Close stellar encounters with planetesimal discs: the dynamics of asymmetry in the β Pictoris system

We numerically investigate the dynamics of how a close stellar fly-by encounter of a symmetrical circumstellar planetesimal disc can give rise to the many kinds of asymmetries and substructures attributed to the edge-on dusty disc of β Pictoris. In addition we present new optical coronagraphic observations of the outer parts of the disc of β Pic, and report that the radial extent is significantly greater than was found in previous measurements. The north-easterly extension of the disc's mid-plane is now measured out to 1835 au from the star; the south-westerly component is measured out to 1450 au. Thus the asymmetry in the length of the former with respect to the latter is approximately 25 per cent. We proceed to use the length asymmetry induced in the distribution of simulation test particles as the principal diagnostic feature when modelling the disc response in order to constrain fly-by parameters. In particular we favour a low-inclination prograde and near-parabolic orbit perturber of mass approximately 0.5 M⊙. These initial conditions suggest that the perturber could have been physically associated with β Pic prior to the encounter. Thus we also consider the possibility that the perturber could be bound to β Pic, a consideration also of general interest where dust discs are known to exist in binary star systems. We show that a further consequence of a low-velocity encounter is that the perturber could have captured planetesimals from the β Pic disc, and we deduce that as a result of this the perturber could display a dust disc that is presently amenable to observation. In some of our models, we can relate groupings of perturbed particles to the large-scale structure of the β Pic disc. The groupings correspond to high-eccentricity and high-inclination particles that reach apocentre and maximum height in the south-west, moderately eccentric and low-inclination particles that reach apocentre in the north-east, and relatively unperturbed particles inside ∼200 au radius.