Size of discs formed by wind accretion in binaries can be underestimated if the role of wind-driving force is ignored

Binary systems consisting of a secondary accreting form a wind-emitting primary are ubiquitous in astrophysics. The phenomenology of such Bondi–Hoyle–Lyttleton (BHL) accretors is particularly rich when an accretion disc forms around the secondary. The outer radius of such discs is commonly estimated from the net angular momentum produced by a density variation of material across the BHL or Bondi accretion cylinder, as the latter is tilted with respect to the direction to the primary due to orbital motion. But this approach has ignored the fact that the wind experiences an outward driving force that the secondary does not. In actuality, the accretion stream falls towards a retarded point in the secondary’s orbit as the secondary is pulled towards the primary relative to the stream. The result is a finite separation or ‘accretion stream impact parameter’ (ASIP) separating the secondary and stream. When the orbital radius ao exceeds the BHL radius rb, the ratio of outer disc radius estimated as the ASIP to the conventional estimate a1/2 o /r 1/2 b > 1. We therefore predict that discs will form at larger radii from the secondary than traditional estimates. This agrees with the importance of the ASIP emphasized by Huarte-Espinosa et al. and the practical consequence that resolving the initial outer radius of such an accretion disc in numerical simulations can be less demanding than what earlier estimates would suggest.