Three-Dimensional MHD Simulation of Circumbinary Accretion Disks. Ii. Net Accretion Rate

Abstract

When an accretion disk surrounds a binary rotating in the same sense, the binary exerts strong torques on the gas. Analytic work in the 1D approximation indicated that these torques sharply diminish or even eliminate accretion from the disk onto the binary. However, recent 2D and 3D simulational work has shown at most modest diminution. We present new MHD simulations demonstrating that for binaries with mass ratios of 1 and 0.1 there is essentially no difference between the accretion rate at large radius in the disk and the accretion rate onto the binary. To resolve the discrepancy with earlier analytic estimates, we identify the small subset of gas trajectories traveling from the inner edge of the disk to the binary and show how the full accretion rate is concentrated onto them as a result of stream-disk shocks driven by the binary torques.

On the Eccentricity Excitation in Post-Main-Sequence Binaries

“…Previously, the reaccretion of the metal-poor gas from the circumbinary disk, in which the refractory component has condensed into dust grains, has been proposed (Waters et al 1992) to explain the extreme Fe depletion seen in many binary post-AGB stars (Van Winckel et al 1995;Maas et al 2005). Efficient accretion from the circumbinary disk is also seen in numerical simulations (MacFadyen & Milosavljević 2008;Farris et al 2014;Shi & Krolik 2015;Muñoz & Lai 2016). Thus, reaccretion should be an important factor in the binary-disk coupling.…”

Accretion and Orbital Inspiral in Gas-Assisted Supermassive Black Hole Binary Mergers

“…Over limited amount of time ( ) n t r c the inner disk should be depleted, reducingṀ b and bringing it in equilibrium with˙¥ M , as previously suggested by Shi & Krolik (2015). This convergence was not observed in simulations of Farris et al (2014) and Shi & Krolik (2015) because of their short duration, althoughṀ b does show signs of a slow decrease in the longer runs of Farris et al (2014).…”

“…Here, we merely caution that the enhancement ofṀ b overṀ in a single SMBH case seen in simulations of Farris et al (2014) and Shi & Krolik (2015) should not be over-interpreted in favor of a scenario with long-lasting˙> ¥ M M b , described in Section 3.2.2. First, given the short duration of these simulations-hundreds to thousands of binary orbital periods (which is at best close to t ν at the inner edge of the disk)-their inferred accretion rates cannot reflect the long-termṀ b .…”

“…Nor does it explain the recent results of Shi & Krolik (2015), who similarly to Farris et al (2014), found an enhancement of accretion by the binary (over the single SMBH), but using a grid-based code (a modified version of ZEUS for MHD) with the excised central region enclosing the binary. The different outcomes could be partly ascribed to the effects of numerical resolution (D'Orazio et al 2013 found that it affectsṀ b ) or numerical scheme used.…”

“…The different outcomes could be partly ascribed to the effects of numerical resolution (D'Orazio et al 2013 found that it affectsṀ b ) or numerical scheme used. Also, the way in whichṀ b has been related toṀ in a single SMBH case is non-unique: while some studies directly compareṀ b toṀ found in identical simulations with a single SMBH 5 (D 'Orazio et al 2013;Farris et al 2014;Shi & Krolik 2015), others use some proxies for the latterṀ, e.g., normalization by the value of Σ at some radius (MacFadyen & Milosavljević 2008;Shi et al 2012).…”