自引力盘中二元黑洞的演化：力矩的剖析
Evolution of binary black holes in self gravitating discs: dissecting the torques
摘要：
研究了气体吸积和引力矩的相互作用对双星系统的角动量收支的影响。特别分析了重力力矩（T_g）的物理来源及其在圆盘中的位置。我们分析了初始准圆形大质量黑洞双星（BHB）的演化的3D SPH模拟居住在中央中空的大质量自引力circumbinary光盘。我们采用不同的腔内热力学和黑洞的数值尺寸，表明：（i）BHB偏心增长是一个普遍的结果，与吸积和所采用的热力学无关;（ii）半长轴衰变既取决于T_g，又取决于吸积引起的盘状双星L-转移的相互作用;（iii）T_g的光谱结构主要是由盘边缘的过密度和盘体中的旋臂引起的，一般不能直接反映双星的周期;（iv）净T_g在BHB共转半径上变号。我们量化的相对重要性，这两个，这似乎取决于的critical属性的instreaming气体，这是至关重要的，在评估光盘二进制L-转移;（V）净扭矩表现为一个纯粹的运动学（非共振）的效果，因为它源于腔，在那里的材料流入和流出的高度偏心轨道。在评估黑洞的命运时，必须考虑到黑洞的吸积和与腔内气流的相互作用。此外，由盘施加的总扭矩通过改变BHB的所有元素（质量、质量比、偏心率、半长轴）来影响L（BHB）。因此，将潮汐扭矩等同于半长轴收缩的常见处方可能对真实的天体物理系统来说是很差的近似。
英文摘要：
We study the interplay between gas accretion and gravity torques in changing a binary elements and its total angular momentum (L) budget. Especially, we analyse the physical origin of the gravity torques (T_g) and their location within the disc. We analyse 3D SPH simulations of the evolution of initially quasi-circular massive black hole binaries (BHBs) residing in the central hollow of massive self-gravitating circumbinary discs. We use different thermodynamics within the cavity and for the numerical size of the black holes to show that (i) the BHB eccentricity growth found previously is a general result, independent of the accretion and the adopted thermodynamics; (ii) the semi-major axis decay depends both on the T_g and on the interplay with the disc-binary L-transfer due to accretion; (iii) the spectral structure of the T_g is predominately caused by disc edge overdensities and spiral arms developing in the body of the disc and, in general, does not reflect directly the period of the binary; (iv) the net T_g changes sign across the BHB corotation radius. We quantify the relative importance of the two, which appear to depend on the thermodynamical properties of the instreaming gas, and which is crucial in assessing the disc-binary L-transfer; (v) the net torque manifests as a purely kinematic (non-resonant) effect as it stems from the cavity, where the material flows in and out in highly eccentric orbits. Both accretion onto the black holes and the interaction with gas streams inside the cavity must be taken into account to assess the fate of the BHB. Moreover, the total torque exerted by the disc affects L(BHB) by changing all the elements (mass, mass ratio, eccentricity, semimajor axis) of the BHB. Common prescriptions equating tidal torque to semi-major axis shrinking might therefore be poor approximations for real astrophysical systems.
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