LONG-TERM EVOLUTION OF SLOWLY ROTATING COLLAPSAR IN SPECIAL RELATIVISTIC MAGNETOHYDRODYNAMICS
LONG-TERM EVOLUTION OF SLOWLY ROTATING COLLAPSAR IN SPECIAL RELATIVISTIC MAGNETOHYDRODYNAMICS
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DOI：
10.1088/0004-637x/704/1/354
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发表时间：
2009-05
期刊：
影响因子：
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通讯作者：
S. Harikae;T. Takiwaki;K. Kotake
中科院分区：
文献类型：
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作者：
S. Harikae;T. Takiwaki;K. Kotake
根据伽玛射线暴（GRBs）的坍缩星模型，我们给出了旋转大质量恒星坍缩的二维磁流体动力学（MHD）模拟的数值结果。在伽玛暴前体演化计算的推动下，我们将重点放在了较低的中心核心角动量上，而不是以往的研究。通过执行包括现实状态方程和中微子冷却在内的狭义相对论模拟，我们跟踪了缓慢旋转的坍缩的长期演化，长达10秒，伴随着射流和吸积盘的形成。我们的研究结果表明，对于具有坍缩功能的GRB祖先来说，存在一个临界初始角动量，在这个角动量之下，物质被迅速吞没到中心物体中，没有形成吸积盘和MHD流出物。当大于标准时，我们发现MHD喷气机的发射有以下两种方式。对于初始磁场较强的模型，吸积盘内部被放大的磁压可以驱动MHD外流，这使得强磁爆炸像一个“磁塔”。对于初始磁场较弱的模式，磁塔首先停止，随后的MHD流出是由赤道到极地吸积物质湍流流入的磁扭曲产生的。不管形成的不同，射流只能达到温和的相对论速度，爆炸能量小于1049尔格。为了在吸积盘加热的极漏斗区域获得更强的中微子能量沉积，我们发现较小的初始角动量是有利的。这是因为引力压缩使盘的温度更高。由于盘内中微子的高不透明度，我们发现ν和ν的光度几乎相当，这有利于提高能量沉积率。我们讨论了能量沉积如何能像磁驱动的过程一样有效。在计算模型中，我们认为初始角动量为j ~ 1.5jlso （jlso：最后稳定轨道的角动量）和初始磁场强度为~ 1010g的模型为grb火球的产生提供了最合理的条件，因为该模型不仅适用于磁塔快速产生MHD流出，而且适用于在真空极性漏斗中获得更强的中微子加热。
We present our numerical results of two-dimensional magnetohydrodynamic (MHD) simulations of the collapse of rotating massive stars in light of the collapsar model of gamma-ray bursts (GRBs). Pushed by recent evolution calculations of GRB progenitors, we focus on lower angular momentum of the central core than those taken mostly in previous studies. By performing special relativistic simulations including both realistic equation of state and neutrino cooling, we follow a long-term evolution of the slowly rotating collapsars up to ∼10 s, accompanied by the formation of jets and accretion disks. Our results show that for the GRB progenitors to function as collapsars, there is a critical initial angular momentum, below which matter is quickly swallowed to the central objects, no accretion disks and no MHD outflows are formed. When larger than the criteria, we find the launch of the MHD jets in the following two ways. For models with stronger initial magnetic fields, the magnetic pressure amplified inside the accretion disk can drive the MHD outflows, which makes the strong magnetic explosions like a “magnetic tower.” For models with weaker initial magnetic fields, the magnetic tower stalls first and the subsequent MHD outflows are produced by the magnetic twisting of the turbulent inflows of the accreting material from the equatorial to the polar regions. Regardless of the difference in the formation, the jets can attain only mildly relativistic speeds with the explosion energy less than 1049 erg. To obtain stronger neutrino energy depositions in the polar funnel regions heated from the accretion disk, we find that smaller initial angular momentum is favorable. This is because the gravitational compression makes the temperature of the disk higher. Due to high neutrino opacity inside the disk, we find that the luminosities of νe and become almost comparable, which is advantageous for making the energy deposition rate larger. We discuss how the energy deposition can be as efficient as the magnetically driven processes for energetizing jets. Among the computed models, we suggest that the model with the initial angular momentum of j ∼ 1.5jlso (jlso: the angular momentum of the last stable orbit) and with initial magnetic field strength of ∼1010 G, provides a most plausible condition for making fireballs for GRBs, because such model is appropriate not only for producing the MHD outflows quickly by the magnetic towers, but also for obtaining the stronger neutrino heating in the evacuated polar funnel.