evolution of LMXBs and IMXBs for a wide range of initial masses and orbital periods using detailed binary stellar evolution calculations, and an integration of these results into a Monte-Carlo binary population synthesis code. Here we present some of the main results of our binary calculations and some preliminary results of the population synthesis study for a ``standard'' reference model. While the inclusion of IMXBs improves the agreement with the observed properties of ``LMXBs'', several significant discrepancies remain, which suggests that additional physical processes need to be included in the model.

Abstract:

We present a method for the calculation of the effects of external irradiation on the geometrical shape of the secondary in a close binary containing a compact star, the source of the radiation, and a normal companion star, where we include the possibility of shadowing by an accretion disc. The model is based on a simple modification of the standard Roche binary potential in which the radiation-pressure force is parameterised using the ratio of the radiation to the gravitational force. We have constructed numerical solutions of approximate 3-dimensional irradiated equipotential surfaces to demonstrate the main geometrical effects of external radiation pressure. For systems in which the inner Lagrangian point is irradiated directly and for sufficiently high irradiation fluxes, the critical condition for which the secondary fills its tidal lobe changes from an inner to an outer critical configuration, where the critical equipotential surface connects to one of the outer rather than the inner Lagrangian point. Such a situation may apply to evaporating binary pulsar systems (e.g. PSR 1957+20), stars orbiting supermassive black holes in AGN and some high-mass X-ray binaries (including Centaurus X-3). For systems containing an accretion disc, which shadows the inner Lagrangian point from the external irradiation, the presence of significant radiation pressure causes a non-axisymmetric deformation of the stellar surface. We have calculated modified Roche potentials to determine the main effects on the optical lightcurves and radial velocity curves for typical binary parameters. Compared to previous studies, the inclusion of irradiation-pressure effects results in changes in the derived system parameters (e.g. component masses, radial velocities) that may be as high as 30 per cent.