Evolution of warped and twisted accretion discs in close binary systems

Astronomy Unit, Queen Mary, University of London, Mile End Road, London E1 4NS, UK e-mail: [M.Fragner;R.P.Nelson]@qmul.ac.uk

Received: 7 August 2009
Accepted: 2 October 2009

Abstract

Context. There are numerous examples of accretion discs in binary systems where the disc midplane is believed to be inclined relative to the binary orbit plane. Examples include the X-ray binaries Her X-1 and SS433, and the young stellar binary HK Tau. Under suitable physical conditions, such a configuration is expected to induce warping and rigid-body precession of the disc.

Aims. We aim to examine the detailed disc structure that arises in a misaligned binary system as a function of the disc aspect ratio h, viscosity parameter α, disc outer radius R, and binary inclination angle . We also aim to examine the conditions that lead to an inclined disc being disrupted by strong differential precession.

Methods. We use a grid-based hydrodynamic code to perform 3D simulations. This code has a relatively low numerical viscosity compared with the SPH schemes that have been used previously to study inclined discs. This allows the influence of viscosity on the disc evolution to be tightly controlled.

Results. We find that for thick discs (h=0.05) with low α, efficient warp communication in the discs allows them to precess as rigid bodies with very little warping or twisting. Such discs are observed to align with the binary orbit plane on the viscous evolution time. Thinner discs with higher viscosity, in which warp communication is less efficient, develop significant twists before achieving a state of rigid-body precession. Under the most extreme conditions we consider (h=0.01, and ), we find that discs can become broken or disrupted by strong differential precession. Discs that become highly twisted are observed to align with the binary orbit plane on timescales much shorter than the viscous timescale, possibly on the precession time.

Conclusions. We find agreement with previous studies that show that thick discs with low viscosity experience mild warping and precess rigidly. We also find that as h is decreased substantially, discs may be disrupted by strong differential precession, but for disc thicknesses that are significantly less (h=0.01) than those found in previous studies (h=0.03).