致密天体并合形成的粘性扩展吸积盘中的丰中子冻结
Neutron-Rich Freeze-Out in Viscously Spreading Accretion Disks Formed from Compact Object Mergers
摘要：
在黑洞-NS双星和中子星-NS双星的合并过程中，形成了质量约为0.001-0.1 Msun的吸积盘。最初，这样的超吸积盘冷却有效的中微子发射和他们的组合物是驱动的对简并条件下捕获的中子丰富。然而，随着磁盘粘性扩散和它的温度下降，冷却变得低效和磁盘成为平流。分析论证和数值模拟表明，一旦发生这种情况，强风可能会赶走大部分磁盘的剩余质量。我们用一维高度积分模拟计算了致密天体合并形成的粘性扩散吸积盘的热演化和核成分。我们表明，冻结出弱平衡必然伴随着磁盘的后期过渡到平流状态。因此，超吸积盘通常冻结出富中子（电子分数Ye ~ 0.2-0.4），并且它们的后期流出强烈合成稀有的富中子同位素。利用太阳系中这些同位素的丰度测量值，我们将银河系中致密天体的合并速率限制为<1 e-5（M_d，0/0.1 Msun）^（-1）/年，其中M_d，0是吸积盘的平均初始质量。因此，要么NS-NS合并率处于当前估计的低端，要么典型合并期间产生的平均盘质量<< 0.1 Msun。我们还指出，如果大多数短持续时间的伽马射线暴（GRB）是由致密天体并合产生的，那么它们的聚束率必须超过f_b ~ 0.13（M_d，0/0.1 Msun），对应于喷流半张角> 30（M_d，0/0.1 Msun）^（1/2）度。这与其他证据一致，即短持续时间的伽玛射线暴流出比长持续时间的伽玛射线暴流出的准直度更低。
英文摘要：
Accretion disks with masses ~0.001-0.1 Msun form during the merger of neutron star (NS)-NS and black hole-NS binaries. Initially, such hyper-accreting disks cool efficiently by neutrino emission and their composition is driven neutron-rich by pair captures under degenerate conditions. However, as the disk viscously spreads and its temperature drops, cooling becomes inefficient and the disk becomes advective. Analytic arguments and numerical simulations suggest that once this occurs, powerful winds likely drive away most of the disk's remaining mass. We calculate the thermal evolution and nuclear composition of viscously spreading accretion disks formed from compact object mergers using one-dimensional height-integrated simulations. We show that freeze-out from weak equilibrium necessarily accompanies the disk's late-time transition to an advective state. As a result, hyper-accreting disks generically freeze out neutron-rich (with electron fraction Ye ~ 0.2-0.4), and their late-time outflows robustly synthesize rare neutron-rich isotopes. Using the measured abundances of these isotopes in our solar system, we constrain the compact object merger rate in the Milky Way to be < 1e-5 (M_d,0/0.1 Msun)^(-1) per year, where M_d,0 is the average initial mass of the accretion disk. Thus, either the NS-NS merger rate is at the low end of current estimates or the average disk mass produced during a typical merger is << 0.1 Msun. We also show that if most short duration gamma-ray bursts (GRBs) are produced by compact object mergers, their beaming fraction must exceed f_b ~ 0.13(M_d,0/0.1 Msun), corresponding to a jet half-opening angle > 30(M_d,0/0.1 Msun)^(1/2) degrees. This is consistent with other evidence that short duration GRB outflows are less collimated than those produced in long duration GRBs.
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