Hydrodynamics of Circumbinary Accretion: Angular Momentum Transfer and Binary Orbital Evolution

Abstract

We carry out 2D viscous hydrodynamical simulations of circumbinary accretion using the moving-mesh code AREPO. We self-consistently compute the accretion flow over a wide range of spatial scales, from the circumbinary disk (CBD) far from the central binary, through accretion streamers, to the disks around individual binary components, resolving the flow down to 2% of the binary separation. We focus on equal-mass binaries with arbitrary eccentricities. We evolve the flow over long (viscous) timescales until a quasi-steady state is reached, in which the mass supply rate at large distancesṀ 0 (assumed constant) equals the time-averaged mass transfer rate across the disk and the total mass accretion rate onto the binary components. This quasi-steady state allows us to compute the secular angular momentum transfer rate onto the binary, J b , and the resulting orbital evolution. Through direct computation of the gravitational and accretional torques on the binary, we find that J b is consistently positive (i.e., the binary gains angular momentum), with l 0 ≡ J b /Ṁ 0 in the range of (0.4 − 0.8)a 2 b Ω b , depending on the binary eccentricity (where a b , Ω b are the binary semi-major axis and angular frequency); we also find that this J b is equal to the net angular momentum current across the CBD, indicating that global angular momentum balance is achieved in our simulations. In addition, we compute the time-averaged rate of change of the binary orbital energy for eccentric binaries, and thus obtain the secular rates ȧ b and ė b . In all cases, ȧ b is positive, i.e., the binary expands while accreting. We discuss the implications of our results for the merger of supermassive binary black holes and for the formation of close stellar binaries.

The Birth of a Massive First-star Binary

“…On the contrary, the massive binary has a large separation as a result of the accretion of high angular momentum gas from the cloud. In fact, we have seen that the separation of the binary increases as it accretes the gas from the circum-binary disk, as suggested in recent simulations of binary accretion (Muñoz et al 2019;Moody et al 2019;Duffell et al 2019). The late-time disk fragmentation leading to such multiple systems may play some roles in formation of close binaries, which can evolve to the progenitors of BH merger events or X-ray binaries.…”

Circumbinary Accretion from Finite and Infinite Disks

“…7; the values of l 0 lie in the range [0.65, 0.85]a 2 b Ω b for all mass ratios explored. As in Muñoz et al (2019), we find that the time-averaged spin torques Ṡ 1 and Ṡ 2 are much smaller than L b (the contribution of Ṡ 1,2 to J b is 3% − 5% for r acc = 0.03a b ).…”

“…While the angular momentum transfer rate of CBDs around circular binaries was consistent with Miranda et al (2017), that of eccentric binaries exhibited significant discrepancies, highlighting the limitations of simulations that exclude the binary cavity. Most importantly, Muñoz et al (2019) showed that the (time-averaged) angular momentum current through the CBD J d is in agreement with the total torque acting directly on the binary J b (including both gravitational and accretion torques), confirming that their simulations are in (quasi-) steady state and that accreting binaries gain angular momentum from the disk. These results have been subsequently confirmed (and extended to 3D inclined disks) in an independent study by Moody et al (2019) using ATHENA++ (Stone et al 2008;White et al 2016).…”

“…where the specific torque due to external forces dl b /dt| ext (including both gravity and accretion) is computed directly from simulation output, as areṀ 1 andṀ 2 (see Muñoz et al 2019, for more details). Just like the binary accretion rateṀ b , the transfer rate of angular momentumJ b also reaches a stationary behavior without long-term trends.…”

“…It is natural to ask what happens to a binary that accretes from a finite disk/torus. In Muñoz et al (2019), we hypothesized that the secular angular momentum transfer rate J b should still follow the net mass accretion rate Ṁ b , provided that the latter changes slowly in time.…”