Collapsars: Gamma-Ray Bursts and Explosions in “Failed Supernovae”
Collapsars: Gamma-Ray Bursts and Explosions in “Failed Supernovae”
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DOI：
10.1086/307790
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发表时间：
1998-10
期刊：
影响因子：
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通讯作者：
A. MacFadyen;S. Woosley
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文献类型：
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作者：
A. MacFadyen;S. Woosley
利用二维流体力学程序(PROMETHUS)，我们研究了旋转氦恒星Mα≳10M☉的持续演化，其中铁核坍塌不会产生成功的出射激波，而是形成一个2-3M☉的黑洞。最详细地探索的模型是一颗35M☉主序星的14M☉氦核心。结果对角动量很敏感。对于J16≡j/(1016cm2 S-1)≲3，物质几乎不受限制地落入黑洞。预计不会有资金外流。对于J16≳20，流入的物质在1000公里外被离心力阻止，在那里中微子的损失可以忽略不计。赤道吸积率很低，爆炸性的氧气燃烧可能会为赤道微弱的爆炸提供动力。然而，对于3≲J16≲20，对于这类恒星来说，一个合理的值是，在一个半径处形成一个致密盘，在这个半径处，引力结合能可以被有效地辐射为中微子，或者通过磁流体动力学过程转换为束流。这些都是产生伽马射线暴(GRB)的最佳候选者。在这里，我们研究了这种圆盘的形成，相关的流型，以及圆盘粘性参数α≈0.001和0.1时的吸积率。沿着旋转轴的流入最初是不受限制的，在最初的几秒钟内会打开一个疏散的通道。与此同时，黑洞被吸积旋转(到MHD 0.9时)，能量在盘中被≈过程耗散，并被中微子辐射。对于α=0.1.1的模型，在30°到45°的极角之间有明显的能量外流。这些外流由盘中的粘性耗散驱动，具有高达几倍于1051ergs的能量和~1M☉的质量，并富含56Ni。它们本身就构成了一次类似超新星的爆炸。同时，通过圆盘的吸积保持了大约10-20个S，但这是时变的(±30%)，这是因为在原子核正在经历光解的区域，外缘的流体动力学不稳定。由于中微子能量沉积效率对吸积率很敏感，这种不稳定性导致极区能量沉积高度可变。在50ms和泛音具有显著功率的这种可变性中的一些可能在突发的时间结构中持续存在。在接下来的时间里，标准α=0.1和J16=10模型的平均吸积速率为0.07M☉S-1。由中微子湮灭沿旋转轴沉积的总能量为(1-14)×1051ergs，这取决于克尔参数的演化和不确定的中微子效率。模拟极地地区能量的沉积，以每极5×1050ergs S-1的恒定速率，导致强相对论外流急流照射到大约1%的天空。这些喷流可能还会受到流经的“喷嘴”两侧的不稳定性的影响。喷流吹走吸积物质，保持高度聚焦，并能够穿透S附近的恒星。当喷流穿过恒星表面后，可以出现高度相对论的气流。由于物质抛射和喷流对吸积率、角动量和圆盘粘性的敏感性，以及观测结果随视角的变化，可能出现大范围的结果，从明亮的伽玛暴971214到微弱的伽玛暴超新星SN 1998bw。当喷流第一次从恒星中爆发出来时，X射线前体也是可能的。虽然只有一小部分超新星会产生伽玛暴，但我们预测，塌陷星总是会产生类似SN1998bw的超新星。然而，在这个模型中，短于几秒的硬的、高能量的伽玛暴将很难产生，可能需要合并中子星和黑洞来解释它们。
Using a two-dimensional hydrodynamics code (PROMETHEUS), we explore the continued evolution of rotating helium stars, Mα ≳ 10 M☉, in which iron-core collapse does not produce a successful outgoing shock but instead forms a black hole of 2-3 M☉. The model explored in greatest detail is the 14 M☉ helium core of a 35 M☉ main-sequence star. The outcome is sensitive to the angular momentum. For j16 ≡ j/(1016 cm2 s-1) ≲ 3, material falls into the black hole almost uninhibited. No outflows are expected. For j16 ≳ 20, the infalling matter is halted by centrifugal force outside 1000 km where neutrino losses are negligible. The equatorial accretion rate is very low, and explosive oxygen burning may power a weak equatorial explosion. For 3 ≲ j16 ≲ 20, however, a reasonable value for such stars, a compact disk forms at a radius at which the gravitational binding energy can be efficiently radiated as neutrinos or converted to beamed outflow by magnetohydrodynamical (MHD) processes. These are the best candidates for producing gamma-ray bursts (GRBs). Here we study the formation of such a disk, the associated flow patterns, and the accretion rate for disk viscosity parameter α ≈ 0.001 and 0.1. Infall along the rotational axis is initially uninhibited, and an evacuated channel opens during the first few seconds. Meanwhile the black hole is spun up by the accretion (to a ≈ 0.9), and energy is dissipated in the disk by MHD processes and radiated by neutrinos. For the α = 0.1 model, appreciable energetic outflows develop between polar angles of 30° and 45°. These outflows, powered by viscous dissipation in the disk, have an energy of up to a few times 1051 ergs and a mass ~1 M☉ and are rich in 56Ni. They constitute a supernova-like explosion by themselves. Meanwhile accretion through the disk is maintained for approximately 10-20 s but is time variable (±30%) because of hydrodynamical instabilities at the outer edge in a region where nuclei are experiencing photodisintegration. Because the efficiency of neutrino energy deposition is sensitive to the accretion rate, this instability leads to highly variable energy deposition in the polar regions. Some of this variability, which has significant power at 50 ms and overtones, may persist in the time structure of the burst. During the time followed, the average accretion rate for the standard α = 0.1 and j16 = 10 model is 0.07 M☉ s-1. The total energy deposited along the rotational axes by neutrino annihilation is (1-14) × 1051 ergs, depending upon the evolution of the Kerr parameter and uncertain neutrino efficiencies. Simulated deposition of energy in the polar regions, at a constant rate of 5 × 1050 ergs s-1 per pole, results in strong relativistic outflow jets beamed to about 1% of the sky. These jets may be additionally modulated by instabilities in the sides of the "nozzle" through which they flow. The jets blow aside the accreting material, remain highly focused, and are capable of penetrating the star in ~10 s. After the jet breaks through the surface of the star, highly relativistic flow can emerge. Because of the sensitivity of the mass ejection and jets to accretion rate, angular momentum, and disk viscosity, and the variation of observational consequences with viewing angle, a large range of outcomes is possible, ranging from bright GRBs like GRB 971214 to faint GRB-supernovae like SN 1998bw. X-ray precursors are also possible as the jet first breaks out of the star. While only a small fraction of supernovae make GRBs, we predict that collapsars will always make supernovae similar to SN 1998bw. However, hard, energetic GRBs shorter than a few seconds will be difficult to produce in this model and may require merging neutron stars and black holes for their explanation.