We perform a series of 2D smoothed particle hydrodynamics ( SPH ) simulations of gas accretion onto binaries via a circumbinary disc , for a range of gas temperatures and binary mass ratios ( q ) .
We show that increasing the gas temperature increases the accretion rate onto the primary for all values of the binary mass ratio : for example , for q = 0.1 and a fixed binary separation , an increase of normalised sound speed by a factor of 5 ( from our “ cold ” to “ hot ” simulations ) changes the fraction of the accreted gas that flows on to the primary from 10 \% to \sim 40 \% .
We present a simple parametrisation for the average accretion rate of each binary component accurate to within a few percent and argue that this parameterisation ( rather than those in the literature based on warmer simulations ) is relevant to supermassive black hole accretion and all but the widest stellar binaries .
We present trajectories for the growth of q during circumbinary disc accretion and argue that the period distribution of stellar “ twin ” binaries is strong evidence for the importance of circumbinary accretion .
We also show that our parametrisation of binary accretion increases the minimum mass ratio needed for spin alignment of supermassive black holes to q \sim 0.4 , with potentially important implications for the magnitude of velocity kicks acquired during black hole mergers .