Cosmic-ray propagation in the bi-stable interstellar medium
Cosmic-ray propagation in the bi-stable interstellar medium
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双稳态星际介质中的宇宙射线传播
DOI：
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发表时间：
2018
影响因子：
6.5
通讯作者：
Y. Dubois
中科院分区：
文献类型：
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作者：
B. Commerçon;A. Marcowith;Y. Dubois
语境。宇宙射线穿过银河系尺度向下传播到恒星形成的较小尺度。宇宙射线与星际介质的其他成分接近能量均分，如果压力梯度出现，可以提供对抗重力的支持。
目标。我们研究宇宙射线在湍流和磁化双稳态星际气体中的传播。研究了宇宙射线捕获和宇宙射线压力梯度发展所需的条件。
方法。我们得出了湍流介质中宇宙射线捕获的临界扩散系数的分析值，该值遵循观察到的比例关系。然后，我们提出了一项数值研究，使用星际气体和宇宙射线混合物的演化进行 3D 模拟，其中在 40 件盒子内通过随机力以不同规模驱动湍流。我们探索了一个大参数空间，其中宇宙射线扩散系数、磁化强度、驱动尺度、湍流强迫的幅度以及初始宇宙射线能量密度都发生变化。
结果。我们发现宇宙射线扩散系数的星际动力学发生了明显的转变，低于从观察到的尺度关系推导出来的临界值。该临界扩散取决于 Dcrit ≃ 3.1 × 1023 cm2 s−1(L/1 pc)q+1 的特征长度尺度 L，其中指数 q 将湍流速度色散 σ 与长度尺度相关联，即 σ ~ Lq。因此，在我们的模拟中，这种转变发生在 Dcrit ≃ 1024–1025 cm2 s−1 附近。在我们参数研究的所有情况下都恢复了转变，并且与我们的简单分析估计非常一致。在捕获的宇宙射线区域中，诱导的宇宙射线压力梯度可以改变气体流动并提供对热不稳定性发展的支持。我们讨论了可以显着降低星际介质内宇宙射线扩散系数的可能机制。
结论。对于扩散系数 D0 ≤ 1025 cm2 s−1 或在宇宙射线压力超过热压力十倍以上的区域，宇宙射线压力梯度可以发展和改变热双稳态气体的演化。这项研究为进一步的工作提供了基础，包括更真实的宇宙射线扩散系数以及局部宇宙射线源。
Context. Cosmic rays propagate through the galactic scales down to the smaller scales at which stars form. Cosmic rays are close to energy equipartition with the other components of the interstellar medium and can provide a support against gravity if pressure gradients develop.
Aims. We study the propagation of cosmic rays within the turbulent and magnetised bi-stable interstellar gas. The conditions necessary for cosmic-ray trapping and cosmic-ray pressure gradient development are investigated.
Methods. We derived an analytical value of the critical diffusion coefficient for cosmic-ray trapping within a turbulent medium, which follows the observed scaling relations. We then presented a numerical study using 3D simulations of the evolution of a mixture of interstellar gas and cosmic rays, in which turbulence is driven at varying scales by stochastic forcing within a box of 40 pc. We explored a large parameter space in which the cosmic-ray diffusion coefficient, the magnetisation, the driving scale, and the amplitude of the turbulence forcing, as well as the initial cosmic-ray energy density, vary.
Results. We identify a clear transition in the interstellar dynamics for cosmic-ray diffusion coefficients below a critical value deduced from observed scaling relations. This critical diffusion depends on the characteristic length scale L of Dcrit ≃ 3.1 × 1023 cm2 s−1(L/1 pc)q+1, where the exponent q relates the turbulent velocity dispersion σ to the length scale as σ ~ Lq. Hence, in our simulations this transition occurs around Dcrit ≃ 1024–1025 cm2 s−1. The transition is recovered in all cases of our parameter study and is in very good agreement with our simple analytical estimate. In the trapped cosmic-ray regime, the induced cosmic-ray pressure gradients can modify the gas flow and provide a support against the thermal instability development. We discuss possible mechanisms that can significantly reduce the cosmic-ray diffusion coefficients within the interstellar medium.
Conclusions. Cosmic-ray pressure gradients can develop and modify the evolution of thermally bi-stable gas for diffusion coefficients D0 ≤ 1025 cm2 s−1 or in regions where the cosmic-ray pressure exceeds the thermal one by more than a factor of ten. This study provides the basis for further works including more realistic cosmic-ray diffusion coefficients, as well as local cosmic-ray sources.
影响因子：
6.5
作者：
H. C. F. Aharonian;E. al.
通讯作者：
H. C. F. Aharonian;E. al.