Binary accretion rates: dependence on temperature and mass-ratio

TL;DR

This study uses 2D SPH simulations to analyze how gas temperature and binary mass ratio influence accretion rates onto binaries, providing a new parametrization relevant to black hole and stellar binary evolution.

Contribution

It introduces a simple, accurate parametrization of accretion rates that accounts for temperature effects, improving predictions for supermassive black hole and stellar binary growth.

Findings

Higher gas temperature increases primary accretion fraction.

Parametrization accurately predicts accretion rates within a few percent.

Minimum mass ratio for black hole spin alignment is raised to ~0.4.

Abstract

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 $\sim40\%$. We present a simple parametrisation for the average accretion rate of each binary component accurate to within a few percent and argue that this parametrisation (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.