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arXiv 1910.04763astro-ph.HEastro-ph.EPastro-ph.SR
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从有限和无限轨道的环二进制增生

Circumbinary Accretion from Finite and Infinite Disks

Diego Muñoz, Dong Lai, Kaitlin Kratter, Ryan Miranda

摘要:

利用{ \footnotesize AREPO}对环双星盘吸积过程进行了二维粘性流体动力学模拟。我们解决了吸积流从一个大规模的CBD到流光和磁盘周围的个别二进制组件。扩展我们最近的研究\citep{mun19},我们认为圆形双星与各种质量比($0.1\leq q_{\rm{b}}\leq1$)和研究吸积从"无限“,稳定供应盘和有限大小,粘性传播环面。对于"无限“盘,可以达到全局稳定状态,并且吸积可变性对于$q_{\rm{b}}>0.5$具有主频率${\sim}0.2Ω_{\rm{b}}$,对于$q_{\rm{b}}<0.5$具有主频率$Ω_{\rm{b}}$,($Ω_{\rm{b}}$是二进制角频率)。我们发现吸积“本征值”$l_0$ --每单位吸积质量从盘到双星的净角动量转移--总是正值,并且落在范围($0.65$ - $0.85)a_{\rm b}^2Ω_{\rm{b}}$(其中$a_{\rm{b}}$为双星分离))内,对质量比和粘度的依赖很弱。当$q_{\rm{b}}\gtrsim0.3$ .从一个有限的环面的吸积可以分为两个阶段:一个初始的瞬态阶段,对应于填充的二进制腔,其次是一个粘性的伪稳定阶段,在此期间的环面粘性扩展和吸积到二进制。在粘性阶段,二元系每单位吸积质量的净扭矩接近$l_0$,即针对“无限”圆盘推导出的值。我们的结论是,从CBD吸积的类似质量的双星获得角动量和扩展在很长的时间尺度。这一结果对超大质量双星黑洞和新形成的双星的合并产生了重大影响。我们提供了一个字的警告,从模拟到空的circumbinary腔瞬态吸积得出的结论。

英文摘要:

We carry out 2D viscous hydrodynamics simulations of circumbinary disk (CBD) accretion using {\footnotesize AREPO}. We resolve the accretion flow from a large-scale CBD down to the streamers and disks around individual binary components. Extending our recent studies \citep{mun19}, we consider circular binaries with various mass ratios ($0.1\leq q_{\rm{b}}\leq1$) and study accretion from ``infinite'', steady-supply disks and from finite-sized, viscously spreading tori. For ``infinite'' disks, a global steady state can be reached, and the accretion variability has a dominant frequency ${\sim}0.2Ω_{\rm{b}}$ for $q_{\rm{b}}>0.5$ and $Ω_{\rm{b}}$ for $q_{\rm{b}}<0.5$, ($Ω_{\rm{b}}$ is the binary angular frequency). We find that the accretion ``eigenvalue'' $l_0$ -- the net angular momentum transfer from the disk to the binary per unit accreted mass -- is always positive and falls in the range ($0.65$-$0.85)a_{\rm b}^2Ω_{\rm{b}}$ (with $a_{\rm{b}}$ the binary separation), depending weakly on the mass ratio and viscosity. This leads to binary expansion when $q_{\rm{b}}\gtrsim0.3$. Accretion from a finite torus can be separated into two phases: an initial transient phase, corresponding to the filling of the binary cavity, followed by a viscous pseudo-stationary phase, during which the torus viscously spreads and accretes onto the binary. In the viscous phase, the net torque on the binary per unit accreted mass is close to $l_0$, the value derived for ``infinite'' disks. We conclude that similar-mass binaries accreting from CBDs gain angular momentum and expand over long time scales. This result significantly impacts the coalescence of supermassive binary black holes and newly formed binary stars. We offer a word of caution against conclusions drawn from simulations of transient accretion onto empty circumbinary cavities.

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