Irradiation pressure effects in close binary systems

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 parametrized using the ratio of the radiation to the gravitational force. We have constructed numerical solutions of approximate three-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 inner Lagrangian points. 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 (HMXBs, 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. This has particularly important consequences for low-mass X-ray binaries, for which the X-ray luminosities can be close to the Eddington limit. We have calculated modified Roche potentials to determine the main effects on the optical light curves 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. We conclude that the proper inclusion of irradiation pressure effects is essential for a reliable analysis of close binary systems in which the secondaries are strongly irradiated.

Spectral modelling of massive binary systems

“…The external radiation pressure is more difficult to treat. We have based our method on the approach of Drechsel et al (1995) and Phillips & Podsiadlowski (2002). This method consists of scaling the mass of the companion in the Roche potential.…”

Thermal timescale mass transfer rates in intermediate-mass X-ray binaries

“…If, however, part of the lost mass forms a circumbinary disk rather than leaves the system (Soberman et al 1997), the disk would extract orbital angular momentum from the binary by tidal torque, enhancing the mass transfer rates and leading to more rapid orbital shrink (Spruit & Taam 2001). It should be also noted that, during the evolution of an IMXB the strong X-ray radiation by the accretor and the accretion disk could illuminate the donor star and cause expansion of the donor and strong stellar wind (Podsiadlowski 1991;Hameury et al 1993;Phillips & Podsiadlowski 2002), which would also lead to a higher mass transfer rate and shorter duration of mass transfer. Given that there is not a generally accepted theory on the irradiation effect, we have not included this in our calculations.…”

Critical study of the distribution of rotational velocities of Be stars

“…Owing to the mutual irradiation and radiation pressure that become significant in close binary systems (Cranmer 1993;Drechsel et al 1995;Phillips & Podsiadlowski 2002;Palate et al 2013), line spectra can change somewhat as a function of the orbital phase and inclination angle of the system. In this discussion we omit the mutual irradiation and radiation pressure.…”