Axisymmetric simulations of magneto-rotational core collapse : dynamics and gravitational wave signal
Axisymmetric simulations of magneto-rotational core collapse : dynamics and gravitational wave signal
复制标题
磁旋转核心塌陷的轴对称模拟：动力学和引力波信号
DOI：
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发表时间：
2005
期刊：
影响因子：
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通讯作者：
Ewald Mueller
中科院分区：
文献类型：
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作者：
M. Obergaulinger;M. Aloy;Ewald Mueller
目标。我们已经进行了全面的参数研究的崩溃的旋转，强磁化的恒星核在轴对称的爱因斯坦四极公式的基础上，以确定其引力波签名。方法.我们使用牛顿显式磁流体动力学欧拉代码的基础上的松弛TVD方法的理想MHD方程的解决方案，并应用约束传输方法，以保证一个发散的磁场的自由演变。我们忽略中微子输运的影响，采用简化的状态方程。初始模型是多面体在旋转平衡与规定程度的微分旋转和旋转能量。初始磁场为纯极向磁场，场强范围为10 ^{10}~{
m G}$至$10^{13}~{
m G}$。核心的演变一直持续到核心反弹后的几十毫秒。结果初始磁场的放大主要是由核心的差分旋转产生一个强大的环向场分量的能量相当于旋转能量。极向场分量在坍缩期间通过压缩而增长，但在核心反弹之后没有显著变化。在大部分的模拟核心的增长时间的磁旋转不稳定性（MRI）是几毫秒的顺序。通过纯Ω发电机或MRI都可以达到的饱和场强为10 ^{15}~{
m G}$在核心的表面。由于MRI，产生向外输送角动量的强极向场分量的片状环流发展，前提是初始场不太弱。弱初始磁场（$la $10 ^{11}~{
m G}$）对地核动力学和引力波信号没有显著影响。强初始字段（$ga$10 ^{12}~{
引起相当大的角动量传输，由此从坍缩的核中提取旋转能量，所述坍缩的核失去离心支撑并进入长期收缩阶段。与相应的非磁性模型相比，反弹时的引力波振幅变化高达百分之几十。如果角动量损失很大，则后反弹模型。如果角动量损失很大，则核心的后反弹平衡状态从离心平衡状态变为压力平衡状态。这种跃迁在引力波信号中留下了印记，即由于离心力而引起的地核反弹所特有的大尺度振荡的振幅减小。在某些模型中，准周期性的大尺度振荡被更高频率的不规则振荡所取代。
这种模式定义了一种新的信号类型，我们称之为IV型引力波信号。准直双极流出产生了一个独特的功能，可以让他们通过引力波天文学的检测：一个大的正四极波振幅的大小类似的反弹信号。
Aims. We have performed a comprehensive parameter study of the collapse of rotating, strongly magnetized stellar cores in axisymmetry to determine their gravitational wave signature based on the Einstein quadrupole formula. Methods. We use a Newtonian explicit magnetohydrodynamic Eulerian code based on the relaxing-TVD method for the solution of the ideal MHD equations, and apply the constraint-transport method to guarantee a divergence-free evolution of the magnetic field. We neglect effects due to neutrino transport and employ a simplified equation of state. The initial models are polytropes in rotational equilibrium with a prescribed degree of differential rotation and rotational energy. The initial magnetic fields are purely poloidal the field strength ranging from $10^{10}~{
m G}$ to $10^{13}~{
m G}$. The evolution of the core is followed until a few ten milliseconds past core bounce. Results. The initial magnetic fields are amplified mainly by the differential rotation of the core giving rise to a strong toroidal field component with an energy comparable to the rotational energy. The poloidal field component grows by compression during collapse, but does not change significantly after core bounce. In large parts of the simulated cores the growth time of the magneto-rotational instability (MRI) is of the order of a few milliseconds. The saturation field strengths that can be reached both via a pure Ω dynamo or the MRI are of the order of $10^{15}~{
m G}$ at the surface of the core. Sheet-like circulation flows which produce a strong poloidal field component transporting angular momentum outwards develop due to MRI, provided the initial field is not too weak. Weak initial magnetic fields ($la$$10^{11}~{
m G}$) have no significant effect on the dynamics of the core and the gravitational wave signal. Strong initial fields ($ga$$10^{12}~{
m G}$) cause considerable angular momentum transport whereby rotational energy is extracted from the collapsed core which loses centrifugal support and enters a phase of secular contraction. The gravitational wave amplitude at bounce changes by up to a few ten percent compared to the corresponding non-magnetic model. If the angular momentum losses are large, the post-bounce model. If the angular momentum losses are large the post-bounce equilibrium state of the core changes from a centrifugally to a pressure supported one. This transition imprints in the gravitational wave signal a reduction of the amplitude of the large-scale oscillations characteristic of cores bouncing due to centrifugal forces.
In some models the quasi-periodic large-scale oscillations are replaced by higher frequency irregular oscillations. This pattern defines a new signal type which we call a type IV gravitational wave signal. Collimated bipolar outflows give rise to a unique feature that may allow their detection by means of gravitational wave astronomy: a large positive quadrupole wave amplitude of similar size as that of the bounce signal.