Preferential accretion of binary stars

The attractive properties of gravity enable matter in dense cores to collapse into stars, spanning seven orders of magnitude in space and time, which makes modelling star formation a challenging multi-scale process. To circumvent this scale problem, stars are replaced by a sub-grid sink particle at a much larger scale. Sink particles are created above a threshold density and acquire mass and momentum through accretion. In models where binary star systems form and migrate to separations of a few cells, the accretion flow is unresolved and the relative accretion rate to the sink particles may become inaccurate. We introduce a new recipe for accretion onto binary sink particles that have overlapping accretion regions, and we implement an algorithm to track the angular momentum of sink particles as a proxy for stellar spin. Our preferential binary accretion recipe uses a virtual binary sink particle for the purpose of accretion and redistributes the accreted mass onto the sink particles according to results from models investigating binary accretion in detail. This solves problems common to current algorithms in many codes: (i) accretion is not suppressed due to large velocity differences between gas and stars, when that velocity is only internal to the binary system; (ii) the accretion rates are smoother for the unresolved close binaries in eccentric orbits; and (iii) non-physical suppression of accretion onto the secondary sink particle, when the primary dominates the potential, is eliminated. We test our implementation by comparing simulations at increasing resolution until the binaries are resolved. While not perfect, the algorithm mitigates undesired properties of current algorithms and is particularly useful for global models of star-forming regions. It may also be applied to other unresolved accreting binaries, such as compact objects in evolved star clusters and binary supermassive black holes in cosmological models.