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

TL;DR

This paper shows that neglecting the wind-driving force in binary accretion models leads to underestimating the size of accretion discs, and provides a revised estimate accounting for the accretion stream impact parameter.

Contribution

It introduces the concept of the accretion stream impact parameter (ASIP) and demonstrates that discs form at larger radii than traditional models predict, improving understanding of wind accretion in binaries.

Findings

Discs form at larger radii than traditional estimates suggest.

The accretion stream impact parameter (ASIP) is a key factor in disc size.

Resolving outer disc radii in simulations can be easier than previously thought.

Abstract

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 toward a retarded point in the secondary's orbit as the secondary is pulled toward 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 a_o exceeds the BHL radius r_b, the ratio of outer disc radius estimated as the ASIP to the conventional estimate a_o^{1/2}/r_b^{1/2}>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. (2013) 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.